INTRODUCTION TO GSMINTRODUCTION• The Global System for Mobile Communications (GSM) is a set of recommendations and specifications for a digital cellular telephone network (known as a Public Land Mobile Network, or PLMN).• These recommendations ensure the compatibility of equipment from different GSM manufacturers, and interconnectivity between different administrations, including operation across international boundaries.• GSM networks are digital and can cater for high system capacities.• They are consistent with the world-wide digitization of the telephone network, and are an extension of the Integrated Services Digital Network (ISDN), using a digital radio interface between the cellular network and the mobile subscriber equipment.
INTRODUCTION TO GSMCELLULAR TELEPHONY• A cellular telephone system links mobile subscribers into the public telephone system or to another cellular subscriber.• Information between the mobile unit and the cellular network uses radio communication. Hence the subscriber is able to move around and become fully mobile.• The service area in which mobile communication is to be provided is divided into regions called cells.• Each cell has the equipment to transmit and receive calls from any subscriber located within the borders of its radio coverage area. Radio Cell Mobile subscriber
INTRODUCTION TO GSMGSM FREQUENCIES• GSM systems use radio frequencies between 890-915 MHz for receive and between 935-960 MHz for transmit.• RF carriers are spaced every 200 kHz, allowing a total of 124 carriers for use.• An RF carrier is a pair of radio frequencies, one used in each direction.• Transmit and receive frequencies are always separated by 45 MHz. UPLINK FREQUENCIES DOWNLINK FREQUENCIES 890 915 935 960 UPLINK AND DOWNLINK FREQUENCY SEPARATED BY 45MHZ
INTRODUCTION TO GSMExtended GSM (EGSM)• EGSM has 10MHz of bandwidth on both transmit and receive.• Receive bandwidth is from 880 MHz to 890 MHz.• Transmit bandwidth is from 925 MHz to 935 MHz.• Total RF carriers in EGSM is 50. UPLINK FREQUENCIES DOWNLINK FREQUENCIES 880 890 915 925 935 960 UPLINK AND DOWNLINK FREQUENCY SEPARATED BY 45MHZ
INTRODUCTION TO GSMDCS1800 FREQUENCIES• DCS1800 systems use radio frequencies between 1710-1785 MHz for receive and between 1805-1880 MHz for transmit.• RF carriers are spaced every 200 kHz, allowing a total of 373 carriers.• There is a 100 kHz guard band between 1710.0 MHz and 1710.1 MHz and between 1784.9 MHz and 1785.0 MHz for receive, and between 1805.0 MHz and 1805.1 MHz and between 1879.9 MHz and 1880.0 MHz for transmit.• Transmit and receive frequencies are always separated by 95 MHz. UPLINK FREQUENCIES DOWNLINK FREQUENCIES 1710 MHz 1785 MHz 1805 MHz 1880 MHz UPLINK AND DOWNLINK FREQUENCY SEPARATED BY 95MHZ
FEATURES OF GSM
FEATURES OF GSMINCREASED CAPACITY• The GSM system provides a greater subscriber capacity than analogue systems.• GSM allows 25 kHz per user, that is, eight conversations per 200 kHz channel pair (a pair comprising one transmit channel and one receive channel).• Digital channel coding and the modulation used makes the signal resistant to interference from cells where the same frequencies are re- used (co-channel interference); a Carrier to Interference Ratio (C/I) level of 12 dB is achieved, as opposed to the 18 dB typical with analogue cellular.• This allows increased geographic reuse by permitting a reduction in the number of cells in the reuse pattern.
FEATURES OF GSMAUDIO QUALITY• Digital transmission of speech and high performance digital signal processors provide good quality speech transmission.• Since GSM is a digital technology, the signals passed over a digital air interface can be protected against errors by using better error detection and correction techniques.• In regions of interference or noise-limited operation the speech quality is noticeably better than analogue.USE OF STANDARDISED OPEN INTERFACES• Standard interfaces such as C7 and X25 are used throughout the system. Hence different manufacturers can be selected for different parts of the PLMN.• There is a high flexibilty in where the Network components are situated.
FEATURES OF GSMIMPROVED SECURITY AND CONFIDENTIALITY• GSM offers high speech and data confidentiality.• Subscriber authentication can be performed by the system to check if a subscriber is a valid subscriber or not.• The GSM system provides for high degree of confidentiality for the subscriber. Calls are encoded and ciphered when sent over air.• The mobile equipment can be identified independently from the mobile subscriber. The mobile has a identity number hard coded into it when it is manufactured. This number is stored in a standard database and whenever a call is made the equipment can be checked to see if it has been reported stolen.
FEATURES OF GSMCLEANER HANDOVERS• GSM uses Mobile assisted handover techique.• The mobile itself carries out the signal strength and quality measurement of its server and signal strength measurement of its neighbors.• This data is passed on the Network which then uses sophisticated algorithms to determine the need of handover.SUBSCRIBER IDENTIFICATION• In a GSM system the mobile station and the subscriber are identified separately.• The subscriber is identified by means of a smart card known as a SIM.• This enables the subscriber to use different mobile equipment while retaining the same subscriber number.
FEATURES OF GSMENHANCED RANGE OF SERVICES• Speech services for normal telephony.• Short Message Service for point ot point transmission of text message.• Cell broadcast for transmission of text message from the cell to all MS in its coverage area. Message like traffic information or advertising can be transmitted.• Fax and data services are provided. Data rates available are 2.4 Kb/ s, 4.8 Kb/s and 9.6 Kb/s.• Supplementary services like number identification , call barring, call forwarding, charging display etc can be provided.
FEATURES OF GSMFREQUENCY REUSE• There are total 124 carriers in GSM ( additional 50 carriers are available if EGSM band is used).• Each carrier has 8 timeslots and if 7 can be used for traffic then a maximum of 868 ( 124 X 7 ) calls can be made. This is not enough and hence frequencies have to be reused.• The same RF carrier can be used for many conversations in several different cells at the same time.• The radio carriers available are allocated according to a regular pattern which repeats over 2 the whole coverage area. 1 3• 4 The pattern to be used depends on traffic 5 7 requirement and spectrum availability. 6 2• Some typical repeat patterns are 4/12, 7/21 etc. 1
NETWORK COMPONENTS H DNMC EIR AUC HLR VLR F C B A XCDR OMC-S MSC IWF UM BSC ABIS OMC-R EC BTS BTS PSTN UM
NETWORK COMPONENTSMobile Switching Centre (MSC)• The Mobile services Switching Centre (MSC) co-ordinates the setting up of calls to and from GSM users.• It is the telephone switching office for MS originated or terminated traffic and provides the appropriate bearer services, teleservices and supplementary services.• It controls a number of Base Station Sites (BSSs) within a specified geographical coverage area and gives the radio subsystem access to the subscriber and equipment databases.• The MSC carries out several different functions depending on its position in the network.• When the MSC provides the interface between PSTN and the BSS in the GSM network it is called the Gateway MSC.• Some important functions carried out by MSC are Call processing including control of data/voice call setup, inter BSS & inter MSC handovers, control of mobility management, Operation & maintenance support including database management, traffic metering and man machine interface & managing the interface between GSM & PSTN
NETWORK COMPONENTSMobile Switching Centre (MSC) – Lucent MSC
NETWORK COMPONENTSMobile Station (MS)• The Mobile Station consists of the Mobile Equipment (ME) and the Subscriber Identity Module (SIM).Mobile Equipment• The Mobile Equipment is the hardware used by the subscriber to access the network.• The mobile equipment can be Vehicle mounted, with the antenna physically mounted on the outside of the vehicle or portable mobile unit, which can be handheld.• Mobiles are classified into five classes according to their power rating. CLASS POWER OUTPUT 1 20W 2 8W 3 5W 4 2W 5 0.8W
NETWORK COMPONENTSSIM• The SIM is a removable card that plugs into the ME.• It identifies the mobile subscriber and provides information about the service that the subscriber should receive.• The SIM contains several pieces of information – International Mobile Subscribers Identity ( IMSI ) - This number identifies the mobile subscriber. It is only transmitted over the air during initialising. – Temporary Mobile Subscriber Identity ( TMSI ) - This number also identifies the subscriber. It can be alternatively used by the system. It is periodically changed by the system to protect the subscriber from being identified by someone attempting to monitor the radio interface. – Location Area Identity ( LAI ) - Identifies the current location of the subscriber. – Subscribers Authentication Key ( Ki ) - This is used to authenticate the SIM card. – Mobile Station International Standard Data Number ( MSISDN ) -
NETWORK COMPONENTSSIM• Most of the data contained within the SIM is protected against reading (eg Ki ) or alterations after the SIM is issued.• Some of the parameters ( eg. LAI ) will be continously updated to reflect the current location of the subscriber.• The SIM card can be protected by use of Personal Identity Number ( PIN ) password.• The SIM is capable of storing additional information such as accumulated call charges. FULL SIZE SIM CARD MINI SIM CARD GSM
NETWORK COMPONENTSMobile Station International Subscribers Dialling Number ( MSISDN ) :• Human identity used to call a MS• The Mobile Subscriber ISDN (MSISDN) number is the telephone number of the MS.• This is the number a calling party dials to reach the subscriber.• It is used by the land network to route calls toward the MSC. CC NDC SN 98 XXX 12345 CC = Country code NDC = National Destination Code SN = Subscriber Number
NETWORK COMPONENTSInternational Mobile Subscribers Identity ( IMSI ) :• Network Identity Unique to a MS• The International Mobile Subscriber Identity (IMSI) is the primary identity of the subscriber within the mobile network and is permanently assigned to that subscriber.• The IMSI can be maximum of 15 digits. MCC MNC MSIN 404 XX 12345..10 MCC = Mobile Country Code ( 3 Digits ) MNC = Mobile Network Code ( 2 Digits ) MSIN = Mobile Subscriber Identity Number
NETWORK COMPONENTSTemporary Mobile Subscribers Identity ( TMSI ) :• The GSM system can also assign a Temporary Mobile Subscriber Identity (TMSI).• After the subscribers IMSI has been initialized on the system, the TMSI can be used for sending messages backwards and forwards across the network to identify the subscriber.• The system automatically changes the TMSI at regular intervals, thus protecting the subscriber from being identified by someone attempting to monitor the radio channels.• The TMSI is a local number and is always allocated by the VLR.• The TMSI is maximum of 4 octets.
NETWORK COMPONENTSEquipment Identity Register ( EIR )• The Equipment Identity Register (EIR) contains a centralized database for validating the international mobile station equipment identity, the IMEI.• The database contains three lists: – The white list contains the number series of equipment identities that have been allocated in the different participating countries. This list does not contain individual numbers but but a range of numbers by identifying the beginning and end of the series. – The grey list contains IMEIs of equipment to be monitored and observed for location and correct function. – The black list contains IMEIs of MSs which have been reported stolen or are to be denied service.• The EIR database is remotely accessed by the MSC’s in the Network and can also be accessed by an MSC in a different PLMN. .
NETWORK COMPONENTSEquipment Identity Register ( EIR ) EIR White List Grey List Black List All Valid Service allowed Service denied assigned ID’s but noted Range 1 MS IMEI 1 MS IMEI 1 Range 2 MS IMEI 2 MS IMEI 2 Range n MS IMEI n MS IMEI n
NETWORK COMPONENTSInternational Mobile Equipment Identity ( IMEI ) :• IMEI is a serial number unique to each mobile• Each MS is identified by an International Mobile station Equipment Identity (IMEI) number which is permanently stored in the Mobile Equipment.• On request, the MS sends this number over the signalling channel to the MSC.• The IMEI can be used to identify MSs that are reported stolen or operating incorrectly. TAC FAC SNR SP 6 2 6 1 TAC = Type Approval Code FAC = Final Assembly Code SNR = Serial Number SP = Spare
NETWORK COMPONENTSHOME LOCATION REGISTER( HLR )• The HLR contains the master database of all subscribers in the PLMN.• This data is remotely accessed by the MSC´´s and VLRs in the network. The data can also be accessed by an MSC or a VLR in a different PLMN to allow inter-system and inter-country roaming.• A PLMN may contain more than one HLR, in which case each HLR contains a portion of the total subscriber database. There is only one database record per subscriber.• The subscribers data may be accessed by the IMSI or the MSISDN.• The parameters stored in HLR are – Subscribers ID (IMSI and MSISDN ) – Current subscriber VLR. – Supplementary services subscribed to. – Supplementary services information (eg. Current forwarding address ). – Authentication key and AUC functionality. – TMSI and MSRN
NETWORK COMPONENTSVISITOR LOCATION REGISTER ( VLR )• The Visited Location Register (VLR) is a local subscriber database, holding details on those subscribers who enter the area of the network that it covers.• The details are held in the VLR until the subscriber moves into the area serviced by another VLR.• The data includes most of the information stored at the HLR, as well as more precise location and status information.• The additional data stored in VLR are – Mobile status ( Busy / Free / No answer etc. ) – Location Area Identity ( LAI ) – Temporary Mobile Subscribers Identity ( TMSI ) – Mobile Station Roaming Number ( MSRN )• The VLR provides the system elements local to the subscriber, with basic information on that subscriber, thus removing the need to access the HLR every time subscriber information is required.
NETWORK COMPONENTSAuthentication Centre ( AUC ) • The AUC is a processor system that perform authentication function. • It is normally co-located with the HLR. • The authentication process usually takes place each time the subscriber initialises on the system. • Each subscriber is assigned an authentication key (Ki) which is stored in the SIM and at the AUC. • A random number of 128 bits is generated by the AUC & sent to the MS. • The authentication algorithm A3 uses this random number and authentication key Ki to produce a signed response SRES( Signed Response ). • At the same time the AUC uses the random number and Authentication algoritm A3 along with the Ki key to produce a SRES. • If the SRES produced by AUC matches the one produced by MS is the same, the subscriber is permitted to use the network.
NETWORK COMPONENTSAUTHENTICATION PROCESS HLR AUC Ki, A3, A8 MS A3 ( RAND, Ki ) = SRES A3 , A8 , A5 , Ki A8 ( RAND, Ki ) = Kc Triples RAND Generated TRIPLES VLR RAND, Kc , SRES RAND Kc SRES SRES SRES = A3 (RAND , Ki ) SRES SRES = SRES BTS A5 , AIR INTERFACE ENCRYPTION Kc = HYPERFRAME NUM A8 (RAND , Ki ) Kc
NETWORK COMPONENTSBase Station Sub-System ( BSS ) :• The BSS is the fixed end of the radio interface that provides control and radio coverage functions for one or more cells and their associated MSs.• It is the interface between the MS and the MSC.• The BSS comprises one or more Base Transceiver Stations (BTSs), each containing the radio components that communicate with MSs in a given area, and a Base Site Controller (BSC) which supports call processing functions and the interfaces to the MSC.• Digital radio techniques are used for the radio communications link, known as the Air Interface, between the BSS and the MS.• The BSS consists of three basic Network Elements (NEs). – Transcoder (XCDR) or Remote transcoder (RXCDR) . – Base Station Controller (BSC). – Base Transceiver Stations (BTSs) assigned to the BSC. .
NETWORK COMPONENTSTranscoder( XCDR )• The speech transcoder is the interface between the 64 kbit/s PCM channel in the land network and the 13 kbit/s vocoder (actually 22.8 kbit/s after channel coding) channel used on the Air Interface.• This reduces the amount of information carried on the Air Interface and hence, its bandwidth.• If the 64 kbits/s PCM is transmitted on the air interface without occupation, it would occupy an excessive amount of radio bandwidth. This would use the available radio spectrum inefficiently.• The required bandwidth is therefore reduced by processing the 64 kbits/s PCM data so that the amount of information required to transmit digitized voice falls to 13kb/s.• The XCDR can multiplex 4 traffic channels into a single 64 kbit/s timeslot. Thus a E1/T1 serial link can carry 4 times as many channels.• This can reduce the number of E1/T1 leased lines required to connect remotely located equipment.• When the transcoder is between the MSC and the BSC it is called a remote transcoder (RXCDR).
