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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.
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.
890 960 935 915 UPLINK FREQUENCIES DOWNLINK FREQUENCIES UPLINK AND DOWNLINK FREQUENCY SEPARATED BY 45MHZ INTRODUCTION TO GSM
UPLINK FREQUENCIES DOWNLINK FREQUENCIES 890 915 935 960 880 925 UPLINK AND DOWNLINK FREQUENCY SEPARATED BY 45MHZ
Extended 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.
INTRODUCTION TO GSM
1710 MHz 1880 MHz 1805 MHz 1785 MHz UPLINK FREQUENCIES DOWNLINK FREQUENCIES UPLINK AND DOWNLINK FREQUENCY SEPARATED BY 95MHZ
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.
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.
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 equ i pment 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.
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 N/W.
M obile S witching C entre (MSC) – Lucent MSC NETWORK COMPONENTS
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 attempti n g 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 ) - This is the telephone number of the mobile.
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.
Equipment Identity Register ( EIR ) White List All Valid assigned ID’s Range 1 Range 2 Range n Black List Service denied MS IMEI 1 MS IMEI 2 MS IMEI n Grey List Service allowed but noted MS IMEI 1 MS IMEI 2 MS IMEI n EIR NETWORK COMPONENTS
TAC FAC SNR 6 2 6 1 TAC FAC SNR SP SP = Type Approval Code = Final Assembly Code = Serial Number = Spare
International 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.
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. .
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.
Base Transceiver Station (BTS ) NETWORK COMPONENTS
MSC BSC BTS12 BTS1 BTS2 BTS4 BTS3 BTS11 BTS13 BTS14 BTS5 BTS6 BTS7 BTS8 BTS9 BTS11 Open ended Daisy Chain Daisy Chain with a fork. Fork has a return loop back to the chain Star Daisy Chain with a fork. Fork has a return loop back to the chain BTS Connectivity NETWORK COMPONENTS
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 analyse 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.
Operation And Maintenance Centre For Radio ( OMC-R ) NETWORK COMPONENTS
Operation And Maintenance Centre For Radio ( OMC-R ) NETWORK COMPONENTS
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 Algorit h m - 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.
MS C lass M ark NETWORK COMPONENTS
MOBILE MAXIMUM RANGE RANGE= TIMING ADVANCE = DELAY OF BITS (0-63) BIT PERIOD= 577/156.25 = 3.693 sec =3.693 * 10e-6 sec VELOCITY= 3 * 10e5 Km/sec RANGE= 34.9 Km TIMIMG ADVANCE * BIT PERIOD* VELOCITY 2
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
Signalling System (C7 or SS7
Packet Switched Data
A bis using the LAPD protocol (Link Access Procedure D )
INTERFACE NAMES Each 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 PCM This 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 to the BTS’s. It represents the physical layer in the OSI model. Each 2 Mb/s link provides 30 traffic channels available to carry speech ,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
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.
NORMAL BURST 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 57 bits 57 bits 26 bits 3 3 FRAME1(4.615ms) FRAME2 Training sequence Data Data Tail Bits Tail Bits Flag Bit Flag Bit Guard Period Guard Period 0.546ms 0.577ms Carries traffic channel and control channels BCCH, PCH, AGCH, SDCCH, SACCH and FACCH. CHANNEL CONCEPT
Data - 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 always 000. Guard period - 8.25 bits long. The receiver can only receive and decode if the burst is received within the timeslot designated for it.Since the MS are moving. Exact synchronization of burst is not possible practically. Hence 8.25bits corresponding to about 30us is available as guard period for a small margin of error. Flag bits - This bit is used to indicate if the 57 bits data block is used as FACCH. Training Sequence - This is a set sequence of bits known by both the transmitter and the receiver( BCC of BSIC). When a burst of information is received the equaliser searches for the training sequence code. The receiver measures and then mimics the distortion which the signal has been subjected to. The rece i ver then compares the received data with the distorted possible transmitted sequence and chooses the most likely one. NORMAL BURST CHANNEL CONCEPT
FREQUENCY CORRECTION BURST 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 142 bits 3 3 FRAME1(4.615ms) FRAME2 Fixed Data Tail Bits Tail Bits Guard Period Guard Period 0.546ms 0.577ms
Carries FCCH channel.
