Wireless is bouncing in human imagination from the very earlier need of communications. Though the term wireless need not apply in the very earlier stages. The effort was to establish connections without physical resources or connections. Like for example the above picture is an army wireless signaling method. To send a message or a signal to someone away, a very common old method was to shout loudly, make some noise , but then you loose the secrecy behind communication, so to overcome, army started using colourful flags, so the advent of wireless communications.
A basic wireless communication model is shown above. The input to a wireless system is the information source, which could be speech , data, video etc. This information goes to a transmitter which modulates the source information to a Radio Frequency Carrier and it is radiated by a transmitter. On the other hand the information is received by a receive antenna from where it goes to the receiver which demodulated the RF carrier and recovers the original information. This information's then goes to the receiving destination, like the telephone speaker or display or video terminal.
Wireless communications are of different types : like Simplex, half duplex and Duplex. SIMPLEX : In simplex type of communications, the direction of transmission is only. In very simple words the user terminal is either a transmitter or a receiver. In these types of communication, there will be a transmitter which transmits an information modulated RF carrier which is received by the receiver which demodulates the information. Common examples of Simplex wireless communication are Pagers, AM/FM/ Television Broadcast services etc.
In Half-Duplex communication, the direction of transmission is in both direction but not simultaneous , it is alternate. There will a transmitter plus receiver on both ends of communication terminals. As shown above "A" will transmit information and "B" will receive, when "B" wants to reply , it will indicate "A" to stop transmission and "B" will then start transmitting and "A" will receive. Common examples of this type of systems are Push To Talk Radios etc.
In Duplex communication both the information terminals will transmit and receive simultaneously. Duplex communication is the current trend for wireless mobile application, this is because the Mobile applications require simultaneous communications. Now in the previous two cases both the transmitter and receivers are tuned to a single RF carrier, but what should happen in this case, can I use the same frequency on both ends for simultaneous communication. No I can't these will interfere with each other. So before we go into the discussion in the frequency domain we need to understand the electromagnetic spectrum.
Let us understand the frequency domain in which wireless communications work. The entire electromagnetic spectrum is divided several Frequency bands like MF, HF, VHF, UHF, SHF and EHF. The common property illustrated above is the wavelength which goes on increasing as the frequency increases. Within this entire band, applications are spread. The spread in applications is based on the properties of electromagnetic waves. Up to 1GHz and also near to 2 Ghz are used for consumer applications like FM radios, Television broadcast, Mobile Telephony etc. From 2 GHz to 100Ghz is the Microwave band. Above 100 Ghz is the Millimeter waves.
Now let us understand why the applications explained on the previous page are spread in specific frequency bands. This is because of the properties of the Radio Frequencies. A common property of Radio frequencies is the loss. Any RF transmission through any media will suffer loss. This loss goes on increasing with higher frequencies. And any RF receiver has a finite sensitivity, the ability to recover the RF signal at its receiver input. So the higher the frequency more will be the loss, and hence low level of signal will come at the receiver, To overcome the poor reception , that is to get level of signal coming at the receiver , the RF frequency should be transmitted at higher power. High Power again increase the health hazards , the effect of which is also proportional to frequency , i.e. the effect of higher frequency transmitted at a certain power X dbm will be higher than a lower frequency transmitted at the sam power of X dbm. So the selection of frequency depends on Application . Based on this the RF spectrum is divided into Application band. The lower frequency bands are used for consumer application where human beings are involved in the transmission and reception of Radio frequencies very closely like Mobile Phones, Domestic Radios , Television broadcast etc. The Application bands are further divided into Technology bands . This is because for certain user applications like Mobile Telephones there are different technologies like Analog , Digital etc., and in many countries different technologies operate simultaneously , so naturally they need to operate in different bands.
As explained earlier the frequency bands are further divided into several application segments. Like Mobile Telephone communications always operates in either the 800- 900 MHz and 1800-1900 MHz band. From these bands, certain specified segments or so called blocks are reserved for certain applications by regulating authorities which reserve this since these blocks are defined by the standard technologies. An example for this is the Mobile Telephone Communications. As mentioned above, these 50 MHz bands are reserved by the regulating authorities as defined by the standards. AMPS/DAMPS band is regulated by TIA , whereas GSM band is reserved by ETSI. What does this mean 50 MHz band to be used for Mobile Communications. But when it is Mobile Communication it is just not one user but several users. This means this needs a further division in the band. So to provide multiple access these 50 MHz bands are further divided into Channels. But how ? Today worldwide there are three channelization methods used , FDMA, TDMA and CDMA.