NETWORK COMPONENTSTRANSCODER(XCDR) - Siemens
NETWORK COMPONENTSTRANSCODING 30 Timeslots 1 traffic channel / TS Each Timeslot =16 X 4 64 Kbps / TS = 64 Kb/s 4 E1 lines = 30 X 4 30 timeslots = 30 x 4 =120 Timeslots =120 traffic channels MSC XCDR BSC Transcoded information from four calls 0 1 2 16 31
NETWORK COMPONENTSBase Station Controller (BSC)• The BSC network element provides the control for the BSS.• It controls and manages the associated BTSs, and interfaces with the Operations and Maintenance Centre (OMC).• The purpose of the BSC is to perform a variety of functions. The following comprise the functions provided by the BSC: – Controls the BTS components.- – Performs Call Processing. – Performs Operations and Maintenance (O & M). – Provides the O & M link (OML) between the BSS and the OMC. – Provides the A Interface between the BSS and the MSC. – Manages the radio channels. – Transfers signalling information to and from MSs.
NETWORK COMPONENTSBase Station Controller (BSC) – Siemens BSC
NETWORK COMPONENTSBase Transceiver Station (BTS)• The BTS network element consists of the hardware components, such as radios, interface modules and antenna systems that provide the Air Interface between the BSS and the MSs.• The BTS provides radio channels (RF carriers) for a specific RF coverage area.• The radio channel is the communication link between the MSs within an RF coverage area and the BSS.• The BTS also has a limited amount of control functionality which reduces the amount of traffic between the BTS and BSC.
NETWORK COMPONENTSBase Transceiver Station (BTS)
NETWORK COMPONENTSBTS Connectivity Open ended Daisy Chain MSC BSC BTS12 BTS13 BTS14 Star BTS11 BTS5 Daisy Chain with BTS1 a fork. Fork has a return loop back Daisy Chain with BTS6 to the chain a fork. Fork has a BTS4 return loop back to the chain BTS2 BTS7 BTS9 BTS11 BTS3 BTS8
NETWORK COMPONENTSOperation And Maintenance Centre For Radio (OMC-R)• The OMC controls and monitors the Network elements within a region.• The OMC also monitors the quality of service being provided by the Network.• The following are the main functions performed by the OMC-R – The OMC allows network devices to be manually removed for or restored to service. The status of network devices can be checked from the OMC and tests and diagnostics invoked. – The alarms generated by the Network elements are reported and logged at the OMC. The OMC-R Engineer can monitor and analyze these alarms and take appropriate action like informing the maintenance personal. – The OMC keeps on collecting and accumulating traffic statistics from the network elements for analysis. – Software loads can be downloaded to network elements or uploaded to the OMC.
NETWORK COMPONENTSOperation And Maintenance Centre For Radio (OMC-R)
NETWORK COMPONENTSBase Station Identity Code• BSIC allows a mobile station to distinguish between neighboring base stations.• It is made up of 8 bits. 7 6 5 4 3 2 1 0 0 0 NCC BCC BCC NCC = National Colour Code( Differs from operator to operator ) BCC = Base Station Colour Code, identifies the base station to help distinguish between Cell’s using the same BCCH frequencies
NETWORK COMPONENTS MS Class Mark• The MS is identified by it’s classmark which the mobile sends during it’s initial message.• The classmark contains the following information – Revision level - Identifies the phase of the GSM specifications the mobiles complies with. – RF Power Capabilities - The maximum power the mobile can transmit. This information is held in the MS Power Class Number. – Ciphering Algorithm - Indicates the ciphering algorithm implemented in the mobile. There is only one algorithm (A5 ) in GSM phase 1, however GSM phase 2 specifies different algorithms (A5/0 to A5/7 ) – Frequency Capability - Indicates the frequency bands the MS can receive and transmit on. – Short Message Capability- Indicates whether the MS is able to receive short messages or not.
MOBILE MAXIMUM RANGE RANGE= TIMIMG ADVANCE * BIT PERIOD* VELOCITY 2 TIMING ADVANCE = DELAY OF BITS (0-63) BIT PERIOD= 577/156.25 = 3.693µsec =3.693 * 10e-6 sec VELOCITY= 3 * 10e5 Km/sec RANGE= 34.9 Km
MULTIPLE ACCESS TECHNIQUES• In order for several links to be in progress simultaneously in the same geographical area without mutual interference , multiple access techniques are deployed.• The commonly used multiple access techniques are – Frequency Division Multiple Access (FDMA ) – Time Division Multiple Access (TDMA ) – Code Division Multiple Access (CDMA )
TERRESTERIAL INTERFACE• The terrestrial interfaces comprises all the connections between the GSM system entities ,apart from the Um or air interface.• The terrestrial interfaces transport the traffic across the system and allows the passage of thousands of data messages to make the system function.• The standard interfaces used are – 2 Mb/s – Signalling System (C7 or SS7 – Packet Switched Data – A bis using the LAPD protocol (Link Access Procedure D )•
INTERFACE NAMESEach interface specified in GSM has a name associated with it. NAME INTERFACE Um MS ----- BTS Abis BTS ----- BSC A MSC ------ BSC B MSC ------ VLR C MSC ------ HLR D VLR ----- HLR E MSC ------ MSC F MSC ------ EIR G VLR ------ VLR H HLR ------ AUC
2 Mbits/s Trunk 30- channel PCMThis interface carries the traffic from the PSTN to the MSC,between MSC’s, from the MSC to the BSC’s and from the BSC’s tothe BTS’s.It represents the physical layer in the OSI model.Each 2 Mb/s link provides 30 traffic channels available to carryspeech ,data or control information.Typical Configuration TS 0 TS 1-15 TS 16 TS 17 - 31 TS 0 - Frame allignment/ Error checking/ Signalling/ Alarms TS 1-15 , 17-31 - Traffic TS 16 - Siganlling
Cell Global Identity ( CGI ) : LAI MCC MNC LAC CI CGI MCC = Mobile Country Code MNC = Mobile Network Code LAC = Location Area Identity CI = Cell Identity
CHANNEL CONCEPTCHANNELS Downlink UplinkPhysical channel - Each timeslot on a carrier is referred to as a physicalchannel. Per carrier there are 8 physical channels.Logical channel - Variety of information is transmitted between the MS andBTS. There are different logical channels depending on the informationsent. The logical channels are of two types • Traffic channel • Control channel
CHANNEL CONCEPT GSM Control Channels Control Channels BCH ( Broadcast channels ) CCCH(Common Control Chan) DCCH(Dedicated Channels) Downlink only Downlink & Uplink Downlink & Uplink BCCH Synch. RACH CBCH SDCCH ACCH Broadcastcontrol channel Channels Random Access Channel Cell Broadcast Standalone dedicated Associated Channel Control Channels control channel SCH FCCH PCH/Synchronisation Frequency AGCH FACCH SACCH Correction channel Fast Associated Slow associated channel Paging/Access grant Control Channel Control Channel
CHANNEL CONCEPTBCH ChannelsBCCH( Broadcast Control Channel ) • Downlink only • Broadcasts general information of the serving cell called System Information • BCCH is transmitted on timeslot zero of BCCH carrier • Read only by idle mobile at least once every 30 secs.SCH( Synchronisation Channel ) • Downlink only • Carries information for frame synchronisation. Contains TDMA frame number and BSIC.FCCH( Frequency Correction Channel ) • Downlink only. • Enables MS to synchronise to the frequency. • Also helps mobiles of the ncells to locate TS 0 of BCCH carrier.
CHANNEL CONCEPTCCCH ChannelsRACH( Random Access Channel ) • Uplink only • Used by the MS to access the Network.AGCH( Access Grant Channel ) • Downlink only • Used by the network to assign a signalling channel upon successfull decoding of access bursts.PCH( Paging Channel ) • Downlink only. • Used by the Network to contact the MS.
CHANNEL CONCEPTDCCH ChannelsSDCCH( Standalone Dedicated Control Channel ) • Uplink and Downlink • Used for call setup, location update and SMS.SACCH( Slow Associated Control Channel ) • Used on Uplink and Downlink only in dedicated mode. • Uplink SACCH messages - Measurement reports. • Downlink SACCH messages - control info.FACCH( Fast Associated Control Channel ) • Uplink and Downlink. • Associated with TCH only. • Is used to send fast messages like handover messages. • Works by stealing traffic bursts.
CHANNEL CONCEPT NORMAL BURST FRAME1(4.615ms) FRAME2 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 0.577ms 0.546ms 3 57 bits 26 bits 57 bits 3Guard Tail Flag Training Flag Tail Guard Data DataPeriod Bits Bit sequence Bit Bits Period Carries traffic channel and control channels BCCH, PCH, AGCH, SDCCH, SACCH and FACCH.
CHANNEL CONCEPTNORMAL BURSTData - Two blocks of 57 bits each. Carries speech, data or control info.Tail bits - Used to indicate the start and end of each burst. Three bits always000.Guard period - 8.25 bits long. The receiver can only receive and decode ifthe burst is received within the timeslot designated for it.Since the MS aremoving. Exact synchronization of burst is not possible practically. Hence8.25bits corresponding to about 30us is available as guard period for asmall margin of error.Flag bits - This bit is used to indicate if the 57 bits data block is used asFACCH.Training Sequence - This is a set sequence of bits known by both thetransmitter and the receiver( BCC of BSIC). When a burst of information isreceived the equaliser searches for the training sequence code. Thereceiver measures and then mimics the distortion which the signal has beensubjected to. The receiver then compares the received data with thedistorted possible transmitted sequence and chooses the most likely one.
CHANNEL CONCEPT FREQUENCY CORRECTION BURST FRAME1(4.615ms) FRAME2 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 0.577ms 0.546ms 3 142 bits 3Guard Tail Tail Guard Fixed DataPeriod Bits Bits Period • Carries FCCH channel. • Made up of 142 consecutive zeros. • Enables MS to correct its local oscillator locking it to that of the BTS.
CHANNEL CONCEPT SYNCHRONISATION BURST FRAME1(4.615ms) FRAME2 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 0.577ms 0.546ms 3 39 bits 64 bits 39 bits 3Guard Tail Encrypted Synchronisation Encrypted Tail GuardPeriod Bits Bits Sequence Bits Bits Period • Carries SCH channel. • Enables MS to synchronise its timings with the BTS. • Contains BSIC and TDMA Frame number.
CHANNEL CONCEPT DUMMY BURST FRAME1(4.615ms) FRAME2 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 0.577ms 0.546ms 3 57 bits 26 bits 57 bits 3Guard Tail Flag Training Flag Tail Guard Data DataPeriod Bits Bit sequence Bit Bits Period • Transmitted on the unused timeslots of the BCCH carrier in the downlink.
CHANNEL CONCEPTACCESS BURST FRAME1(4.615ms) FRAME2 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 0.577ms 8 41 bits 36 bits 3 68.25 bits Tail Synchronisation Encrypted Tail Guard Bits Sequence Bits Bits Period• Carries RACH.• Has a bigger guard period since it is used during initial access and the MS does not know how far it is actually from the BTS.
CHANNEL CONCEPTNEED FOR TIMESLOT OFFSET BSS Downlink 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 MS Uplink 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7• If Uplink and Downlink are aligned exactly, then MS will have to transmit and receive at the same time. To overcome this problem a offset of 3 timeslots is provided between downlink and uplink
CHANNEL CONCEPTNEED FOR TIMESLOT OFFSET BSS Downlink 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 0 MS Uplink 5 6 7 0 1 2 3 4 5 6 7 0 1 2 3 4 5 3 timeslot offset • As seen the MS does not have to transmit and receive at the same time. This simplifies the MS design which can now use only one synthesizer.
CHANNEL CONCEPT26 FRAME MULTIFRAME STRUCTURE 4.615 msec 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 T T T T T T T T T T T T S T T T T T T T T T T T T I 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 120 msec• MS on dedicated mode on a TCH uses a 26-frame multiframe structure.• Frame 0-11 and 13-24 used to carry traffic.• Frame 12 used as SACCH to carry control information from and to MS to BTS.• Frame 25 is idle and is used by mobile to decode the BSIC of neighbor cells.
CODINGSPEECH CODING• The transmission of speech is one of the most important service of a mobile cellular system.• The GSM speech codec, which will transform the analog signal(voice) into a digital representation, has to meet the following criterias • A good speech quality, at least as good as the one obtained with previous cellular systems. • To reduce the redundancy in the sounds of the voice. This reduction is essential due to the limited capacity of transmission of a radio channel. • The speech codec must not be very complex because complexity is equivalent to high costs.• The final choice for the GSM speech codec is a codec named RPE- LTP (Regular Pulse Excitation Long-Term Prediction).
CODINGSPEECH CODING• This codec uses the information from previous samples (this information does not change very quickly) in order to predict the current sample.• The speech signal is divided into blocks of 20 ms. These blocks are then passed to the speech codec, which has a rate of 13 kbps, in order to obtain blocks of 260 bits.
CODINGCHANNEL CODING• Channel coding adds redundancy bits to the original information in order to detect and correct, if possible, errors ocurred during the transmission.• The channel coding is performed using two codes: a block code and a convolutional code.• The block code receives an input block of 240 bits and adds four zero tail bits at the end of the input block. The output of the block code is consequently a block of 244 bits.• A convolutional code adds redundancy bits in order to protect the information. A convolutional encoder contains memory. This property differentiates a convolutional code from a block code.• A convolutional code can be defined by three variables : n, k and K.• The value n corresponds to the number of bits at the output of the encoder, k to the number of bits at the input of the block and K to the memory of the encoder.
CODINGCHANNEL CODING ( Cont )• The ratio, R, of the code is defined as R = k/n. Example - Lets consider a convolutional code with the following values: k is equal to 1, n to 2 and K to 5. This convolutional code uses then a rate of R = 1/2 and a delay of K = 5, which means that it will add a redundant bit for each input bit. The convolutional code uses 5 consecutive bits in order to compute the redundancy bit. As the convolutional code is a 1/2 rate convolutional code, a block of 488 bits is generated. These 488 bits are punctured in order to produce a block of 456 bits. Thirty two bits, obtained as follows, are not transmitted : C (11 + 15 j) for j = 0, 1, ..., 31 k=1 Convolution code R = k/n = 1/2 n=2 1 bit input 2 bit input• The block of 456 bits produced by the convolutional code is then passed to the interleaver
CODING CHANNEL CODING FOR GSM SPEECH CHANNELS• Before applying the channel coding, the 260 bits of a GSM speech frame are divided in three different classes according to their function and importance.• The most important class is the class 1a containing 50 bits.Next important is the class 1b, which contains 132 bits.The least important is the class 2, which contains the remaining 78 bits.• The different classes are coded differently.• First of all, the class 1a bits are block-coded. Three parity bits, used for error detection, are added to the 50 class 1a bits.The resultant 53 bits are added to the class 1b bits.• Four zero bits are added to this block of 185 bits (50+3+132). A convolutional code, with r = 1/2 and K = 5, is then applied, obtaining an output block of 378 bits.• The class 2 bits are added, without any protection, to the output block of the convolutional coder. An output block of 456 bits is finally obtained.