Made up of 142 consecutive zeros.
Enables MS to correct its local oscillator locking it to that of the BTS.
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.
BSS Downlink MS Uplink 5 0 3 timeslot offset CHANNEL CONCEPT NEED FOR TIMESLOT OFFSET
T 15 T 5 T 9 T 10 T 11 S 12 T 13 T 14 T 6 T 7 T 8 T 0 T 1 T 2 T 3 T 4 T 16 T 17 T 18 T 19 T 20 T 21 T 22 T 23 T 24 I 25 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 120 msec 4.615 msec 26 FRAME MULTIFRAME STRUCTURE
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.
Example - Let's 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
The block of 456 bits produced by the convolutional code is then passed to the interleaver
Convolution code R = k/n = 1/2 k=1 1 bit input n=2 2 bit input CODING
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.
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.
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.
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.
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
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.
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.
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 confirm s 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 employ s 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 use s 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 lower.
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 commence s 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.
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.
I n 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.
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.
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 s ecs .
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_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 PLMN's 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 PLMN's, without making the process take too long.
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 MS's 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.
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.
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.
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 PLMN's are available. This monitoring is done so as to minimise interruptions to the monitoring of the PCH.
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.
PAGING Example cch_conf = 0 bs_ag_blk_res = 1 bs_pa_mfrms = 2 If cch_conf = 1 minimum = 2 If cch_conf = 6 Maximum = 81 * 4 M in time between pages = 2 * 235.5 = 471ms Max time between pages = 9 * 235.5 =2.1195 sec
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.
AVAILABLE PAGING BLOCKS ON 1 CCCH_GROUP Maximum AGCH reservation for non-combined multiframe = 7 Available paging blocks = 2 Maximum AGCH reservation for combined multiframe = 1 Available paging blocks = 2 Minimum AGCH reservation for non-combined multiframe = 0 Available paging blocks = 9 Minimum AGCH reservation for combined multiframe = 0 Available paging blocks = 3 No of paging blocks will have a range of 2 - 9
CALCULATION OF CCCH AND PAGING GROUP NO CCCH_GROUP = [ ( IMSI mod 1000) mod (BS_CC_CHANS * N ) ] div N Paging group no = [ ( IMSI mod 1000) mod (BS_CC_CHANS * N ) ] mod N
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.
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.
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 15 T 5 T 9 T 10 T 11 S 12 T 13 T 14 T 6 T 7 T 8 T 0 T 1 T 2 T 3 T 4 T 16 T 17 T 18 T 19 T 20 T 21 T 22 T 23 T 24 I 25 T 15 T 5 T 9 T 10 T 11 S 12 T 13 T 14 T 6 T 7 T 8 T 0 T 1 T 2 T 3 T 4 T 16 T 17 T 18 T 19 T 20 T 21 T 22 T 23 T 24 I 25 T 15 T 5 T 9 T 10 T 11 S 12 T 13 T 14 T 6 T 7 T 8 T 0 T 1 T 2 T 3 T 4 T 16 T 17 T 18 T 19 T 20 T 21 T 22 T 23 T 24 I 25 T 15 T 5 T 9 T 10 T 11 S 12 T 13 T 14 T 6 T 7 T 8 T 0 T 1 T 2 T 3 T 4 T 16 T 17 T 18 T 19 T 20 T 21 T 22 T 23 T 24 I 25 NUMBER OF NEIGHBORS HANDOVER
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.
INTERFERENCE ON IDLE CHANNEL HANDOVER
The 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 RXLEV ii) Measurements performed in BSS - Uplink RXLEV - Uplink RXQUAL - MS-BS distance - Interference level in unallocated time slots Every SACCH multiframe (480 ms) a new processed value for each of the measurements is calculated.. HANDOVER HANDOVER
Handover takes place between different cell which are controlled by the different BSC and each BSC is controlled by different MSC.