All the mobile Technologies don't need very large bandwidth for single user communication. Technologies have well defined the bandwidth required for each call. Like AMPS needs 30 Khz of bandwidth in one direction for communication. Similarly GSM needs 200 Khz for one direction communication. This means if the AMPS band is divided in 30 Khz channels, we can have 1666 Freq channels and similarly for GSM we can have 250 Freq channels. And this is what is practically done. This method of dividing a Frequency band into small bandwidth user channels is termed as Frequency Division Multiple Access ( FDMA ). The term itself specifies Multiple Accesses ( means users ) in a band by dividing into several small Frequency Channels.
It was found later that division by Frequency itself was not sufficient to meet the capacity requirements. So each FDMA channel is further divided into time domain into timeslots. This means now multiple user will use the same FDMA channel , but will not at the same instance. Each user will be given a timeslot, which is of a fixed time period. One user will accomplish communication in that time period and then will remain idle for remaining timeslots which are used by other users. With this the capacity is increased, i.e. now more users can use the same old number of FDMA channels. This method of increasing multiple accesses is termed as Time Division Multiple Access ( TDMA ). For example: AMPS FDMA Channels were divided in 3 timeslots and a new technology was created which was DAMPS. So with DAMPS now we have 4998 Channels. And Similarly in GSM we have 8 timeslots on each FDMA Channel , so we have 2000 Channels.
Code Division Multiple Access is a completely different method of Multiple Access. In CDMA the FDMA Channel is of very large bandwidth i.e. 1.25 MHz. This 1.25 MHz channel is divided into 64 code channels. Each code channel can be used by a different user. All the users will communicate at the same time and will only receive and transmit information correlated to its code. We will discuss CDMA in more details in later sections.
What access methods are we going to use for WLL. First of all WLL as mentioned earlier is not a technology so it uses available wireless telephone technologies with some implementation modifications. WLL needs Duplex Communication . Now lets refresh , in duplex communication the transmission and reception occurs at the same time, so naturally it cannot occur on the same resource or channel. So at least two channels are required for one call. Now what type of two channels. These Duplex channels should be based on the existing Channelization methods.
The above figures illustrates two methods of Duplex Communication. Frequency Division Duplex (FDD) : In this we use the existing the FDMA technique. With FDMA we have divided the application band into several small bandwidth channels. Now for each call, the user will transmit on one FDMA channel and receiver on another FDMA channel. So for each call a pair of FDMA Channels are required . TDD : With this method the transmission and reception takes place on the same frequency but at not at the same time. Each FDMA channel is divided into separate Transmit and Receive Timeslot. The user may first transmit for a specific period and then receive in the next time period. Though this theoretically is not duplex communication but practically it is since the user does not recognize this alternate transmission and reception, since the time periods are very small in microseconds.
To provide wireless communication access to subscribers, we need a pair of RF channels per active call. In a large city , with several thousand subscribers, number of channels required would be so large that they can not be accomodated on one site. Also, there is a limit to availability of spectrum. If an operators get a 5 MHz band, then he has only 25 channels to use !!! With 25 channels loaded at one site, he can offer cellular service to 2000 subscribers at most . ( In GSM , each RF channels accomodates 8 traffic channels ). To overcome this limitation, you have to create zones of coverage, which are called as CELLS.
Each cell has a Base Station Transeiver ( BTS ) at the chosen site. The location of BTS site depends upon several factors.. 1. Coverage in prime localities / hot spots is the most important criteria in choosing the BTS site. 2. Terrain - Type of buildings in the area to be covered. 3. Availability of proper site - COST factor 4. Availability of open space, power supply , security 5. Nearby installations of Cellular / other transmitters. 6. Access to the network - leased lines / Microwave link / Optical link. "Number of BTS" sites is a measure of the size .
As a convention and for simplicity in representation, Cells in Cellular system are shown as Hexagons. The actual cell covered by a Base station takes a very different shape depending upon the terrain, obstuctions and transmitting antenna charecteristics. The cells are of different sizes and shapes. To cover a densly populated areas, smaller cells are used. Where as large cells cover a low subscriber density areas. On the highways and main roads, cell size is optimised to cover larger length of the cell along the road. This is made possible by using a highly directive antenna.