CODINGSpeech Channel Coding 260 bits Class 1a Class 1b 50 bits 132 bitsParity Tailcheck Class 1a 3 Class 1b 4 bits 50 bits 132 bits Class 2 78 bits Convolution coding 378 bits 456 bits
CODING CHANNEL CODING FOR CONTROL CHANNELS• In GSM the signalling information is just contained in 184 bits.• Forty parity bits, obtained using a fire code, and four zero bits are added to the 184 bits before applying the convolutional code (r = 1/2 and K = 5). The output of the convolution code is then a block of 456 bits which does not need to be punctured. Parity 184 bits bitsFire Tailcode bits 184 bits 40 bits 4 Convolution coding 456 bits
CODINGCHANNEL CODING FOR DATA CHANNELS• In data information is contained in 240 bits.• Four tails bits are added to the 240 bits before applying the convolutional code (r = 1/2 and K = 5). The output of the convolutional code is then a block of 488 bits which when punctuated yields 456 bits. 240 bits Tail bits 240 bits 4 Convolution coding 488 bits Punctuate 456 bits
INTERLEAVINGINTERLEAVING• An interleaving rearranges a group of bits in a particular way.• It is used in combination with FEC codes( Forward Error Correction Codes ) in order to improve the performance of the error correction mechanisms.• The interleaving decreases the possibility of losing whole bursts during the transmission, by dispersing the errors.• As the errors are less concentrated, it is then easier to correct them.
INTERLEAVING GSM SPEECH CHANNEL INTERLEAVING• A burst in GSM transmits two blocks of 57 data bits each.• Therefore the 456 bits corresponding to the output of the channel coder fit into 8 ‘57 data’ bits (8 * 57 = 456). The 456 bits are divided into eight blocks of 57 bits.• The first block of 57 bits contains the bit numbers (0, 8, 16, .....448), the second one the bit numbers (1, 9, 17, .....449), etc.• The last block of 57 bits will then contain the bit numbers (7, 15, .....455).• The first four blocks of 57 bits are placed in the even-numbered bits of four consecutive bursts.• The other four blocks of 57 bits are placed in the odd-numbered bits of the next four bursts.• The interleaving depth of the GSM interleaving for speech channels is eight.• A new data block also starts every four bursts. The interleaver for speech channels is called a block interleaver.
INTERLEAVING CONTROL CHANNEL INTERLEAVING• A burst in GSM transmits two blocks of 57 data bits each.• Therefore the 456 bits corresponding to the output of the channel coder fit into four bursts (4*114 = 456).• The 456 bits are divided into eight blocks of 57 bits. The first block of 57 bits contains the bit numbers (0, 8, 16, .....448), the second one the bit numbers (1, 9, 17, .....449), etc. The last block of 57 bits will then contain the bit numbers (7, 15, .....455).• The first four blocks of 57 bits are placed in the even-numbered bits of four bursts.• The other four blocks of 57 bits are placed in the odd-numbered bits of the same four bursts.• Therefore the interleaving depth of the GSM interleaving for control channels is four and a new data block starts every four bursts.• The interleaver for control channels is called a block rectangular interleaver.
INTERLEAVING DATA INTERLEAVING• A particular interleaving scheme, with an interleaving depth equal to 22, is applied to the block of 456 bits obtained after the channel coding.• The block is divided into 16 blocks of 24 bits each, 2 blocks of 18 bits each, 2 blocks of 12 bits each and 2 blocks of 6 bits each.• It is spread over 22 bursts in the following way : • the first and the twenty-second bursts carry one block of 6 bits each • the second and the twenty-first bursts carry one block of 12 bits each • the third and the twentieth bursts carry one block of 18 bits each • from the fourth to the nineteenth burst, a block of 24 bits is placed in each burst• A burst will then carry information from five or six consecutive data blocks. The data blocks are said to be interleaved diagonally.
MODULATIONCIPHERING• Ciphering is used to protect signaling and user data.• A ciphering key is computed using the algorithm A8 stored on the SIM card, the subscriber key and a random number delivered by the network (this random number is the same as the one used for the authentication procedure).• A 114 bit sequence is produced using the ciphering key, an algorithm called A5 and the burst numbers.• This bit sequence is then XORed with the two 57 bit blocks of data included in a normal burst.• In order to decipher correctly, the receiver has to use the same algorithm A5 for the deciphering procedure.MODULATION• Modulation is done using 0.3 GMSK
SIGNALLING SYSTEMWHAT IS SIGNALLING ?• The term signaling is used in many contexts.• In technical systems, it very often refers to the control of different procedures.• With reference to telephony, signaling means the transfer of information and the instructions relevant to control and monitor telephony connections.
SIGNALLING SYSTEM C7GENERAL INTRODUCTION• Today’s global telecom networks are included in very complex technical systems.• Naturally, a system of this type requires extensive signaling, both internally in different nodes (for example, exchanges) and externally between different types of network nodes.• During this training we will focus on external signaling.• Thus, the term signaling in the following slides always refers to external signaling traffic.• The main purpose of using signaling in modern telecom networks – where different network nodes must cooperate and communicate with each other – is to enable transfer of control information between nodes in connection with: –Traffic control procedures as set-up, supervision, and release of telecommunication connections and services
GENERAL INTRODUCTION• Database communication, for example, database queries concerning specific services, roaming in cellular networks, etc.• Network management procedures, for example, blocking or deblocking trunks.• Traditionally, external signaling has been divided into two basic types – Access signaling (for example, Subscriber Loop Signaling) This means signaling between a subscriber terminal (telephone) and the local exchange. – Trunk signaling (that is, Inter-Exchange Signaling) This is used for signaling between exchanges.
SIGNALING IN TELECOMMUNICATION NETWORK SIGNALLING ACCESS SIG TRUNK SIGNALLING SUBSCRIBER LINE SIG. CHANNEL ASSOCIATED SIG. DIGITAL SUBSCRIBER SIG. COMMON CHANNEL SIG.
Access Signaling• There are many types of access signaling, for example, PSTN analogue subscriber line signaling, ISDN Digital Subscriber Signaling System (DSS1), and signaling between the MS and the network in the GSM system.• Signaling on the analogue subscriber line between a telephony subscriber and the Local Exchange (LE) is performed by means of on/off hook signals, dialed digits, information tones (dial tone, busy tone, etc.), recorded announcements, and ringing signals.• The dialed digits can be sent in two different ways: as decadic pulses (used for old-type rotary-dial telephones), or as a combination of two tones (used for modern pushbutton telephones). The latter system is known as the Dual Tone Multi Frequency (DTMF).• The information tones (dial tone, ringing tone, busy tone, etc.) are audio signals used to keep the calling party (the A-subscriber) informed about what is going on in the network during the set-up of a call.
Access Signaling• Digital Subscriber Signaling System No. 1 (DSS1) is the standard access signaling system used in ISDN. It is also called a D-channel signaling system• D-channel signaling is defined for digital access lines only.• The signaling protocols are based on the OSI (Open System Interconnection) reference model, layers 1 to 3.• Consequently, the signaling messages are transferred as data packets between the user terminal and the local exchange.• Due to the much more complex service environment at the ISDN user’s site, the amount of signaling information and the number of variations
Trunk Signaling• The Inter-exchange Signaling information is usually transported on one of the time slots in a PCM link, either in association with the speech channel or independently.• There are two commonly used methods for Inter Exchange Signaling. Channel Associated Signaling (CAS) – In CAS, the speech channel (in-band), or a channel closely associated with a speech channel (out-band), is used for signaling. Common Channel Signaling (CCS) – In this case a dedicated channel, completely separate from the speech channel, is used for signaling. Due to the high capacity, one signaling channel in CCS can serve a large number of speech channels.• In a GSM network, CCITT Signaling System No. 7 is used.• Signaling System No. 7 is a Common Channel Signaling system.
CHANNEL ASSOCIATED SIGNALING (CAS)• Channel Associated Signaling (CAS) means that the signaling is always sent on the same connection (PCM link) as the traffic.• The signaling is associated with the traffic channel.• In a 2 Mb/s PCM link, 30 time slots are used for speech, whereas TS 0 is used for synchronization and TS 16 is used for the line signaling.• All 30 traffic connections share TS 16 in a multiframe consisting of 16 consecutive frames.• On TS 16, each traffic channel has a permanently allocated recurring location for line signaling, where two traffic channels share TS 16 in one frame.
COMMON CHANNEL SIGNALING (CCS)• In CCS, signaling messages (or data packets) are transmitted over time slots in a PCM link reserved for the purpose of signaling.• The system is designed to use a common data channel (or signaling link) as the carrier of all signals, required by a large number of traffic channels.• In 1968, CCITT specified a Common Channel Signaling system called CCS System No. 6, which was designed especially for international analogue telephony networks.• However, very few installations of this system remain today. It has, as already mentioned, been replaced by Signaling System No. 7.• The first version of SS7 (1980) was designed for telephony and data.• In the 80’s the demand for new services dramatically increased and the SS7 was therefore developed to meet the signaling requirements, specified for all these new services.• Today SS7 is used in many different networks and related services typically betn PSTN, ISDN, PLMN & IN services throughout the world.
OSI REFERENCE MODEL• The Signaling System No. 7, which is a type of packet switched data communication system, is structured in a modular and layered way.• Such a design of SS7 is similar to the Open System Interconnection model.• Open Systems are systems that use standardized communication procedures developed from the reference model.• Thus, all such open systems are able to communicate with each other.• The word “system” can refer to computers, exchanges, data networks, etc.
OSI MODEL REFERENCE DIAGRAM APPLICATION APPLICATION PRESENTATION PRESENTATION SESSION SESSION TRANSPORT TRANSPORT NETWORK NETWORK LINK LINK PHYSICAL PHYSICAL
COMMUNICATION PROCESS• Each layer has its own specified functions and provides specific services for the layers above.• It is important to define the interfaces between different layers and the functions within each layer.• The way a function is realized within a layer is not predicted.• Logically, the communication between functions always takes place on the same level according to the protocols for that level.• Only functions on the same level can “talk to each other”.• In the transmitting system, the protocol for each layer adds information to the data received from the layer above.• The addition usually consists of a header and/or a trailer.• In the receiving system, the additions are used, for example, to identify bits or data fields carrying information for that specific layer only.• These fields are decoded by layer functionality and are removed when delivering the message to the applications orlayers above.
• When the data reaches the application layer on the receiving side, it consists of only the data that originated in the application layer of the sending system.• Logically, each layer communicates with the corresponding layer in the other system.• This communication is called Peer-to-Peer communication and is controlled by the layer’s protocol.DESCRIPTION OF LAYERSApplication Layer• This layer provides services for support of the user’s application process and for control of all communication between applications.• Examples of layer 7 functions are file transfer, message handling, directory services, and operation and maintenance.
Presentation Layer• This layer defines how data is to be represented, that is, the syntax.• The presentation layer transforms the syntax used in the application into the common syntax needed for the communication between applications.• Layer 6 contains data compression.Session Layer• This layer establishes connections between presentation layers in different systems.• It also controls the connection, the synchronization and the disconnection of the dialogue.• It allows the presentation layer to determine checkpoints, from which the retransmission will start when the data transmission has been interrupted.
Transport Layer• This layer guarantees that the bearer service has the quality required by the application in question.• Examples of functions are error detection and correction (end-to- end), and flow control.• The transport layer optimizes the data communication, for example by multiplexing or splitting data streams before they reach the network.
Network Layer• The basic network layer service is to provide a transparent channel.• This means that the application requesting a channel ignores network problems and the related signal exchange because that is the task of the lower levels.• It just requires an open channel, transparent for the transmission of data, between transport layers in different systems.• The Network Layer establishes, maintains, and releases connections between the nodes in the network and handles addressing and routing of circuits. Data Link Layer• This layer provides an essentially error-free point-to-point circuit between network layers.• The layer contains resources for error detection, error correction, flow control, and retransmission.
Physical Layer• This layer provides mechanical, electrical, functional, and procedural resources for activating, maintaining, and blocking physical circuits for the transmission of bits between data link layers.• The physical layer contains functions for converting data into signals compatible with the transmission medium.• For the communication between only two exchanges, layers 1 and 2 are sufficient.• For the communication between all exchanges in the network, layer 3 must be added because it provides addressing and routing.
SIGNALING SYSTEM NO. 7 INTRODUCTION• The Signaling System (SS)No. 7 is an elaborate set of recommendations defining protocols for the internal management of digital networks.• These recommendations were introduced in 1980 and revised in 1984 and 1988 in different-colored books (yellow, red, and blue).• CCITT SS No. 7 is intended primarily for digital networks, both national and international, where the high transmission rates (64 kbps) can be exploited.• It may also be used on analogue lines especially on international trunks (CCITT SS No 6).• CCS was initially meant for telephony only, but has now evolved into non-telephony and non-connection related applications (for example, location updating of a mobile subscriber).• A dialogue with a database or between two databases is a typical application for CS in GSM.
• Thus, there is a need for a generic system that is able to support a wide variety of applications in telecommunication.• The variety of applications is increasing as new types of telephony systems and a wider use of databases in the network become necessary (mobile telephony networks, ISDN, IN, etc.).• Even though the standardization of SS7 is now the responsibility of ITU-T, for traditional and historical reasons, the system is often called “CCITT No. 7 signaling system”.• The signaling system used in GSM follows the CCITT recommendations.• The modular layer structure allows flexible usage of the specifications.
USER PARTS• The User Parts (UPs) contain functions dealing with the processing of signal information before and after it is transmitted through the signaling network.• The MTP provides the means of reliable transport and delivery of UP information across the SS7 network.• It also has the ability to react to system and network failures that affect the information from the UPs and take necessary action to ensure that the information is safely conveyed.• The User does not mean the subscriber involved in the call, but the user of the MTP.• The MTP is a common transport system developed to serve one or more User Parts in the same node.• Every Signaling Point(SP) consists of MTP & a number of its users.• Only UPs of the same type can communicate with each other.• To forward signaling messages between UPs, located in different nodes, the MTP is used.
USERS OF SIGNALING SYSTEM CCITT NO 7MAP CAP BSSAP ISUP TUP TCAP SCCP MTP CCITT SS NO. 7 PROTOCOLS IN GSM
MTP user partsISUP (ISDN User Part)• It provides control-functions and signaling, needed in an ISDN, to deal with ISDN subscriber calls and related functions.TUP (Telephony User Part)• It provides all necessary functions and signaling for dealing with a telephony user.• TUP is being replaced by ISUP in telecommunication networks.DUP (Digital User Part)• This UP is used for purposes such as file transfer and related signaling.
SCCP• The MTP was designed for the real-time applications of telephony.• The connectionless nature of the MTP provides a low-overhead facility suiting the requirements of telephony.• Regarding GSM, other applications such as network management need services such as expanded addressing capability and reliable message transfer.• The SCCP was developed to meet these requirements.• The SCCP also sends its messages through the MTP.• The SCCP provides functions for completely new services, for example, non-circuit-related signaling.• Some functions, not directly related to users, but necessary for network control, are used.• The main reason is that they are necessary for serving applications in higher layers and for maintenance purposes.
SCCP• These functions use SCCP services: Transaction Capabilities (TC) – First introduced in 1984, TC provides the mechanisms for transaction-oriented applications and functions. Operation and Maintenance Application Part (OMAP) – Specifies network management functions and messages related to operation and maintenance.