HANDOVER TYPES Inter-MSC Handover BSS1 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 MSC. BSS2 MSC1 BTS2 MSC2 HANDOVER
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
MCC MNC LAC 3 digits 1-2 digits Max 16 bits 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.
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 15 T 5 T 9 T 10 T 11 S 12 T 13 T 14 T 6 T 7 T 8 T 0 T 1 T 2 T 3 T 4 T 16 T 17 T 18 T 19 T 20 T 21 T 22 T 23 T 24 I 25 T 15 T 5 T 9 T 10 T 11 S 12 T 13 T 14 T 6 T 7 T 8 T 0 T 1 T 2 T 3 T 4 T 16 T 17 T 18 T 19 T 20 T 21 T 22 T 23 T 24 I 25 T 15 T 5 T 9 T 10 T 11 S 12 T 13 T 14 T 6 T 7 T 8 T 0 T 1 T 2 T 3 T 4 T 16 T 17 T 18 T 19 T 20 T 21 T 22 T 23 T 24 I 25 T 15 T 5 T 9 T 10 T 11 S 12 T 13 T 14 T 6 T 7 T 8 T 0 T 1 T 2 T 3 T 4 T 16 T 17 T 18 T 19 T 20 T 21 T 22 T 23 T 24 I 25 DISCONTINOUS TRANSMISSION SID
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.
Following factors are used for calculation of paging 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.
SYS INFORMATION MESSAGES
CALCULATION OF PAGING GROUP Total 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_mfrms Range of Paging Groups on 1 CCCH_Group Minimum available Paging Groups = Min pag blocks * min bs_pa_mfrms = 2 * 2 = 4 Maximum available Paging Groups = Max pag blocks * max bs_pa_mfrms = 9 * 9 = 81 SYS INFORMATION MESSAGES
AVAILABLE PAGING BLOCKS ON 1 CCCH_GROUP Maximum AGCH reservation for non-combined multiframe = 7 Available paging blocks = 2 Maximum AGCH reservation for combined multiframe = 1 Available paging blocks = 2 Minimum AGCH reservation for non-combined multiframe = 0 Available paging blocks = 9 Minimum AGCH reservation for combined multiframe = 0 Available paging blocks = 3 No of paging blocks will have a range of 2 - 9 SYS INFORMATION MESSAGES
CALCULATION OF CCCH AND PAGING GROUP NO CCCH_GROUP = [ ( IMSI mod 1000) mod (BS_CC_CHANS * N ) ] div N Paging group no = [ ( IMSI mod 1000) mod (BS_CC_CHANS * N ) ] mod N SYS INFORMATION MESSAGES
The system initiates the release of a channel by sending a channel release message to the MS and will start timer TT3109.
SACCH messages on the downlink are disconnected.
On the receipt of the channel release, the MS starts internal timer T3110 and disconnects the main signalling link.
When T3110 times out or when the signalling 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 T3109 Start timer T3110 Disconnect Stop timer T3109 Start timer T3111 UA Stop T3110 on the receipt of UA T3111 expired Release Radio Resources
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.
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.
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.
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.
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).
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 .
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 Eric s son and so on.
INTRODUCTION TO RF PLANNING
Network Planning Tool (PLANET) INTRODUCTION TO RF PLANNING
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.
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.
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.
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 * log 10 (P 1 /P 2 )
The powers P 1 ad P 2 must be in the same units. If the units are not compatible, then they should be transformed.
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.}
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 40 .
Experimentally it has been determined that the standard deviation, , of the mean received signal strength, r, lies between 8 to 12 dB with the higher generally found in large urban areas.
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
Diversity techniques Interleaving In time domain Frequency Hopping In Frequency domain Spatial diversity In spatial domain Polarisation diversity In polarisation domain
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 Systems Transmission media 1 Transmission T media 2 Peak Fade Receiver Information CONCEPT OF DIVERSITY ANTENNA SYSTEMS
Diversity Antenna Systems Combining Combined signal fed to receiver Signal 2 Signal 1
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) .
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.