Distance between two cells using same frequency is given by D=R * SQRT(3 * N ). This distance is a deciding factor on what cluster size has to be used. A small cluster size would give a smaller D, which means interference form cell using same freq is going to be higher. A large value of n ( eg. 12 ) would give larger value of D , which is good in terms of interference. But this would affect the number of frequencies available for loading per cell. For N=3 ,
The Global System for Mobile communications (GSM) is a huge, rapidly expanding and successful technology. Less than five years ago, there were a few 10's of companies working on GSM. Each of these companies had a few GSM experts who brought knowledge back from the European Telecommunications Standards Institute (ETSI) committees designing the GSM specification. Now there are 100's of companies working on GSM and 1000's of GSM experts. GSM is no longer state-of-the-art. It is everyday-technology, as likely to be understood by the service technician as the ETSI committee member. GSM evolved as a mobile communications standard when there were too many standards floating around in Europe. Analog cellular was in use for several years in different parts of world. Even today there are few networks of Analog cellular. The experience of analog cellular helped in developing specifications for a Digital Cellular standard. The work on GSM specs took a completed decade before practical systems were implemented using these specs. GSM is quickly moving out of Europe and is becoming a world standard. HP has become expert in GSM through our involvement in Europe. With excellent internal communications, HP is in an excellent position to help our customers, in other regions of the world, benefit from our GSM knowledge. In this presentation we will understand the basic GSM network elements and some of the important features. Since this is a very complex system, we have to do develop the knowledge in a step by step approach.
GSM is truly becoming the GLOBAL System for Mobile Communications. It's been clear for a long time that GSM would be used across Europe. Now, many countries around the world, who have been delaying their decision, have selected GSM. GSM has become a Pan Asian standard and is going to be used in much of South America.
PCN started in the UK with Mercury One-to-One and Hutchison Microtel (Orange) offering the first two networks to use DCS1800. The UK PCNs have had overwhelming success with their competitive business tariffs and cheap off-peak calls. Germany's E net followed the UK PCNs. DCS1800 is becoming more widespread with systems in Thailand, Malaysia, France, Switzerland and Australia. Further systems are planned in Argentina, Brazil, Chile, France, Hungary, Poland, Singapore and Sweden. Even the USA, which has shunned the adoption of GSM900, is about to use the GSM based PCS1900 for it's PCS system. In the USA, GSM will share the allocated bands with other systems based on CDMA, NAMPS and IS-136 TDMA. The PCS1900 licences already cover approximately half of the US population, and we are likely to see this increase to close to full coverage as remaining licences are granted.
GSM Requirements The quality of Voice in the GSM system must be better then that acheived by the 900Mhz analogue systems over all the operating conditions. The system must offer encryption of user information. The system must operate in the entire frequency band 890-915Mhz and 935-960MHz. An international standardized signalling system must be usd to allow the nterconnection of mobile switching centres and location registers. Minimize modifications to the existing fixed public networks. Design the system so handsets costs are minimised Handsets must be able to be used in all participating countries. Maximum flexibility for other services like ISDN System should maximize the functions and services available to cater for the special nature of mobile communications.
evelopment of GSM. The seven companies are: American Personal Communications (APC), American Portable Telecom, Bell South Personal Communications, Intercel, Omnipoint, Pacific Bell Mobile Services and Western Wireless Co. Many of the large GSM manufacturers are also backing PCS1900 including Nokia, Ericsson, Matra, AEG, Northern Telecom. In technical terms PCS1900 will be identical to DCS1800 except for frequency allocation and power levels. The first commercial PCS system was launched by APC, under the name of Sprint Spectrum on 15th November 1995, based on PCS1900. The majority of US PCS licences will becom operational over the next two years. Other systems are also on trial in the US, including DECT.
w services such as data and fax to GSM and DCS1800.