OSI Model CCITT SS NO 7 Model ASEAPPLICATION USER PARTS TCAPPRESENTATION SESSION TRANSPORT SCCP NETWORK SIGNALLING NETWORK NSP MTP LINK SIGNALLING LINK PHYSICAL SIGNALLING DATA LINK
Mobile originated call MS BSS MSC Channel Request (RACH) Immediate Assignment [ Reject ] (AGCH) SDCCH Seizure CM Service Request + Connection Request < CMSREQ > Connection [ Confirmed / Refused ] Link Establishment Authentication Request Authentication Response DT1 <CICMD> Ciphering Mode Command S Ciphering Mode Complete DT1 <CICMP> D C Identity Request C Identity Response H Setup Call Proceeding Connection Management Assignment Request Assignment Request [ Failed ] Assignment Command Assignment [ Complete / Failure ] Assignment [ Complete / Failure ] T C H TCH Seizure
Mobile terminated call MS BSS MSC UDT < PAGIN > Paging Request (PCH) Paging Channel Request (RACH) Immediate Assignment [ Reject ] (AGCH) SDCCH Seizure Paging Response + Connection Request < PAGRES > Connection [ Confirmed / Refused ] Link Establishment Authentication Request Authentication Response DT1 <CICMD> Ciphering Mode Command S Ciphering Mode Complete DT1 <CICMP> D C Identity Request C Identity Response H Setup Call Confirmed Connection Management Assignment Request Assignment Request [ Failed ] Assignment Command Assignment [ Complete / Failure ] Assignment [ Complete / Failure ] T C H TCH Seizure
RF POWER CONTROL• RF power control is employed to minimise the transmit power required by MS or BS while maintaining the quality of the radio links.• By minimising the transmit power levels, interference to co-channel users is reduced.• Power control is implemented in the MS as well as the BSS.• Power control on the Uplink also helps to increase the battery life.
POWER CONTROL IN THE MS• The RF power level employed by the MS is indicated by means of the 5 bit TXPWR field sent either in the layer 1 header of each downlink SACCH message block, or in a dedicated signalling block.• The MS confirms the power level that it is currently employing by setting the MS_TXPWR_CONF field in the uplink SACCH L1 header to its current power setting. The value of this field is the power setting actually used by the mobile for the last burst of the previous SACCH period.• The MS employs the most recently commanded RF power level appropriate to the channel for all transmitted bursts on either a TCH (including handover access burst), FACCH,SACCH or SDCCH.• When accessing a cell on the RACH (random access) and before receiving the first power command during a communication on a DCCH or TCH (after an IMMEDIATE ASSIGNMENT), the MS uses either the power level defined by the MS_TXPWR_MAX_CCH parameter broadcast on the BCCH of the cell, or the maximum TXPWR of the MS as defined by its power class, whichever is the
1111111 indicates this field does not have any TA value 8 7 6 5 4 3 2 1 Spare Ordered MS Power Level Octet 1 Spare Ordered Timing Advance Octet 2 POWER CONTROL MS• The range over which a MS is capable of varying its RF output power is from its maximum output down to 20mW, in steps of nominally 2dB.• 0 - 43dBm…….15 - 13dBm.
TIMING OF POWER CHANGE BY MS• Upon receipt of a command on the SACCH to change its RF power level (TXPWR field) the MS changes to the new level at a rate of one nominal 2dB power step every 60ms (13 TDMA frames), i.e. a full range change of 15 steps should take about 900ms .• The change commences at the first TDMA frame belonging to the next reporting period . The MS changes the power one nominal 2 dB step at a time, at a rate of one step every 60 ms following the initial change, irrespective of whether actual transmission takes place or not.• In case of channel change the commanded power level is applied on the new channel immediately.
BSS POWER CONTROL• Power control at BSS is optional.• The range over which the BS is capable of reducing its RF output power from its maximum level is nominally 30dB, in 15 steps of nominally 2dB.
RADIO LINK FAILURE• The criterion for determining Radio Link Failure in the MS is based on the success rate of decoding messages on the downlink SACCH.• The radio link failure criterion is based on the radio link counter S.• If the MS is unable to decode a SACCH message, S is decreased by 1.• If a SACCH message is decoded successfully, S is increased by 2.• If S reaches 0 a radio link failure is assumed & the MS aborts the conn.• The RADIO_LINK_TIMEOUT parameter is transmitted by each BS in the BCCH data. 4 Decoded 3 Not Decoded 2 1 0 SACCH Blocks
RADIO LINK FAILURE• The MS continues transmitting as normal on the uplink until S reaches 0.• The algorithm will start after the assignment of a dedicated channel and S is initialized to RADIO_LINK_TIMEOUT.• The aim of determining radio link failure in the MS is to ensure that calls with unacceptable voice/data quality, which cannot be improved either by RF power control or handover, are either re- established or released in a defined manner.• In general the parameters that control the forced release should be set such that the forced release will not normally occur until the call has degraded to a quality below that at which the majority of subscribers would have manually released. This ensures that, for example, a call on the edge of a radio coverage area, although of bad quality, can usually be completed if the subscriber wishes.
CELL SELECTION AND RE-SELECTION• In Idle mode (i.e. not engaged in communicating with a BS), an MS will do the cell selection and re-selection procedures .• The procedures ensure that the MS is camped on a cell from which it can reliably decode downlink data and with which it has a high probability of communications on the uplink. The choice of cell is determined by the path loss criterion. Once the MS is camped on a cell, access to the network is allowed.• An MS is said to be camped on a cell when it has determined that the cell is suitable and stays tuned to a BCCH + CCCH of that cell. While camped on a cell, an MS may receive paging messages or under certain conditions make random access attempts on a RACH of that cell, and read BCCH data from that cell.• The MS will not use the discontinuous reception (DRX) mode of operation (i.e. powering itself down when it is not expecting paging messages from the network) while performing the selection and reselection algorithm. However use of powering down is permitted at all other times in idle mode.
CELL SELECTION AND RE-SELECTION• For the purposes of cell selection and reselection, the MS is required to maintain an average of received signal strengths for all monitored frequencies. These quantities termed the "receive level averages” is the averages of the received signal strengths measured in dBm.• The cell selection and reselection procedures make use of the "BCCH Allocation" (BA) list. There are in two BA lists which may or may not be identical, depending on choices made by the PLMN operator.• (i) BA (BCCH) - This is the BA sent in System Information Messages on the BCCH. It is the list of BCCH carriers in use by a given PLMN in a given geographical area. It is used by the MS in cell selection and reselection.• (ii) BA (SACCH) - This is the BA sent in System Information Messages on the SACCH and indicates to the MS which BCCH carriers are to be monitored for handover purposes.• When the MS goes on to a TCH or SDCCH, it starts monitoring BCCH carriers in BA (BCCH) until it gets its first BA (SACCH) message.
CELL SELECTION - NO BCCH DATA AVAILABLE• The MS searches all 124 RF channels in the GSM system, takes readings of RSS on each RF channel, and calculate the received level average for each.• The averaging is based on at least five measurement samples per RF carrier spread over 3 to 5 secs.• The MS tunes to the carrier with the highest average RSS & determines whether or not this carrier is a BCCH carrier.• If it is a BCCH carrier, the MS attempts to synchronise to this carrier and read the BCCH data. The MS camps on the cell provided it can successfully decode the BCCH data and this data indicates that it is part of the selected PLMN, that the cell is not barred (CELL_BAR_ACCESS = 0) & that the parameter C1 is greater than 0.• If the cell is part of the selected PLMN but is barred or C1 is less than zero, the MS uses the BCCH Allocation obtained from this cell and subsequently only searches these BCCH carriers. Otherwise the MS tune to the next highest carrier and so on.
CELL SELECTION - NO BCCH DATA AVAILABLE• CELL_BAR_ACCESS may be employed to bar a cell that is only intended to handle handover traffic etc. For example of this could be an umbrella cell which encompasses a number of microcells.• If at least the 30 strongest RF channels have been tried, but no suitable cell has been found, provided the RF channels which have been searched include at least one BCCH carrier, the available PLMNs shall be presented to the user, otherwise more RF channels shall be searched until at least one BCCH carrier is found.• 30 RF channels are specified to give a high probability of finding all suitable PLMNs, without making the process take too long.
CELL SELECTION - BCCH INFORMATION AVAIL.• The MS stores the BCCH carriers in use by the PLMN selected when it was last active in the GSM network. A MS may also store BCCH carriers for more than one PLMN which it has selected previously (e.g. at national borders or when more than one PLMN serves a country).• If an MS includes a BCCH carrier storage option it searches only for BCCH carriers in the list.• If an MS decodes BCCH data from a cell of the selected PLMN but is unable to camp on that cell, the BA of that cell is examined. Any BCCH carriers in the BA which are not in the MSs list of BCCH carriers to be searched is added to the list.• If no suitable cell has been found after all the BCCH carriers in the list have been searched, the MS acts as if there were no stored BCCH carrier information. Since information concerning a number of channels is already known to the MS, it may assign high priority to measurements on those of the 30 strongest carriers from which it has not previously made attempts to obtain BCCH information, and omit repeated measurements on the known ones.
PATH LOSS CRITEREON( C1)• This parameter is used to ensure that the MS is camped on the cell with which it has the highest probability of successful communication on uplink and downlink.• The path loss criterion parameter C1 used for cell selection and reselection is defined by: C1 = (A - Max(B,0)) where A = Received Level Average - RXLEV_ACCESS_MIN B = MS_TXPWR_MAX_CCH - P RXLEV_ACCESS_MIN =Minimum received level at the MS required for access to the system. MS_TXPWR_MAX_CCH = Maximum TXPWR level an MS may use when accessing the system. P = Maximum RF output power of the MS.• All values are expressed in dBm.
PATH LOSS CRITEREON( C1)A = + Good Downlink - Poor DownlinkB = - Good Downlink + Poor DownlinkMonitoring of Received Level and BCCH data• In Idle Mode an MS continues to monitor all BCCH carriers as indicated by the BCCH Allocation .• A running average of received level in the preceding 5 to 60 seconds is be maintained for each carrier in the BCCH Allocation.• For the serving cell receive level measurement samples is taken at least for each paging block of the MS and the receive level average is determined using samples collected over a period of 5 s or five consecutive paging blocks of that MS, whichever is the greater period.
Monitoring of Received Level and BCCH data• At least 5 received level measurement samples are required per receive level average value. New sets of receive level average values is calculated as often as possible.• The same number of measurement samples is taken for all non serving cell BCCH carriers, and the samples allocated to each carrier is as far as possible uniformly distributed over each evaluation period.• The list of the 6 strongest carriers is updated at least every minute and may be updated more frequently.• In order to minimise power consumption, MSs that employ DRX (i.e. power down when paging blocks are not due) monitor the signal strengths of non-serving cell BCCH carriers during the frames of the Paging Block that they are required to listen to. Received level measurement samples can thus be taken on several non-serving BCCH carriers and on the serving carrier during each Paging Block.• The MS includes the BCCH carrier of the current serving cell (i.e. the cell the MS is camped on) in this measurement routine.
Monitoring of Received Level and BCCH data• The MS has to decode the full BCCH data of the serving cell at least every 30 seconds.• The MS attempts to decode the BCCH data block that contains the parameters affecting cell reselection for each of the 6 strongest non- serving cell BCCH carriers at least every 5 minutes.• When the MS recognizes that a new BCCH carrier has become one of the 6 strongest, the BCCH data shall be decoded for the new carrier within 30 seconds.• The MS attempts to check the BSIC for each of the 6 strongest non serving cell BCCH carriers at least every 30 seconds, to confirm that it is monitoring the same cell.• If a change of BSIC is detected then the carrier is treated as a new carrier and the BCCH data redetermined.• When requested by the user, the MS monitors the 30 strongest GSM carrier to determine, within 15 seconds, which PLMNs are available. This monitoring is done so as to minimise interruptions to the monitoring of the PCH.
CALL RE-ESTABLISHMENT• In the event of a radio link failure, call re-establishment may be attempted if it is enabled in the database.• The received level measurement samples taken on surrounding cells and on the serving cell BCCH carrier in the last 5 seconds is averaged, and the carrier with the highest average received level which is part of a permitted PLMN is taken.• A BCCH data block containing the parameters affecting cell selection is read on this carrier.• If the parameter C1 is greater than zero, it is part of the selected PLMN, the cell is not barred, and call re-establishment is allowed, call re-establishment is attempted on this cell.• If the above conditions are not met, the carrier with the next highest average received level is taken, and the MS repeats the above procedure.• If the cells with the 6 strongest average received level values are tried but cannot be used, the call re-establishment attempt is abandoned.
bs_ag_blk_res• To ensure that some of the blocks are always left clear for access grant messages the parameter bs_ag_blk_res is used to input the number of blocks to be reserved for this purpose.• The reserved blocks is not be used for paging whatever the demand.• If more than one timeslot exists within a cell, this parameter will reserve the indicated number of blocks on each timeslot.• This parameter is broadcast on the BCCH.• This parameter is used to calculate the number of paging groups available. COMBINED CCCH BLOCKS AGCH BLOCKS PCH BLOCKS No 9 0 9 No 9 1 8 No 9 2 7 No 9 3 6 No 9 4 5 No 9 5 4 No 9 6 3 No 9 7 2 Yes 3 0 3 Yes 3 1 2 Yes 3 2 1
Bs_pa_mfrms• Used to indicate the number of 51 frame multiframes between transmission of paging messages to MS of the same group.• Is transmitted on BCCH.• Used by the MS to calculate its paging group. 8 16 24 32 8 Value 7 15 23 31 7 0 = 2 multiframes 6 14 22 30 6 1 = 3 multiframes 5 13 21 29 5 2 = 4 multiframes 4 12 20 28 4 3 11 19 27 3 3 = 5 multiframes 2 10 18 26 2 4 = 6 multiframes 1 9 17 25 1 5 = 7 multiframes AGCH AGCH AGCH AGCH AGCH 6 = 8 multiframes BCCH BCCH BCCH BCCH BCCH 7 = 9 multiframes
PAGINGExamplecch_conf = 0bs_ag_blk_res = 1bs_pa_mfrms = 2If cch_conf = 1minimum = 2If cch_conf = 6Maximum = 81 * 4Min time between pages = 2 * 235.5 = 471msMax time between pages = 9 * 235.5 =2.1195 sec
max_retran• An MS requests resources from the network by transmitting an ``access burst´´ containing the channel request message.• For a single request, channel request will be repeated upto M + 1 times where M = max_retran. max_retrans M 1 1 2 2 3 4 4 7
tx_integer• To reduce the chances of collision the wait period is randomised for each MS.• After the first channel request is sent the next is repeated after a random wait period in the set (S, S+1,….., S+T-1)• Wait period from this set is chosen randomly from this set. TX INTEGER S FOR NON- S FOR COMB RACH SLOTS COMB CCCH CCCH 3, 8, 14 55 41 4, 9, 16 76 52 5, 10, 20 109 58 6, 11, 25 163 86 7, 12, 32 127 115
AVAILABLE PAGING BLOCKS ON 1 CCCH_GROUPMaximum AGCH reservation for non-combined multiframe = 7 Available paging blocks = 2 Maximum AGCH reservation for combined multiframe = 1 Available paging blocks = 2Minimum AGCH reservation for non-combined multiframe = 0 Available paging blocks = 9 Minimum AGCH reservation for combined multiframe = 0 Available paging blocks = 3No of paging blocks will have a range of 2 - 9
CALCULATION OF CCCH AND PAGING GROUP NOCCCH_GROUP = [ ( IMSI mod 1000) mod (BS_CC_CHANS * N ) ]div NPaging group no = [ ( IMSI mod 1000) mod (BS_CC_CHANS *N ) ] mod N
HANDOVERHANDOVER• The GSM handover process uses a mobile assisted technique for accurate and fast handovers, in order to: – Maintain the user connection link quality. – Manage traffic distribution• The overall handover process is implemented in the MS,BSS & MSC.• Measurement of radio subsystem downlink performance and signal strengths received from surrounding cells, is made in the MS.• These measurements are sent to the BSS for assessment.• The BSS measures the uplink performance for the MS being served and also assesses the signal strength of interference on its idle traffic channels.• Initial assessment of the measurements in conjunction with defined thresholds and handover strategy may be performed in the BSS. Assessment requiring measurement results from other BSS or other information resident in the MSC, may be perform. in the MSC.
HANDOVERHANDOVER (Cont) • The MS assists the handover decision process by performing certain measurements. • When the MS is engaged in a speech conversation, a portion of the TDMA frame is idle while the rest of the frame is used for uplink (BTS receive) and downlink (BTS transmit) timeslots. • During the idle time period of the frame, the MS changes radio channel frequency and monitors and measures the signal level of the six best neighbor cells. • Measurements which feed the handover decision algorithm are made at both ends of the radio link.
HANDOVERMS END• At the MS end, measurements are continuously signalled, via the associated control channel, to the BSS where the decision for handover is ultimately made.• MS measurements include: –Serving cell downlink quality (bit error rate (BER) estimate). –Serving cell downlink received signal level, and six best neighbor cells downlink received signal level.• The MS also decodes the Base Station ID Code (BSIC) from the six best neighbor cells, and reports the BSICs and the measurement information to the BSS.
HANDOVERBTS END• The BTS measures the uplink link quality, received signal level, and MS to BTS site distance.• The MS RF transmit output power budget is also considered in the handover decision.• If the MS can be served by a neighbor cell at a lower power, the handover is recommended.• From a system perspective, handover may be considered due to loading or congestion conditions. In this case, the MSC or BSC tries to balance channel usage among cells.
HANDOVERMS IDLE TIME REPORTING• During the conversation, the MS only transmits and receives for one eighth of the time, that is during one timeslot in each frame.• During its idle time (the remaining seven timeslots), the MS switches to the BCCH of the surrounding cells and measures its signal strength.• The signal strength measurements of the surrounding cells, and the signal strength and quality measurements of the serving cell, are reported back to the serving cell via the SACCH once in every SACCH multiframe.• This information is evaluated by the BSS for use in deciding when the MS should be handed over to another traffic channel.• This reporting is the basis for MS assisted handovers.
HANDOVER MEASUREMENT IN ACTIVE MODEDownlink Frame 24 Frame 25 Idle Frame Frame 0 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 0 1 2 1 2 3 1 2 4 1 2 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 0 1 2 Frame 24 Frame 25 Idle Frame Frame 0 Uplink 1. MS receives and measures signal strength on serving cell(TS2). 2. MS transmits 3. MS measures signsl strength for at least one neighbor cell. 4. MS reads BSIC on SCH for one of the 6 strongest neighbor.
HANDOVERNUMBER OF NEIGHBORS• Maximum 32 averaging of RSS takes place.• Practically a cell neighbors can be equipped for a cell.• If high numbers of neighbors are equipped, then the accuracy of RSS is decreased as should have 8 to 10 neighbors. T T T T T T T T T T T T S T T T T T T T T T T T T I 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 T T T T T T T T T T T T S T T T T T T T T T T T T I 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 T T T T T T T T T T T T S T T T T T T T T T T T T I 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 T T T T T T T T T T T T S T T T T T T T T T T T T I 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25
HANDOVERNUMBER OF NEIGHBORS• In one SACCH multiframe there are 104 TDMA frames.• Out of this 104 frames 4 frames are idle and are used to decode the BSIC.• Remaining 100 TDMA frames are used to measure RSS( Received Signal Strength) of the neighbor.• If 25 neigbors are equipped, then in one SACCH multiframe each neigbor is measured 100/25 = 4 times and averaged out. This produces a less accurate value.• If 10 neigbors are equipped, then in one SACCH multiframe each neigbor is measured 100/10 = 10 times and averaged out. This produces a more accurate value.
HANDOVERINTERFERENCE ON IDLE CHANNEL• GSM causes its own time interference.• The MS has a omni-directional antenna. Much of the MS power goes to the server but a lot is interfering with surrounding cells using the same channel.• The TDMA frames of adjacent cell are not aligned since they are not synchronised. Hence the uplink in the surrounding cell suffers from interference. Channel 10 Cell 1 UPLINK CELL1 1 2 3 4 5 UPLINK CELL2 1 2 3 4 5 Channel 10 Cell 2
HANDOVERINTERFERENCE ON IDLE CHANNEL • The BSS keeps on measuring the interference on the idle timeslots.• Ambient noise is measured and recorded 104 times in one SACCH multiframe.• These measurements are averaged out to produce one figure.• The BSS then distributes the idle timeslots into band 0 to band 5.• Since the BSS knows the interference level on idle timeslots, it uses this data to allocate the best channel first and the worst last. Inteference on idle channel measured on Idle Timeslot by BSS 0 1 2 3 4 5 6 7
HANDOVERHANDOVERThe following measurements is be continuously processed in the BSS :i) Measurements reported by MS on SACCH - Down link RXLEV - Down link RXQUAL - Down link neighbor cell RXLEVii) Measurements performed in BSS - Uplink RXLEV - Uplink RXQUAL - MS-BS distance - Interference level in unallocated time slotsEvery SACCH multiframe (480 ms) a new processed value for each ofthe measurements is calculated..
HANDOVERHANDOVER CONDITIONSHandover is done on five conditions – Interference – RXQUAL – RXLEV – Distance or Timing Advance – Power BudgetInterference - If signal level is high and still there is RXQUAL problem,then the RXQUAL problem is because of interference.RXQUAL - It is the receive quality. It ranges from 0 to 7 , 0 being thebest and 7 the worstRXLEV - It is the receive level. It varies from -47dBm to -110dBm.Timing Advance - Ranges from 0 to 63.Power budget - It is used to save the power of the MS.
HANDOVERHANDOVER TYPESIntra-Cell Handover BSC 0 1 2 3 4 5 6 7 Call is handed BTS from timeslot 3 to timeslot 5• Handover takes place in the same cell from one timeslot to another timeslot of the same carrier or different carriers( but the same cell).• Intra-cell handover is triggered only if the cause is interference.• Intra-cell handover can be enabled or disabled in a cell.
HANDOVERHANDOVER TYPESIntra-BSC Handover BSC1 0 1 2 3 4 5 6 7 BTS1 Call is handed from timeslot 3 of cell1 to timeslot 1 of cell2 . Both the cells are controlled by the same BSC. 0 1 2 3 4 5 6 7• Handover takes place between different cell which are controlled by the same BSC.
HANDOVERHANDOVER TYPESInter-BSC Handover BSS1 0 1 2 3 4 5 6 7 BTS1 Call is handed from timeslotMSC of cell1 to timeslot 1 of cell2 Both the cells are controlled by the different BSC. BSS2 0 1 2 3 4 5 6 7 BTS2• Handover takes place between different cell which are controlled by the different BSC.
HANDOVERHANDOVER TYPESInter-MSC HandoverMSC1 BSS1 0 1 2 3 4 5 6 7 BTS1 Call is handed from timeslot 3 of cell1 to timeslot 1 of cell2 . Both the cells are controlled by the different BSC, each BSC being controlled by different MSCMSC2 BSS2 0 1 2 3 4 5 6 7 BTS2• Handover takes place between different cell which are controlled by the different BSC and each BSC is controlled by different MSC.
LOCATION UPDATE• MSC should always know the location of the MS so that it can contact it by sending pages whenever required.• The mobile keeps on informing the MSC about its current location area or whenever it changes from one LA to another.• This process of informing the MSC is known as location updating.• The new LA is updated in the VLR.• LAI = MCC + MNC + LAC 3 digits 1-2 digits Max 16 bits MCC MNC LAC MCC = Mobile country code. MNC = Mobile Network Code. LAC = Location area code. Identifies a location area within a GSM PLMN network. The maximum length of LAC is 16 bits. Thus 65536 different LA can be defined in one GSM PLMN.
LOCATION UPDATE TYPES• Normal location update• Periodic location update• IMSI attachNormal Location Update• Mobile powers on and is idle.• Reads the LAI broadcast on the BCCH.• Compares with the last stored LAI and if it is different does a location update.
IMSI ATTACH• Saves the network from paging a MS which is not active in the system.• When MS is turned off or SIM is removed the MS sends a detach signal to the Network. It is marked as detached.• When the MS is powered again it reads the current LAI and if it is same does a location update type IMSI attach.• Attach/detach flag is broadcast on the BCCH sys info.PERIODIC LOCATION UPDATE• Many times the MS enters non-coverage zone.• The MS will keep on paging the MS thus wasting precious resources.• To avoid this the MS has to inform the MSC about its current LAI in a set period of time.• This time ranges from 0 to 255 decihours.• Periodic location timer value is broadcast on BCCH sys info messages.
DISCONTINOUS TRANSMISSION • During conversation user talks alternatively. • In DTX mode of operation the transmitter are switched on only for frames containing useful information. • Helps to increase battery life and reduce interference level. T T T T T T T T T T T T S T T T T T T T T T T T T I 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 T T T T T T T T T T T T S T T T T T T T T T T T T I 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25SID T T T T T T T T T T T T S T T T T T T T T T T T T I 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 T T T T T T T T T T T T S T T T T T T T T T T T T I 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25
IMPLEMENTATION OF DTXVoice Activity Detector ( VAD ) 20 ms speech VAD Speech / No speech block• Determines which specific block of 20ms from the speech coder contains speech.• Removes stationary noise.• Inserts comfort noise.• The frames containing this background noise are called SID frames and are sent in blocks of 8 frames within every 104 frame block.
SYSTEM INFORMATION MESSAGES
SYS INFORMATION MESSAGESBROADCAST MESSAGES • System information is data about the network which the MS needs to be able to communicate with the network in a appropriate manner. • System information messages are sent on the BCCH and SACCH. • There are six different types of system information messages. • System information messages 1 to 4 are broadcast on the BCCH and are read by the MS in idle mode. • System information message 5 and 6 are sent on the SACCH to the MS in dedicated mode. • System information messages 1 to 4 are broadcast on the BCCH in a cyclic mode over 8 BCCH multiframes, i.e. 8 * 51 frames. • Every message is sent at least after every 1.8 sec.
SYS INFORMATION MESSAGESBROADCAST MESSAGES System BCCH Information Multiframe 1 0 2 1 3 2 and 6 4 3 and 7 What is sent is optional on BCCH Multiframe 4 and 5 • System information 5 and 6 are sent on the SACCH immediately after HO or whenever nothing else is being sent. • Downlink SACCH is used for system information messages while Uplink SACCH is used for measurement reports.
SYS INFORMATION MESSAGESSYSTEM INFORMATION 1When frequency hopping is used in cell MS needs to know whichfrequency band to use and what frequency within the band it shoulduse in hopping algorithm.Cell Channel Description Cell allocation number :- Informs the band number of the frequency channels used. 00 - Band 0 ( Current GSM band ) Cell allocation ARFCN :- ARFCN’s used for hopping. It is coded in a bitmap of 124 bits. 124 123 122 121 016 015 014 013 012 011 010 009 008 007 006 005 004 003 002 001
SYS INFORMATION MESSAGESSYSTEM INFORMATION 1RACH Control Parameters Access Control Class :- Bitmap with 16 bits. All MS spread out on class 0 - 9. Priority groups use class 11-15. A bit set to 1 barres access for that class. Bit 10 is used to tell the MS if emergency call is allowed or not. 0 - All MS can make emergency call. 1 - MS with class 11-15 only can make emergency calls. Cell barred for access :- 0 - Yes 1 - No
SYS INFORMATION MESSAGESSYSTEM INFORMATION 1RACH Control Parameters Re-establishment allowed :- 0 – Yes 1 - No max_retransmissions :- Number of times the MS attempts to access the Network [ 1,2,4 or 7 ]. tx_integer :- Number of slots to spread access retransmissions when a MS attempts to access the system. Emergency Call Allowed :- Yes / No
SYS INFORMATION MESSAGESSYSTEM INFORMATION 2• Contains list of BCCH frequencies used in neighbor cells.• MS uses this list to measures the signal strength of the neighbors. Neighbor Cell Description BA Indicator :- Allows to differentiate measurement results related to different list of BCCH frequencies sent to the MS. BCCH Allocation number :- Band 0 is used. BCCH ARFCN number :- Bitmap 1 -124 1 = Set 0 = Not set PLMN permitted RACH Control Parameters
SYS INFORMATION MESSAGESSYSTEM INFORMATION 3Location Area Identity 8 7 6 5 4 3 2 1 MCC DIG 2 MCC DIG 1 Octet A 1 1 1 1 MCC DIG 3 Octet B BCD MNC DIG 2 MNC DIG 1 Octet C LAC Octet D Binary LAC Octet ECell Identity 8 7 6 5 4 3 2 1 CI Octet F Binary CI Octet G
SYS INFORMATION MESSAGESSYSTEM INFORMATION 3Control Channel Description Attach / Detach 0 = Allowed 1 = Not allowed cch_conf :- Defines multiframe struture cch_conf Physical Channels Combined No of CCH 0 1 timeslot (0) NO 9 1 1 timeslot (0) YES 3 2 2 timeslots (0, 2) NO 18 4 3 timeslots (0, 2, 4) NO 27 6 4 timeslots (0, 2, 4, 6) NO 36 bs_agblk :- Number of block reserved for AGCH [ 0-7 ]. Ba_pmfrms :- Number of 51 frame multiframes between transmisiion of paging messages to MS of the same group. T3212 :- Periodic location update timer [ 1-255 deci hours].
SYS INFORMATION MESSAGESSYSTEM INFORMATION 3Cell Options dtx pwrc :- Power control on the downlink. 0 = Not used 1 = Used Radio link timeout :- Sets the timer T100 in the MS.Cell Selection Parameters Rxlev_access_min :- Minimum received signal level at the MS for which it is permitted to access the system. 0-63 = -110 dBm to -47dBm mx_txpwr_cch :- Maximum power the MS will use when accessing the system. Cell_reselect_hysteresis :- Used for cell reselection.RACH Control Parameters
SYS INFORMATION MESSAGESSYSTEM INFORMATION 4Location Area IdentificationCell Selection Parameters Rxlev_access_min mx_txpwr_cch Cell_reselect_hysteresisRACH Control Parameters max_retransmissions tx_integer Cell barred for access Re-establishment allowed Emergency Call Allowed Access Control Class
SYS INFORMATION MESSAGESSYSTEM INFORMATION 4Channel Description Channel type :- Indi. channel type SDCCH or CBCH( SDCCH/8). Subchannel number :- Indicates the subchannel. Timeslot number :- Indicates the timeslot for CBCH [0 - 7]. Training Sequence Code :- The BCC part of BSIC[0 - 7 ]. Hopping Channel(H) :- Informs if CBCH channel is hopping or single. 0 - Single RF Channel 1 - RF hopping channel ARFCN :- If H = 0 MAIO :- If H = 1 , informs the MS where to start hopping. Values [0 - 63]. HSN :- If H = 1 , informs the MS in what order in what order the hopping should take place. Values [ 0 - 63]. HSN = 0 Cyclic Hopping. MA :- Indicates which RF Channels are used for hopping. ARFCN numbers coded in bitmap.