The first generation of mobile telephony systems began with the analogue FM, and frequency division duplex (FDD) systems, AMPS, TACS and NMT. These systems are still heavily subscribed in many parts of the world, but are now rapidly being replaced by second generation systems. The second generations systems, such as GSM, IS-95 CDMA, D-AMPS and PDC offer higher speech quality, higher capacity, data, spectral efficiency, security and better roaming capability. While these systems are still growing, the standards bodies and manufacturers are looking to the future and the third generation (3G). The contenders for third generation systems are: EUROPE - ETSI W-CDMA ASIA - ARIB W-CDMA US - UWCC D-AMPS IS-136 - CDG cdma2000
The concept for a 3G system includes: wireless multimedia, bandwidth on demand, integration of satellite and terrestrial services, and continous worldwide coverage. A broad spectrum of terminal complexity is anticipated, including “machine users”, e.g. vending machines reporting stock status. The International Telecommunications Union (ITU) has been holding discussions on “Future Public Land Mobile Telephony Systems” (FLMTS) since 1978. This acronym has now been replaced by the term IMT-2000. “IMT” stands for International Mobile Telecommunications and the “2000” represents the approximate operating frequency and approximate timescale for deployment
BTS : Base Transceiver Station BSC : Base Station Controller MS : Mobile Station TRAU : Transcoder Rate Adaptation Unit MSC : Mobile Switching Center HLR : Home Location Register VLR : Visiting Location Register AUC : Authentication Centre EIR : Equipment Identity Register SMSC : Short Message Service Center VMSC : Voice Mail Service Center BC : Billing Center OMC : Operation
Mobile Station is rthe gadget used by a GSM subscriber. Mobile Station provides the user the GSM communication services for Voice and Data. GSM Mobile Phones are made according toGSM standards. Each and every Phone produced in the world by any manufacturer complies to GSM standards. Any make Mobile Phone can be used on any GSM Network. GSM phone cannot be used, until it is activated by the Service Provider. This activation is done by providing a SIM card to the user. SIm card is plugged inside the MS. This SIM card has an identification which is unique to the SIM card. MS also has other identification which is its unique IMEI number. IMEI number is also soterd in a central database called EIR . Mobile Sation locks to a Base Station, the one closest to it when it turns ON. Mobile station also scans neigbor cells signals when it is idle and also when it is on call. On the other hand, the network also know the whereabouts of the Mobile and updates it in its databases the VLR and HLR. Mobile transmits and receives voice at 13 kbps over air interface.
Mobile Stations major discrimination is on its Power Class. Power Class of the Mobile is the maximum power which the mobile can transmit under extreme condition when commanded by the BTS. The different power capabilites of the Mobile Phones are 20W, 8W, 5W, 2W and 0.8W. The most commonly used class of mobile is 4, i.e 2Watts . The importance behind the Output Power is that it defines the accessibility of the Mobile Phone in areas of poor coverage. High Power Class may not transmit always at full power , hence this results into excess talk time and standby time. Actual ouptut powr when the Mobile is on call is varied as commanded by the BTS , which is based on the Path Loss
The Mobile Station has two indentities associated with it : MSISDN: Mobile Sation International Dialled Number) : This is the human identitiy associated with the Mobile Phone. This identitiy is used by the users to identify their subscription numbers and also by others to call Mobile Phones. This identity is not stored anywhere in the Mobile Phone. It is availble in the Network database (HLR). The objective of this identity is to route the call over the fixed and Mobile Phone network. It is made up of CC ( Country Code ) which provides uniqueness in routing calls to GSM operations country. Within the country the NDC ( National Destination Code ) provides uniques to the different operators. Finally the SN( Subscriber Number) discriminates each subcriber within a network. IMEI ( International Mobile Equipment Identity ) : This is a unique number stored inside the ROM of the Mobile Phone. Its uniqueness lies in the TAC (Type Approval Code ) which is alloted by the MOU to a new mobile which passed the confrmation specifications. After which the manufacturer may discrimate individual phones by FAC (Final Assemble Code) , SNR (Serial Number) and also by the SP ( Spare digit ) typical used for software version
SIM card as defined by GSM is a removable module which can be insertd inside the Mobile Phone when the user wnats to use the MS. It is available in two sizes the Credit Card size ; known as ISO SIM or the Stamp size ; known as Pug in SIM. SIM card has a unique information on it which is its idnetity the IMSI ( International Mobile Subscriber Identity ). This identity is unique to each SIM used in any of the GSM Networks. this identity si used by the Network to accomplish Signaling with the MS and also to d signaling with other GSM networks for information transfer related toa particular connection. The IMSI is a 15 digit identity which consists of MCC ( Mobile Country Code ) which is unique to each GSM Operations country, the MNC( Mobile Country Code) which discriminates the networks within a MCC , further the MSIN ( Mobile Subscriber Identity Number ) which discriminates individual subscribers ina network. SIM also has a two level protection to prevent misuse. SIM can be optionally by the user be set for a PIN code . Whenever the Mobile is turned ON or the SIM is inserted , the Mobile will ask the user to enter the PIN code. If three false entries of SIM card are entered then the SIM is blocked . The the user needs to enter a Pin Unblocking Key ( PUK) . Ten fakse entries of PUK will permanently disable the SIM card permanently.