SYS INFORMATION MESSAGESSYSTEM INFORMATION 5Sent on the SACCH on the downlink to the MS in dedicated mode.Neighbour Cell Description BA-IND :- Used by the Network to discriminate measurements results related to different lists of BCCH carriers sent by the MS( Type 2 or 5). Values 0 or 1 ( different from type 2). BCCH Allocation number :- 00 - Band 0 (Current GSM band). BCCH ARFCN :- Neighboring cells ARFCN’s. Sent as a bitmap. 0 = ARFCN not used 1 = ARFCN used 124 123 122 121 016 015 014 013 012 011 010 009 008 007 006 005 004 003 002 001
SYS INFORMATION MESSAGESSYSTEM INFORMATION 6• MS in dedicated mode needs to know if the LA has changed.• MS may change between cells with different Radio link timeout and DTX. Cell Identity Location Area Identification Cell Options dtx pwrc Radio link timeout PLMN permitted
SYS INFORMATION MESSAGESPAGING• Whenever the Network wants to contact the MS, it sends messages on the paging channel.• Paging is sent on the PCH and it occupies 4 bursts.• MS has to monitor the paging channel to receive paging messages.• MS does not monitor all paging channel but only specific paging channels.• There are three types of paging messages Paging No of MS No of MS Total no of Type using IMSI using TMSI MS 1 2 - 2 2 1 2 3 3 - 4 4
SYS INFORMATION MESSAGESCALCULATION OF PAGING GROUPFollowing factors are used for calculation of paging group• CCCH_group – cch_conf in System Information 3 defines the number of CCCH used in the cell. – CCCH can be allocated only TN 0, 2, 4, 6. – Each CCCH carries its own paging group of MS. – MS will listen to paging messages of its specific group.• bs_pa_mfrms• bs_ag_blk_res
SYS INFORMATION MESSAGESCALCULATION OF PAGING GROUPTotal number of paging groups on 1 CCCH_GROUP(N)No of paging groups N = Paging blocks * Repitition of paging blocks = [ CCCH - bs_ag_blk_res ] * bs_pa_mfrmsRange of Paging Groups on 1 CCCH_GroupMinimum available Paging Groups = Min pag blocks * min bs_pa_mfrms =2*2 =4Maximum available Paging Groups = Max pag blocks * max bs_pa_mfrms =9*9 = 81
SYS INFORMATION MESSAGESAVAILABLE PAGING BLOCKS ON 1 CCCH_GROUPMaximum AGCH reservation for non-combined multiframe = 7 Available paging blocks = 2 Maximum AGCH reservation for combined multiframe = 1 Available paging blocks = 2Minimum AGCH reservation for non-combined multiframe = 0 Available paging blocks = 9 Minimum AGCH reservation for combined multiframe = 0 Available paging blocks = 3No of paging blocks will have a range of 2 - 9
SYS INFORMATION MESSAGESCALCULATION OF CCCH AND PAGING GROUP NOCCCH_GROUP = [ ( IMSI mod 1000) mod (BS_CC_CHANS * N ) ]div NPaging group no = [ ( IMSI mod 1000) mod (BS_CC_CHANS *N ) ] mod N
SOME KEY DATABASE PARAMETERS
TIMER T3101• The MS requests for resources by sending channel request on RACH.• The BSS allocates a SDCCH, if available, and sends a IMMEDIATE ASSIGNMENT message on the downlink. This message contains the details of allocated SDCCH, TSC , TA etc.• As soon as the BSS allocates and sends message on the AGCH, it starts a timer T3101.• The MS logs on the SDCCH and sends a message on the UPLINK.• As soon as the BSS receives this message the timer T3101 is stopped.• If no message is received by the BSS from the MS and timer T3101 expires, then the BSS releases the allocated SDCCH resource.• This timer is set in millisecs.
TIMER T3101Successful SDCCH Access Unsuccessful SDCCH AccessMS MS CELL CELL RACH RACH IMMEDIATE ASSIGNMENT SDCCH ALLOCATED IMMEDIATE ASSIGNMENT SDCCH ALLOCATED (AGCH) START TIMER T3101 (AGCH) START TIMER T3101 IF MS SENDS CL2I ON CL2I SDCCH STOP TIMER T3101 IF T3101 EXPIRES AND BSS DOES NOT RECEIVE CL2I ON SDCCH RELEASE ALLOCATED RESOURCES
Wait_indication parameter & Timer T3122• The MS requests for resources by sending channel request on RACH.• The BSS allocates a SDCCH, if available, and sends a IMMEDIATE ASSIGNMENT message on the downlink. This message contains the details of allocated SDCCH, TSC , TA etc.• If no SDCCH is available, the BSS sends a IMMEDIATE ASSIGNMENT REJECT message to the MS.• As soon as the MS receives the IMMEDIATE ASSIGNMENT REJECT message, it starts a timer T3122 and sets it equal to wait_parameter_indication.• Till the timer T3122 is running, no channel request leaves the MS.• The next channel request is sent only after the expiry of T3122.• This wait_indication_parameter can be set from 0 to 255 secs.
Wait_indication parameter & Timer T3122 MS CELL RACH NO SDCCH AVAILABLE IMMEDIATE ASSIGNMENT SET T3122 IN MS REJECT (AGCH) EQUAL TO WAIT_INDICATION INACTIVE MODEIF T3122 EXPIRES,MS CAN NOW SENDA FRESH REQUEST RACH
CALL QUEING• The MS requests for resources by sending a request to the BSS on RACH.• The BSS assigns a SDCCH and the call setup takes place on the SDCCH.• It is possible that after the call setup, their may not be a TCH available for the MS due to congestion.. Assig. Req• The Network is able to place this call in a queue along with Queuing Indication Start T11 other MS awaiting assignment of a TCH. Assign. Compl / Fail• Stop T11 The length of the queue can be set in the database.• The time for which a MS can T11 expires. Clr Req. stay in the queue can be set by setting the value of T11 timer.
TCH RESOURCE REPORTING• The number of dedicated traffic channel in use within a BSS may be useful information for the MSC.• The number of TCH currently allocated can be indicated with a BSSMAP REOURCE INDICATION MESSAGE.• The message can be sent to the MSC in different modes.• In one-off mode, the MSC asks for a report and the BSS generates one.• In the periodic mode, the MSC tells the BSS that it wants reports at regular interval.• In spontaneous mode, the report is generated in response to TCH demand.
TCH RESOURCE REPORTING 100% TCH Usage No of TCH free LWM Resource Indication to MSC No Handover request entertained HWM Resource Indication to MSC Traffic Usage Handover request entertained
T3109• The MS and BSS monitor the appearance of SACCH messages.• If an uplink failure occurs and the threshold of lost SACCH messages is reached, the BSS will activate timer T3109.• The BSS will stop sending SACCH messages to the MS.• The MS will not receive any SACCH messages and hence T100 will expire.• When T100 expires the MS will return to ideal mode.• The BSS will release the channel resources once T3109 expires.• For the downlink failure same pattern is followed except that T100 expires first.• T3109 should be set higher than T100 ensuring that the system holds on to the radio link long enough for MS to release it first. Otherwise it will be possible to have two MS on the same TCH.
T3109 LADDER DIAGRAM MS BSS SACCH SACCH NOT DECODED LINK_FAIL EXCEEDED DEACTIVATE D/L SACCH START TIMER T3109 START TIMER T100 SACCH DEACTIVATED ON D/L TIMER T3109 STILL RUNNING T100 EXPIRES. RELEASE TIMER T3109 EXPIRES RADIO RESOURCES BSS RELEASES RADIO RESOURCE
T3110 AND T3111 - Normal Release• The system initiates the release of a channel by sending a channel release message to the MS and will start timer T3109.• SACCH messages on the downlink are disconnected.• On the receipt of the channel release, the MS starts internal timer T3110 and disconnects the main signaling link.• When T3110 times out or when the signaling is disconnect confirmed, the MS deactivates all RF links and returns to BCCH idle mode.• When the BSS receives disconnect message, it stops timer T3109 and starts T3111.• When T3111 has expired all RF links are terminated.• If T3109 times out all RF channels are deactivated and are then free to be allocated.• T3109 should be greater than T3110.
T3110 AND T3111 - Normal Release Channel Release Start timer T3109Start timer T3110 Disconnect Stop timer T3109 Start timer T3111 UAStop T3110 on the receipt of UA T3111 expired Release Radio Resources
INTRODUCTION TO FREQUENCY HOPPING f1 f4 Modulated f3 RF signal f2 f1 1 63250 00 /// 2 3 Information is 4 5 6 7 8 9 M 0 # OT * O R OL Voice A transmitted by different frequencies at different timeslot
Introduction to Frequency Hopping• Can be used to improve the quality of the network• Also can be used to increase the capacity of the Network thereby reducing the number of sites required for CAPACITY.The way it works• Each burst is transmitted on a different frequency• Both mobile and base station follow the same hopping sequence
Introduction to Frequency Hopping• Fading – Causes quality deterioration – Is frequency dependent• FH diversifies the impact of fading and improves quality.• The immunity to fading increases by exploiting its frequency selectivity, because using different frequencies the probability of being continuously affected by fading is reduced, so the transmission link quality is improved.• This improvement is much more noticeable for slow moving mobiles.•
Increased Immunity to fading• In a cellular urban environment in most cases multipath propagation will be present and, as a consequence of that, important short term variations in the received level are frequent . This is called Rayleigh fading which results in quality degradation because some of the information will be corrupted.• For a fast moving mobile, the fading situation can be avoided from one burst to another because it also depends on the position of the mobile so the problem is not so serious.• For a stationary one the reception may be permanently affected resulting in a very bad quality, even a drop call.• Once the information is received by the mobile or the base station, the only way to cope with the disturbance produced by the fading (errors in the information bits) are the decoding and deinterleaving processes, with an effectiveness limited by the number of errors they have to deal with.
Increased Immunity to fading• Interference• A result of frequency reuse & irregular terrain and sites• FH diversifies the impact of interference and improves quality• The situation of permanent interference coming from neighbour cells transmitting the same or adjacent frequencies is avoided using Frequency Hopping because the calls will spend the time moving through different frequencies not equally affected by interfering signals. This effect is called Interference Averaging.
Increased Immunity to fading• Considering a non hopping system, the set of calls on the interferer cells which can interfere with the wanted call is fixed for the duration of those calls and some calls will be found with very good quality (no interference problems) whereas some others with very bad quality (permanent interference problems).• With hopping, that set of interfering calls will be continually changing and the effect is that calls tend to experience an average quality rather than extreme situations of either good or bad quality (all the calls will suffer from a controlled interference but only for short and distant periods of time, not for all the duration of the call).• This interference averaging means again spreading the raw bit errors (BER caused by the interference) in order to have a random distribution of them instead of bursts of errors, and therefore enhance the effectiveness of decoding and deinterleaving processes to cope with the BER and lead to a better value of FER.
TYPES OF HOPPINGBase Band Hopping (BBH)• The radio units transmit always the same frequency• Number of frequencies for hopping = Number of carriers• The radio units are always transmitting a fixed frequency and frequency hopping is performed by moving the information for every call among the available radio units in a cell on a per burst basis.• In reception the call is always processed by the same radio unit (the one where the call started).• The number of frequencies to hop over is limited by the number of radio units equipped in the cell.• The BCCH carrier can hop in timeslots 1 to 7 (without power control/DTX).
TYPES OF HOPPINGSynthesiser Frequency Hopping (SFH)• The radio units change (retune) the frequency every burst.• The call always stays in the same radio unit.• Number of frequencies for hopping > Number of carriers.• The radio units can hop over a range of different frequencies( 64 in case of Motorola).• Hybrid combiners are required in the base station (Cavity Combiners can not be used with SFH).• The BCCH carrier can never hop.
Hopping Parameters For frequency hopping operability, GSM defines the following set of parameters:• Mobile Allocation (MA): Set of frequencies the mobile is allowed to hop over. MA is a subset of all the frequencies allocated by the system operator to the cell (cell allocation) although it can be the same. Eg:- If the operator has frequencies from 1 -32, then he can use 1-15 for BCCH and 17-32 for hopping ( MA).• Hopping Sequence Number (HSN): Determines the hopping order used in the cell. 64 different HSNs can be assigned, where HSN = 0 provides a cyclic hopping sequence and HSN = 1 to 63 provide various pseudorandom hopping sequences.• Mobile Allocation Index Offset (MAIO): Determines inside the hopping sequence which frequency the mobile starts to transmit on.• Frequency Hopping Indicator (FHI): Defines a hopping system made up by an associated set of frequencies (MA) to hop over and a hopping sequence (HSN).
INTRODUCTION TO RF PLANNING
INTRODUCTION TO RF PLANNING• Designing a cellular system - particularly one that incorporates both Macrocellular and Microcellular networks is a delicate balancing exercise.• The goal is to achieve optimum use of resources and maximum revenue potential whilst maintaining a high level of system quality.• Full consideration must also be given to cost and spectrum allocation limitations.• A properly planned system should allow capacity to be added economically when traffic demand increases.• As every urban environment is different, so is every macrocell and microcell network. Hence informed and accurate planning is essential in order to ensure that the system will provide both the increased capacity and the improvement in network quality where required, especially when deploying Microcellular systems.
• RF planning plays a critical role in the Cellular design process.• By doing a proper RF Planning by keeping the future growth plan in mind we can reduce a lot of problems that we may encounter in the future and also reduce substantially the cost of optimization.• On the other hand a poorly planned network not only leads to many Network problems , it also increases the optimization costs and still may not ensure the desired quality.
INTRODUCTION TO RF PLANNINGTOOLS USED FOR RF PLANNING• Network Planning Tool• CW Propagation Tool• Traffic Modeling Tool• Project Management Tool
INTRODUCTION TO RF PLANNINGNetwork Planning Tool• Planning tool is used to assist engineers in designing and optimizing wireless networks by providing an accurate and reliable prediction of coverage, doing frequency planning automatically, creating neighbor lists etc.• With a database that takes into account data such as terrain, clutter, and antenna radiation patterns, as well as an intuitive graphical interface, the Planning tool gives RF engineers a state-of-the-art tool to: – Design wireless networks – Plan network expansions – Optimize network performance – Diagnose system problems• The major tools available in the market are Planet, Pegasos, Cell Cad.• Also many vendors have developed Planning tools of their own like Netplan by Motorola, TEMS by Ericsson and so on.
INTRODUCTION TO RF PLANNINGNetwork Planning Tool (PLANET)
INTRODUCTION TO RF PLANNINGPropagaton Test Kit• The propagation test kit consists of – Test transmitter. – Antenna ( generally Omni ). – Receiver to scan the RSS (Received signal levels). The receiver scanning rate should be settable so that it satisfies Lee’s law. – A laptop to collect data. – A GPS to get latitude and longitude. – Cables and accessories. – Wattmeter to check VSWR.• A single frequency is transmitted a predetermined power level from the canditate site.• These transmitted power levels are then measured and collected by the Drive test kit. This data is then loaded on the Planning tool and used for tuning models.
INTRODUCTION TO RF PLANNINGPropagaton Test Kit
INTRODUCTION TO RF PLANNINGTraffic Modeling Tool• Traffic modelling tool is used by the planning engineer for Network modelling and dimensioning.• It helps the planning engineer to calculate the number of network elements needed to fulfil coverage, capacity and quality needs.• Netdim by Nokia is an example of a Traffic modelling tool.
INTRODUCTION TO RF PLANNINGProject Management Tool• Though not directly linked to RF Design Planning, it helps in scheduling the RF Design process and also to know the status of the project• Site database : This includes RF data, site acquisition,power, civil ,etc.• Inventory Control• Fault tracking• Finance Management
PRELIMINARY WORK Propagation tool setup • Set up the planning tool hardware. This includes the server and or clients which may be UNIX based. • Setup the plotter and printer to be used. Terrain, Clutter, Vector data acquisition and setup • Procure the terrain, clutter and vector data in the required resolution. • Setup these data on the planning tool. • Test to see if they are displayed properly and printed correctly on the plotter.
PRELIMINARY WORKSetup site tracking database • This is done using Project management or site management databases. • This is the central database which is used by all relevant department, viz. RF, Site acquisition, Power, Civil engineering etc, and avoids data mismatch.Load master lease site locations in database • If predetermined friendly sites that can be used are available, then load this data into the site database.
PRELIMINARY WORKMarketing Analysis and GOS determination • Marketing analysis is mostly done by the customer. • Growth plan is provided which lists the projected subscriber growth in phases. • GOS is determined in agreement with the customer (generally the GOS is taken as 2%) • Based on the marketing analysis, GOS and number of carriers as inputs, the network design is carried out.Zoning Analysis • This involves studying the height restrictions for antenna heights in the design area.
PRELIMINARY WORKSet Initial Link Budget • Link Budget Analysis is the process of analyzing all major gains and losses in the forward and reverse link radio paths. • Inputs • Base station & mobile receiver sensitivity parameters • Antenna gain at the base station & mobile station. • Hardware losses(Cable, connector, combiners etc). • Target coverage reliabilty. • Fade margins. • Output • Maximum allowable path loss.