Apart from the IMSI, SIM also contains some other essential information which is required to establish successful communication with the network like the subscriber key and algorithms are used for authentication and Ciphering. SIM also contains a list of preferred networks which the Mobile hunts for when it turns on. Ther are several other features and contents on the SIM which are personalized by the Service Provider which activates the SIM card. MS also stores some information on the SIM card EEPROM like its location information and some timer values.
The above figure represents a generic cell site architecture. The BTS is installed ina cabin/shelter or a room. There is an AC Mains Panel to which the AC mains enters and is properly through fuse panels is distributed to the DC Power Supply unit, which rectifies this AC and provides DC power Supply to the Base Station. Typical Operating Voltage Levels of Macro BTS is 48V. There is also a Battery Backup to provide power to the BTS in the event of the Main Power Failure. The swicthing from Mains to Batteries is also done through the Power Supply Unit. All the elements inside the cabinet generate failure alarms and are connected to an Alarm Interface Panel which is then connected to the BTS. The BTS on the other side is connected to the Antenna System, which comprises of Transmit and Receive Antennas. The number of Antennas depends on the loading on the Base Station , basically the number of sectors it controls.The BTS is also connected on a Abis Interface ( E1/T1) to the BSC. The transport media for this could be coaxial, fibre or most commonly used Digital Microwave Radio. We will now in further slides understand the architecture inside the BTS cabinet and also the differenet variations in the Antenna System. Then we will also see the Abis configurations.
Base Transceiver Station which is the key element inside the cell site has set of tanscivers which transmit and receive RF energy with the Mobiles in its area. On BTS will have mimnum cell or more than one cell. The number of transceivers inside the BTs will depend upon the capacity inside the cell. BTS is connected on a G.703 Abis Interface to the BSC. BTS transmits and receivers voice at 13 Kbps over the air interface to the Mobiles. BTS also carries out certain pther functions with the Mobiles like controlling the Transmit Power and Timing instance of the Mobile and also commanding handovers to the Mobile Stations.
re are two TRX's os tow Receiver Carreir swill also come, and these two receive carrieres will be received b tow Antennas. Each Antenna wil receive two signals, so four signals come to the Splitter. Each trasncverneeds inputs from both antennas , the splitter will split this enerege coming at inout to both the TRX's. PCM Interface. This is a G.703 interface which connects the BTS to the BSC, and his interface in GSM Trmonlgyy is termed as Abis interface. Alarm Interface. All the external Alamrsm a re connected to this interaface , from hermessages realted to alarm are sen on the )&M interface to the BSC. TRAU :Transcodr Rate Adaptation Unit . If ranscoding is used locally inside the BTS , then this unit will be presnet to code the speech from 13 Kbps to 64 Kbps. If transcoding is to be done at hte BSc or MSC , then this unit will not be present..
The block diagram illustrates a typical architecture for a single sector BTS with three transmitter / receiver pairs (TRXs) - i.e. this BTS has three frequencies on which it will transmit on the downlink and receive on the uplink. Base stations will have their link to the rest of the network, the Abis link, provided either over a standard 2mbits (1.544 in the US) link or via a microwave link. Additionally interfaces may be provided to access internally generated BTS alarms and some troubleshooting information via a propietary NEM specific port. The CPU carries out the processing associated with these links. The frequency reference unit generates the timing to be used by the TRXs. This is normally derived from the Abis link, but may also be an alternate external reference e.g. a GPS receiver, or an internal frequency reference. The TRX can be divided into logic, RF and amplification functions. The logic function is responsible for communication between the TRX and other parts of the BTS. It also performs uplink and downlink digital signal processing such as channel coding, interleaving, ciphering, burst formatting and equalization. The RF function contains the actual radio transmitter and reciever. The combiner / distribution unit is the interface between the TRXs and the antenna system, combining transmitted signals from the TRXs and distributing received signals to all TRXs. By using multiple combiners, antenna diversity can be introduced to reduce the impact of fading.