PRELIMINARY WORKInitial cell radius calculation • Using link budget calculation, the maximum allowable path loss is calculated. • Using Okumura hata emprical formula, the initial cell radius can be calculated.Initial cell count estimates • Once the cell radius is known, the area covered by one site can be easily calculated. • By dividing the total area to be covered by the area of each cell, a initial estimate of the number of cells can be made.
INITIAL SURVEYMorphology Definition • Morphology describes the density and height of man made or natural obstructions. • Morphology is used to more accurately predict the path loss. • Some morphology area definitions are Urban, Suburban, rural, open etc. • Density also applies to morphology definitions like dense urban, light suburban, commercial etc. • This basically leads to a number of sub-area formation where the link budget will differ and hence the cell radius and cell count will differ.
Morphology Drive Test • This drive test is done to prepare generic models for network design. • Drive test is done to characterize the propagation and fading effects. • The objective is to collect field data to optimize or adjust the prediction model for preliminary simulations. • A test transmitter and a receiver is used for this purpose. • The received signals are typically sampled ( around 50 samples in 40λ ).
Propagation Tool Adjustment • The data collected by drive testing is used to prepare generic models. • For a given network design there may be more than one model like dense-urban, urban, suburban, rural, highway etc. • The predicted and measured signal strengths are compared and the model adjusted to produce minimum error. • These models are then used for initial design of the network.
INITIAL DESIGNComplete Initial Cell Placement • Planning of cell sites sub-area depending on clutter type and traffic required.Run Propagation Analysis • Using generic models prepared by drive testing & prop test, run predictions for each cell depending on morphology type to predict the coverage in the given sub-areas. • Planning tool calculates the path loss and received signal strength using Co-ordinates of the site location, Ground elevation above mean sea level, Antenna height above ground, Antenna radiation pattern (vertical & horizontal) & antenna orientation, Power radiated from the antenna.
INITIAL DESIGNReset Cell Placement( Ideal Sites) • According to the predictions change the cell placements to design the network for contigious coverage and appropriate traffic.System Coverage Maps • Prepare presentations as follows • Background on paper showing area MAP which include highways, main roads etc. • Phase 1 sites layout on transparency. • Phase 1 sites composite coverage prediction. • Phase 2 sites layout transparency. • Phase 2 composite coverage prediction on transparency. • If more phases follow the same procedure.
INITIAL DESIGNDesign Review With The Client • Initial design review has to be carried out with the client so that he agrees to the basic design of the network. • During design review, first put only the background map which is on paper. Then step by step put the site layout and coverage prediction. • Display may show some coverage holes in phase 1 which should get solved in phase 2 .
SELECTION OF SITESPrepare Initial Search Ring • Note the latitude and longitude from planning tool. • Get the address of the area from mapping software. • Release the search ring with details like radius of search ring, height of antenna etc.Release search rings to project managementVisit friendly site locations • If there are friendly sites available that can be used (infrastructure sharing), then these sites are to be given preference. • If these sites suite the design requirements, then visit these sites first.
SELECTION OF SITESSelect Initial Anchor Sites • Initial anchor sites are the sites which are very important for the network buildup, Eg - Sites that will also work as a BSC.Enter Data In Propagation Tool • Enter the sites exact location in the planning tool.Perform Propagation Analysis • Now since the site has been selected and the lat/lon of the actual site ( which will be different from the designed site) is known, put this site in the planning tool and predict coverage. • Check to see that the coverage objectives are met as per prediction.
SELECTION OF SITESReset / Review Search Rings • If the prediction shows a coverage hole ( as the actual site may be shifted from the designed site), the surrounding search rings can be resetted and reviewed.Candidate site Visit( Average 3 per ring) • For each proposed location, surveys should carried out and at least 3 suitable site candidates identified. • Details of each candidate should be recorded on a copy of the Site Proposal Form for that site. Details must include: » Site name and option letter Site location (Lat./Long) » Building Height » Site address and contact number » Height of surrounding clutter » Details of potential coverage effecting obstructions or other comments(A, B, C,...)
SELECTION OF SITESDrive Test And Review Best Candidate • In order to verify that a candidate site, selected based on its predicted coverage area, is actually covering all objective areas, drive test has to be performed. • Drive test also points to potential interference problems or handover problems for the site. • The test transmitter has to be placed at the selected location with all parameters that have been determined based on simulations. • Drive test all major roads and critical areas like convention centers, major business areas, roads etc. • Take a plot of the data and check for sufficient signal strength, sufficient overlaps and splashes( least inteference to other cells).
SELECTION OF SITESDrive Test Integration • The data obtained from the drive test has to be loaded on the planning tool and overlapped with the prediction. This gives a idea of how close the prediction and actual drive test data match. • If they do not match ( say 80 to 90 %) then for that site the model may need tuning.Visit Site With All Disciplines( SA, Power, Civil etc ) • A meeting at the selected site takes place in which all concerned departments like RF Engineering, Site acquisition, Power, Civil Engineer, Civil contractor and the site owner is present. • Any objections are taken care off at this point itself.
Select Equipment Type For Site • Select equipment for the cell depending on channel requirements • Selection of antenna type and accessories.Locate Equipment On Site For Construction Drawing • Plan of the building ( if site located on the building) to be made showing equipment placement, cable runs, battery backup placement and antenna mounting positions. • Antenna mounting positions to be shown separately and clearly. • Drawings to be checked and signed by the Planner, site acquisition, power planner and project manager.
Perform Link Balance Calculations • Link balance calculation per cell to be done to balance the uplink and the downlink path. • Basically link balance calculation is the same as power budget calculation. The only difference is that on a per cell basis the transmit power of the BTS may be increased or decreased depending on the pathloss on uplink and downlink.EMI Studies • Study of RF Radiation exposure to ensure that it is within limits and control of hazardous areas. • Data sheet to be prepared per cell signed by RF Planner and project manager to be submitted to the appropriate authority.
Radio Frequency Plan/ PN Plan • Frequency planning has to be carried out on the planning tool based on required C/I and C/A and interference probabilities.System Interference Plots • C/I, C/A, Best server plots etc has to be plotted. • These plots have to be reviewed with the customer to get the frequency plan passed.Final Coverage Plot • This presentation should be the same as design review presentation. • This plot is with exact locations of the site in the network.
Identification of coverage holes • Coverage holes can be identified from the plots and subsequent action can be taken(like putting a new site) to solve the problem.
BASIC DEFINITIONSIsotropic RF Source • A point source that radiates RF energy uniformly in all directions (I.e.: in the shape of a sphere) • Theoretical only: does not physically exist. • Has a power gain of unity I.e. 0dBi.Effective Radiated Power (ERP) • Has a power gain of unity i.e. 0dBi • The radiated power from a half-wave dipole. • A lossless half-wave dipole antenna has a power gain of 0dBd or 2.15dBi.Effective Isotropic Radiated Power (EIRP) • The radiated power from an isotropic source EIRP = ERP + 2.15 dB
BASIC DEFINITIONS• Radio signals travel through space at the Speed of Light C = 3 * 108 meters / second• Frequency (F) is the number of waves per second (unit: Hertz)• Wavelength (λ) (length of one wave) = (distance traveled in one second) (waves in one second) λ= C / F If frequency is 900MHZ then wavelength λ = 3 * 108 900 * 106 = 0.333 meters
BASIC DEFINITIONSdB• dB is a a relative unit of measurement used to describe power gain or loss.• The dB value is calculated by taking the log of the ratio of the measured or calculated power (P2) with respect to a reference power (P1). This result is then multiplied by 10 to obtain the value in dB. dB = 10 * log10(P1/P2)• The powers P1 ad P2 must be in the same units. If the units are not compatible, then they should be transformed.• Equal power corresponds to 0dB.• A factor of 2 corresponds to 3dB If P1 = 30W and P2 = 15 W then 10 * log10(P1/P2) = 10 * 10 * log10(30/15) =2
BASIC DEFINITIONSdBm• The most common "defined reference" use of the decibel is the dBm, or decibel relative to one milliwatt.• It is different from the dB because it uses the same specific, measurable power level as a reference in all cases, whereas the dB is relative to either whatever reference a particular user chooses or to no reference at all.• A dB has no particular defined reference while a dBm is referenced to a specific quantity: the milliwatt (1/1000 of a watt).• The IEEE definition of dBm is "a unit for expression of power level in decibels with reference to a power of 1 milliwatt."• The dBm is merely an expression of power present in a circuit relative to a known fixed amount (i.e., 1 milliwatt) and the circuit impedance is irrelevant.}
BASIC DEFINITIONSdBm• dBm = 10 log (P) (1000 mW/watt) where dBm = Power in dB referenced to 1 milliwatt P = Power in watts• If power level is 1 milliwatt: Power(dBm) = 10 log (0.001 watt) (1000 mW/watt) = 10 log (1) = 10 (0) =0• Thus a power level of 1 milliwatt is 0 dBm.• If the power level is 1 watt 1 watt Power in dBm = 10 log (1 watt) (1000 mW/watt) = 10 (3) = 30
BASIC DEFINITIONSdBm• dBm = 10 log (P) (1000 mW/watt) where dBm = Power in dB referenced to 1 milliwatt P = Power in watts• If power level is 1 milliwatt: Power(dBm) = 10 log (0.001 watt) (1000 mW/watt) = 10 log (1) = 10 (0) =0• Thus a power level of 1 milliwatt is 0 dBm.• If the power level is 1 watt 1 watt Power in dBm = 10 log (1 watt) (1000 mW/watt) = 10 (3) = 30
BASIC DEFINITIONSdBm• dBm = 10 log (P) (1000 mW/watt)• The dBm can also be negative value.• If power level is 1 microwatt Power in dBm = 10 log (1 x 10E-6 watt) (1000 mW/watt) = -30 dBm• Since the dBm has a defined reference it can be converted back to watts if desired.• Since it is in logarithmic form it may also be conveniently combined with other dB terms.
BASIC DEFINITIONSdBµv/m• To convert field strength in dbµv/m to received power in dBm with a 50Ω optimum terminal impedance and effective length of a half wave dipole λ/π 0dBu = 10 log[(10-6)2(1000)(λ/π)2/(4*50)] dBm At 850MHZ 0dBu = -132 dBm 39dBu = -93 dBm
FREE SPACE PROPAGATION• Friis Formula Pr = Pt GtGrλ2 Pt Lp Pr (4πd)2 d Gt Gr• Propagation Loss Lp = 10log [4πd / λ]2 The square term is the propagation exponent. It is greater than 2 when obstructions exist.• Propagation Loss in dB: L p = 32.44 + 20Log(d) +20Log(f) f = MHz d = km
PROPAGATION MECHANISMSReflection• Occurs when a wave impinges upon a smooth surface.• Dimensions of the surface are large relative to λ.• Reflections occur from the surface of the earth and from buildings and walls.Diffraction (Shadowing)• Occurs when the path is blocked by an object with large dimensions relative to λ and sharp irregularities (edges).• Secondary “wavelets” propagate into the shadowed region.• Diffraction gives rise to bending of waves around the obstacle.Scattering• Occurs when a wave impinges upon an object with dimensions on the order of λ or less, causing the reflected energy to spread out or“scatter” in many directions.• Small objects such as street lights, signs, & leaves cause scattering
MULTIPATH• Multiple Waves Create “Multipath”• Due to propagation mechanisms, multiple waves arrive at the receiver• Sometimes this includes a direct Line-of-Sight (LOS) signal
MULTIPATHMultipath Propagation• Multipath propagation causes large and rapid fluctuations in a signal• These fluctuations are not the same as the propagation path loss.Multipath causes three major things• Rapid changes in signal strength over a short distance or time.• Random frequency modulation due to Doppler Shifts on different multipath signals.• Time dispersion caused by multipath delays• These are called “fading effects• Multipath propagation results in small-scale fading.
WHAT IS FADING ?• The communication between the base station and mobile station in mobile systems is mostly non-LOS.• The LOS path between the transmitter and the receiver is affected by terrain and obstructed by buildings and other objects.• The mobile station is also moving in different directions at different speeds.• The RF signal from the transmitter is scattered by reflection and diffraction and reaches the receiver through many non-LOS paths.• This non-LOS path causes long-term and short term fluctuations in the form of log-normal fading and rayleigh and rician fading, which degrades the performance of the RF channel.
WHAT IS FADING ?Signal Power (dBm) Large scale fading component Small scale fading component
LONG TERM FADING• Terrain configuration & man made environment causes long-term fading.• Due to various shadowing and terrain effects the signal level measured on a circle around base station shows some random fluctuations around the mean value of received signal strength.• The long-term fades in signal strength, r, caused by the terrain configuration and man made environments form a log-normal distribution, i.e the mean received signal strength, r, varies log- normally in dB if the signal strength is measured over a distance of at least 40λ.• Experimentally it has been determined that the standard deviation, σ, of the mean received signal strength, r, lies between 8 to 12 dB with the higher σ generally found in large urban areas.
RAYLEIGH FADING• This phenomenon is due to multipath propagation of the signal.• The Rayleigh fading is applicable to obstructed propagation paths.• All the signals are NLOS signals and there is no dominant direct path.• Signals from all paths have comparable signal strengths.• The instantaneous received power seen by a moving antenna becomes a random variable depending on the location of the antenna.
RICEAN FADING• This phenomenon is due to multipath propagation of the signal.• In this case there is a partially scattered field.• One dominant signal.• Others are weaker.
DIVERSITY ANTENNA SYSTEMS
Diversity Antenna SystemsNEED OF DIVERSITY Building Building Building
Diversity Antenna SystemsNEED OF DIVERSITY• In a typical cellular radio environment, the communication between the cell site and mobile is not by a direct radio path but via many paths.• The direct path between the transmitter and the receiver is obstructed by buildings and other objects.• Hence the signal that arrives at the receiver is either by reflection from the flat sides of buildings or by diffraction around man made or natural obstructions.• When various incoming radiowaves arrive at the receiver antenna, they combine constructively or destructively, which leads to a rapid variation in signal strength.• The signal fluctuations are known as ‘multipath fading’.
Diversity Antenna SystemsMultipath Propagation• Multipath propagation causes large and rapid fluctuations in a signal• These fluctuations are not the same as the propagation path loss.Multipath causes three major things• Rapid changes in signal strength over a short distance or time.• Random frequency modulation due to Doppler Shifts on different multipath signals.• Time dispersion caused by multipath delays• These are called “fading effects• Multipath propagation results in small-scale fading.
Diversity Antenna SystemsDIVERSITY TECHNIQUE• Diversity techniques have been recognised as an effective means which enhances the immunity of the communication system to the multipath fading. GSM therefore extensively adopts diversity techniques that include Interleaving Diversity techniques In time domain Frequency Hopping In Frequency domain Spatial diversity In spatial domain Polarisation diversity In polarisation domain
Diversity Antenna SystemsCONCEPT OF DIVERSITY ANTENNA SYSTEMS• Spatial and polarisation diversity techniques are realised through antenna systems.• A diversity antenna system provides a number of receiving branches or ports from which the diversified signals are derived and fed to a receiver. The receiver then combines the incoming signals from the branches to produce a combined signal with improved quality in terms of signal strength or signal-to-noise ratio (S/N).• The performance of a diversity antenna system primarily relies on the branch correlation and signal level difference between branches.
Diversity Antenna SystemsCONCEPT OF DIVERSITY ANTENNA SYSTEMS Fade Transmission media 1 Information Receiver Transmission Tmedia 2 Peak
Diversity Antenna SystemsCombiningCombined signal fed to receiver Signal 2 Signal 1 Combined signal Signal 1 Signal 2 Signal Strength Time
Diversity Antenna SystemsSPATIAL DIVERSITY ANTENNA SYSTEMS• The spatial diversity antenna system is constructed by physically separating two receiving base station antennas.• Once they are separated far enough, both antennas receive independent fading signals. As a result, the signals captured by the antennas are most likely uncorrelated.• The further apart are the antennas, the more likely that the signals are uncorrelated.• The types of the configuration used in GSM networks are: • horizontal separation • vertical separation • composite separation.