BSC : Base Station Controller which has several BTS's connected to it. Depending upon the number of BTS's there will be BSC's. The BSC's will be finally connected to the MSC. So for example, when a call is made from a fixed line to mobile, it is routed from PSTN to MSC, then to BSC and BTS and to Mobile. The BSC controls various functions of BTS and Mobile, like assigning a channel for call, handover etc.
The Base Station Controller though controls the BTS which is a radio equipment, but the BSC itself is not a radio equipment It has no radio elements inside it. The BSC has processor boards and PCM interfaces to provide connections to all the BTS's and also on the other side to the MSC. The BSC has a major role to play in swithcing traffic ciruits from the Abis to the A interface. The BSC may also have optionally a TRAU, if transcoding is to be done at the BSC.
Abis is the PCM interface between the BSC and MSC. Physically this is a G.703 interface and could be E1 or T1. in all our further dsicussions we will consider E1. Abis carrier the traffic and signalling information for all the transceivers inside the BTS. It also carries O&M information between the BSC and BTS, like the control caommands coming from the BSCand traffic reports orginated by the BTS. Abis used the HDLC protocol for signalling which is LAPD ( Link Access Protocol on D channel ). LAPD has severla modes of operation. What modes means is how the signalling circuits are distributed over the E1 interface, wether each TRX has seperate signalling circuits or several TRX signalling information is concentrated or multiplexed on limited signaling circuits.
The first LAPD mode illustrated above is the LAPD basic mode. In this mode each TRX has a separate signaling circut of 64 Kbps. Each signalling ciruit has two immediate 64 Kbps Traffic ciruits. GSM used 13 kbps of speech rate on the air interface, to which some TRAU information is added and it becomed 16 Kbps. Four such 16 kbps traffic channels are mapped on one 64 Kbps ciruit. EAch TRX has 8 traffic channels. So for each Transceiver two 64 kbps ciruits are required for Traffic and one for Signalling. So with this mode, 10 Transceivers can be accomodated on one E1.
In this mode which is LAPD Concentrated Signaling information for certain number of TRX's are concentrated on a single 64 kbos circuit. There are two different methods of concentration. The above figure illustratesone method in which on one 64 kbps ciruit the signaling information for 4 TRX's are concentrated. This is typically done by creating 16 kbps subchannels. So with this metod 13 TRXs signaling as well as speech can be accomodated on a single E1 Link.
In this type of concentrated LAPD mode , signaling for all the Transcevier are concentrated on one 64 kbps circuit. With this 15 TRX's signaling and Speech can be accomodated on 1 E1 link. This method is becoming very poplular and is adopted by many of the NEMS.
LAPD multiplexed is a mode in which Signaling for each TRX is on a 16 kbps ciruit which is multiplexed wiht 3 speech channels of 16 Kbps. So for each TRX two 64 kbps circuits are required.
TRAU is the Transcoder Rate Adpatation Unit in short referred to as Transcoder which provides the function of coding speech from 13 kbps to 64 kbps and vice versa. The Mobile Switching Center (MSC) is based on ISDN switching. The Fixed network is also ISDN based . ISDN has speech rae of 64 kbp but the GSM Speech rate is 13 kbps, so in order to do the switching the 13 kbps speech channels should be transcoded to 64 kbps and vice versa in the reverse direction. This function is accomplished by the Transcoder. The Trasncoder does not actually gets 13 kbps inputs, because to the 13 kbps the BTS adds 3 kbps of additional information and makes it 16 kbps. The location of the TRAU is implementation dependent, but it is always preferred to locate the TRAU as close to the MSC as possible, since saves on connectivity resources.