Diversity Antenna SystemsPOLARISATION DIVERSITY ANTENNA SYSTEMS• A single (say vertical) polarised electromagnetic wave is converted to a wave with two orthogonal polarised fields while it is propagating through scattering environment.• It has also been found that the two fields exhibit some extent of decorrelation.
Diversity Antenna SystemsDUAL POLARISED ANTENNAS• A dual-polarisation antenna consists of two sets of radiating elements which radiate or, in reciprocal, receive two orthogonal polarised fields.• The antenna has two input connectors which separately connects to each set of the elements.• The antenna has therefore the ability to simultaneously transmit and receive two orthogonally polarised fields. H/V Slant 45°
Diversity Antenna SystemsADVANTAGES OF DUAL POLARISED ANTENNAS• The best advantage of using the dual polarisation antenna is the reduction in the number of antennas per sector.• Reduced size of the headframe of the supporting structure• Reduced windload and weight.• Reduced difficulty in site acquisition and installation.• Cost saving – Requiring slim tower – Requiring less installation time. – Cost of one dual polarisation antenna is generally lower than that of two – Single polarised antennas
Diversity Antenna SystemsDUAL POLARISED ANTENNA CONFIGURATIONS DUAL POLE ANTENNA DUAL POLE ANTENNA DUAL POLE ANTENNA SINGLE POLE ANTENNA T R T R T RTX RX RX RX RX TX RX TX RX TX
WHAT IS INTERFERNCE ?• Interference is the sum of all signal contributions that are neither noise not the wanted signal.
EFFECTS OF INTERFERENCE• Interference is a major limiting factor in the performance of cellular systems.• It causes degradation of signal quality.• It introduces bit errors in the received signal.• Bit errors are partly recoverable by means of channel coding and error correction mechanisms.• The interference situation is not reciprocal in the uplink and downlink direction.• Mobile stations and base stations are exposed to different interference situation.
SOURCES OF INTERFERENCE• Another mobile in the same cell.• A call in progress in the neighboring cell.• Other base stations operating on the same frequency.• Any non-cellular system which leaks energy into the cellular frequency band.
TYPES OF INTERFERENCE• There are two types of system generated interference – Co-channel interference – Adjacent channel interferenceCo-Channel Interference• This type of interference is the due to frequency reuse , i.e. several cells use the same set of frequency.• These cells are called co-channel cells.• Co-channel interference cannot be combated by increasing the power of the transmitter. This is because an increase in carrier transmit power increases the interference to neighboring co-channel cells.• To reduce co-channel interference, co-channel cells must be physically separated by a minimum distance to provide sufficient isolation due to propagation or reduce the footprint of the cell.
Co-Channel Interference• Some factors other then reuse distance that influence co-channel interference are antenna type, directionality, height, site position etc,• GSM specifies C/I > 9dB. Carrier f1 Interferer f1 dB C I Distance
Co-Channel Interference D C1 C1 C2 C2 C3 C3• In a cellular system, when the size of each cell is approximately the same, co-channel interference is independent of the transmitted power and becomes a function of cell radius(R) and the distance to the centre of the nearest co-channel cell (D).
Co-Channel Interference• Q = D / R = √3N• By increasing the ratio of D/R, the spatial seperation between the co- channel cells relative to the coverage distance of a cell is increased. In this way interference is reduced from improved isolation of RF energy from the co-channel cell.• The parameter Q , called the co-channel reuse ratio, is related to the cluster size.• A small value of Q provides larger capacity since the cluster size N is small whereas a large value of Q improves the transmission quality.
Adjacent-Channel Interference• Interference resulting from signals which are adjacent in frequency to the desired signal is called adjacent channel interference.• Adjacent channel interference results from imperfect receiver filters which allow nearby frequencies to leak into the passband.• Adjacent channel interference can be minimized through careful filtering and channel assignments.• By keeping the frequency separation between each channel in a given cell as large as possible , the adjacent interference may be reduced considerably.
Adjacent-Channel Interference Carrier f1 Interferer f2 dB A C Distance
COUNTERING INTERFERENCEPOWER CONTROL• RF power control is employed to minimise the transmit power required by MS or BS while maintaining the quality of the radio links.• By minimising the transmit power levels, interference to co-channel users is reduced.• Power control is implemented in the MS as well as the BSS.• Power control on the Uplink also helps to increase the battery life.• Power received by the MS is continously sent in the measurement report.• Similarly uplink power received from the MS by the BTS is measured by the BTS.• Complex algorithm evaluate this measurements and take a decision subsequently reducing or increasing the power in the Uplink or the downlink.
COUNTERING INTERFERENCESECTORIZATION• For 120 degrees sectored site as compared to an omni site almost 1/3rd interference is received in the uplink.• The more selective and directional is the antenna, the smaller is the interference.• Reduction in interference results in higher capacity in both links.
REPEATERSINTRODUCTION Donor side antenna Mobile side antenna Repeater receives Donor signal at Repeater amplifies ~ -90dBm the signal and rebroadcasts the signal Donor Cell Poor Coverage area • Repeater units are designed to receive signals from a donor site, amplify and rebroadcast the donor sites signals into poor coverage areas or to extend the coverage range of the donor site. • These repeater are bi-directional and do not translate frequency and subsequently are limited in output power and gain. • Repeaters provide between 50 to 80 dB of gain.
REPEATERSINTRODUCTION • There are two types of repeater band selective and channel selective. • Band selective repeater amplifies a band of frequency. Hence it amplifies any frequency that falls within its band. • Channel selective repeater allows selection of a number of individual channels to amplify and rebroadcast. • Typically a channel selective repeater allows selection of 2 to 4 channels. • If the GSM900 or DCS1800 network incorporates frequency hopping, then only band selective repeaters should be used.
Other NetworksTRANSMISSION SYSTEMS
Introduction To Transmission Systems• Transmission systems form the backbone of any networks.• Normally transmission systems include SDH, PDH, ATM, Microwaves, leased lines.• In GSM normally the core network is located in the same premises and are mostly interconnected by fixed wireline. In huge network consisting of many MSC located at different places the interconnection may be through any of the transmission systems mentioned above.• The Access network consists of BSC’s with many BTS’s connected to them in various transmission topologies. Normal practice is to connect various BSC’c to the MSC via fiber and different BTS’s connected to BSC via microwave in Daisy chain, star or any other topology. However there can be many different ways of implementation.
E1• 2.048 Mbps circuit provides high speed, digital transmission for voice, data, and video signals at 2.048 Mbps.• 2.048 Mbps transmission systems are based on the ITU-T specifications G.703, G.732 and G.704, and are predominant in Europe, Australia, Africa, South America, and regions of Asia.• The primary use of the 2.048 Mbps is in conjunction with multiplexers for the transmission of multiple low speed voice and data signals over one communication path rather then over multiple paths.• The most common line code used to transmit the 2.048 Mbps signal is known as HDB3 (High Density Bipolar 3) which is a bipolar code with a specific zero suppression scheme where no more then three consecutive zeros are allowed to occur.
Typical implementation of a E1
The 2.048 Mbps Framing Format• The 2.048 Mbps signal typically consists of multiplexed data and/or voice which requires a framing structure for receiving equipment to properly associate the appropriate bits in the incoming signal with their corresponding channels.• The 2.048 Mbps frame is broken up into 32 timeslots numbered 0-31.• Each timeslot contains 8 bits in a frame, and since there are 8000 frames per second, each time slot corresponds to a bandwidth of 8 x 8000 = 64 kbps.• Time slot 0 is allocated entirely to the frame alignment signal (FAS) pattern, a remote alarm (FAS Distant Alarm) indication bit, and other spare bits for international and national use.
E1• The FAS pattern (0011011) takes up 7 bits (bits 2-8) in timeslot 0 of every other frame.• In those frames not containing the FAS pattern, bit 3 is reserved for remote alarm indication (FAS Distant Alarm) which indicates loss of frame alignment when it is set to 1.• The remaining bits in timeslot 0 are allocated as shown in the following Figure.• If the 2.048 Mbps signal carries no voice channels, there is no need to allocate additional bandwidth to accommodate signaling.• Hence, time slot 1-31 are available to transmit data with an aggregate bandwidth of 2.048 Mbps - 64 kbps (TSO) = 1.984 Mbps.
E1 TS 16 Multiframe Format
E1• If there are voice channels on the 2.048 Mbps signal, it is necessary to take up additional bandwidth to transmit the signalling information.• ITU-T Recommendation G.704 allocates time slot 16 for the transmission of the channel-associated signalling information.• The 2.048 Mbps can carry up to thirty 64 kbps voice channels in time slot 1-15 and 17-31.• Voice channels are numbered 1-30; voice channels 16-30 are carried in time slot 17-31.• However, the 8 bits in time slot 16 are not sufficient for all 30 channels to signal in one frame. Therefore, a multiframe structure is required where channels can take turns using time slot 16.
E1• Since two channels can send their ABCD signalling bits in each frame, a total of 15 frames are required to cycle through all of the 30 voice channels.• One additional frame is required to transmit the multiframe alignment signal (MFAS) pattern, which allows receiving equipment to align the appropriate ABCD signalling bits with their corresponding voice channels.• This results in the TS-16 multiframe structure where each multiframe contains a total of 16 2.048 Mbps, numbered 0-15.• Figure on the previous slide shows the TS-16 multiframe format for the 2.048 Mbps signal as defined by the ITU-T Recommendation G.704.
E1• As can be seen in Figure , time slot 16 of frame 0 contains the 4-bit long multiframe alignment signal (MFAS) pattern (0000) in bits 1-4. The “Y” bit is reserved for the remote alarm (MFAS Distant Alarm) which indicates loss of multiframe alignment when it is set to 1.• Time slot 16 of frames 1-15 contains the ABCD signalling bits of the voice channels.• Time slot 16 of the nth frame carries the signalling bits of the nth and (n+15)th voice channels. For example, frame 1 carries the signalling bits of voice channels 1 and 16, frame 2 carries the signalling bits of channels 2 and 17 etc.• It is also important to note that the frame alignment signal (FAS) is transmitted in time slot 0 of the even numbered frames.
T1 Introduction• T1 is a digital communications link that enables the transmission of voice, data, and video signals at the rate of 1.544 million bit per second (Mb/s).• Introduced in the 1960s, it was initially used by telephone companies who wished to reduce the number of telephone cables in large metropolitan areas.• T1 simplifies the task of networking different types of communications equipment since it can carrz both voice and data on the same link.
T1 Introduction• To illustrate, Figure 1 on the next page shows what a company’s communications network might look like without T1• Figure 1 shows that telephone, facsimile, and computer applications would all require separate lines.• Typically, voice and low-speed data applications are serviced by analog lines, while high-speed data applications are serviced by digital facilities.• Figure 2 on the next page depicts the same network with a T1 link installed.
T1 FIGURE 1
T1 FIGURE 2
PDH Overview• Long-established analog transmission systems that proved inadequate were gradually replaced by digital communications networks.• In many countries, digital transmission networks were developed based upon standards collectively known today as the Plesiochronous Digital Hierarchy (PDH).• Although it has numerous advantages over analog, PDH has some shortcomings: provisioning circuits can be labor-intensive and time-consuming, automation and centralized control capabilities of telecommunication networks are limited, and upgrading to emerging services can be cumbersome.• A major disadvantage is that standards exist for electrical line interfaces at PDH rates, but there is no standard for optical line equipment at any PDH rate, which is specific to each manufacturer.
PDH Overview• This means that fiber optic transmission equipment from one manufacturer may not be able to interface with other manufacturers’ equipment.• As a result, service providers are often required to select a single vendor for deployment in areas of the network, and are locked into using the network control and monitoring capabilities of that vendor.• Reconfiguring PDH networks can be difficult and labor- intensive - resulting in costly, time-consuming modifications to the network whenever new services are introduced or when more bandwidth is required.
SDH Overview• Bellcore (the research affiliate of the Bell operating companies in the United States) proposed a new transmission hierarchy in 1985.• Bellcore’s major goal was to create a synchronous system with an optical interface compatible with multiple vendors, but the standardization also included a flexible frame structure capable of handling either existing or new signals and also numerous facilities built into the signal overhead for embedded operations, administration, maintenance and provisioning (OAM&P) purposes.• The new transmission hierarchy was named Synchronous Optical Network (SONET).• The International Telecommunication Union (ITU) established an international standard based on the SONET specifications, known as the Synchronous Digital Hierarchy (SDH), in 1988.
SDH Overview• The SDH specifications define optical interfaces that allow transmission of lower-rate (e.g., PDH) signals at a common synchronous rate.• A benefit of SDH is that it allows multiple vendors’ optical transmission equipment to be compatible in the same span.• SDH also enables dynamic drop-and-insert capabilities on the payload; PDH operators would have to demultiplex and remultiplex the higher-rate signal, causing delays and requiring additional hardware.• Since the overhead is relatively independent of the payload, SDH easily integrates new services, such as Asynchronous Transfer Mode (ATM) and Fiber Distributed Data Interface (FDDI), along with existing European 2, 34, and 140 Mbit/s PDH signals, and North American 1.5, 6.3, and 45 Mbit/s signals.
SDH Overview STM1 data rate calculation• SDH multiplexing combines low-speed digital signals such as 2, 34, and 140 Mbit/s signals with required overhead to form a frame called Synchronous Transport Module at level one (STM-1).• Figure 1 shows the STM-1 frame, which is created by 9 segments of 270 bytes each.• The first 9 bytes of each segment carry overhead information; the remaining 261 bytes carry payload.• When visualized as a block, the STM-1 frame appears as 9 rows by 270 columns of bytes.• The STM-1 frame is transmitted row #1 first, with the most significant bit (MSB) of each byte transmitted first.
SDH Overview STM1 data rate calculation• This formula calculates the bit rate of a framed digital signal:• bit rate = frame rate x frame capacity• In order for SDH to easily integrate existing digital services into its hierarchy, it operates at the basic rate of 8 kHz, or 125 microseconds per frame, so the frame rate is 8,000 frames per second.• The frame capacity of a signal is the number of bits contained within a single frame.• Figure 2 shows: frame capacity = 270 bytes/row x 9 rows/frame x 8 bits/byte = 19,440 bits/frame• The bit rate of the STM-1 signal is calculated as follows: bit rate = 8,000 frames/second x 19,440 bits/frame = 155.52 Mbit/s
SDH Overview Multiplexing of STM frames• As the Figure coming on the next slide shows, the ITU has specified that an STM-4 signal should be created by byte interleaving four STM-1 signals.• The basic frame rate remains 8,000 frames per second, but the capacity is quadrupled, resulting in a bit rate of 4 x 155.52 Mbit/ s, or 622.08 Mbit/s.• The STM-4 signal can then be further multiplexed with three additional STM-4s to form an STM-16 signal.• Table 1 lists the defined SDH frame formats, their bit rates, and the maximum number of 64 kbit/s telephony channels that can be carried at each rate.
SDH Overview Multiplexing of STM frames STM1 A STM1 B STM4 4:1 STM1 C STM1 D
SDH Overview Multiplexing of STM frames FRAME BIT MAX NUMBER OF FORMAT RATE TELEPHONY CHANNELS STM - 1 155.52 Mbits/S 1920 STM - 4 622.08 M bits/S 7680 STM - 16 2.488 Gbits/S 30720
Microwave Overview• Normally used for point to point transmission• Used mainly in the GHz range.• Normally distance between radios is less than 50Kms.