The transcoder may be located with the MSC,BSC or BTS.If it is located at the MSC,the 13 kbps channels are transmitted to the BSS by bit stuffing them to a data transfer rate of 16 kbps and then fitting four of them into each 64 kbps circuit. This type of transcoding when done at the MSC is known as remote transcoding since the transcoder device is remote to the BTS.Thus with this type of transcoding each 2.048 Mbps stream from the MSC can carry 120 GSM speech channels.thus increasing channel capacity and thereby reducing cost.
MSC : Mobile Switching Centre is the heart of the GSM network. This element is similar to any PSTN exchange. It does the switching of call, which in other words means connecting , maintaining and release of mobile to mobile and mobile to PSTN and PSTN to Mobile calls. MSC has the database of all the subscribers in the network. It has different sub units for maintaining these databases which are the HLR and VLR. The subscriber database include the subcriber no's and identities like IMSI , MSISDN etc. It also has the listed subcribed features against each subscriber no. All the BSC's in the network are connected to the MSC. MSC connects all the calls coming in and going out to PSTN ( fixed line phones ) through the BSC's. MSC is weighted on the basis of how many subscribers it can support. For example an MSC of 1 lac subscribers means one MSC is enough till subscriber base increases upto 1 lac, beyond which another MSC will be required
As the number of subscribers increase, it will exceed the capcaity of one MSC, so another MSC's will be require, and another when the subscribers agian doubel up. All the MSC's in the network have to communicate with other and also with the outside world. It will be very complicated to connect every MSC to each other and also each MSC to PSTN, so the concept of GMSC ( Gateway MSC ) is used which as the name says is a gateway to all the MSC's to the outside world.
MSC is the entity which establishes calls. To establish calls and other user related services, it needs to know who are users ( list of all subscribers ) and thier secret identitiies, and what services they are allowed. This results into huge database taking into consideration a huge size of GSM subscibers. So to accomplish calls and other services, MSC communicates with HLR and gets some data pertaining to the subscriber. HLR will have a series of subscriber numbers to used in the network, but all all them may not havebeen activated or issued.
VLR has a database of those susbcriber no's which are activiated , which means those no's for which the SIM is issued and the subscriber has turned on at least once. So when a SIm is issued , and the Mobile the turns On, the number of the mobile is ckecked with HLR and then registered in VLR. VLR also has temporayr data pertaining to each susbcriber. Temporary data means this data is always changing, which includes the mobile status (like on/off ), and the location of the mobile, that is in which cells is the mobile, so that the incoming calls can be diverted to those cells. In the case where there are more than one MSC, and also in case of raoming , when the mobile comes from some other MSC, it is not known to the visiting MSC, so it is checked with the HLR, and then temporarily registered in the VLR.
When a subcriber takes service from an operator, it is issued with a SIM card, this SIM card as we discussed earlier has an IMSI corresponding to which is the MSISDN ( 98xxx.... ) which is known to the subscriber. Now IMSI is an identity which is used by the network to communicate with the mobile. So it can be easily be captured and a bogus or fraud SIM can be manufactured easily. TO avoid this the process of Authentication is used, which is the verification of SIM before a service is alloted to a subscriber. This is done with the help of a Secret Key ( code ) which is avialble on the SIM. This Key associated with every SIM which means with every IMSI and is different for each SIM. This pair of IMSI and Key is also available at the HLR -AUC. The AUC associated with the HLR does this job of verification of SIM. The Mobile is asked to send its secret key by performing some operations on it by using some number send by AUC. The secret key is then send in this coded form to the MSC , where it is verified.
EIR is the records of the valid IMEI's . Valid IMEI's mean those mobile equipments which are type approved. MSC may ask a mobile to send its IMEI, which it checks with the EIR, to find its validity. In the EIR, the IMEI's are classifiefd into three categories. White List : This contians the IMEI of type approved mobiles. Black List : List of IMEI's which shold be barred because either they are stolen or are not functioning proprerly Grey List : List of IMEI's who are to evaluated before they are put in black list.
Billing Centre is the network which generates the billing statement for each subscriber in the network. The MSC sents the billing information of each subscriber to the BC which will have subscriber identity, called number, date and time stamp and duration. The BC then produces the billing amount based on the data received from the MSC. The BC will the information about the units of charge and the susbsidised charge time ( non-peak, midniht hours) , date ( low rate holidays, national holidays, seasonal low rate days , etc ). The BC amy or may not be connected directly to the MSC. If it is connected directly to the MSC, the MSC will send the billing data at regular pre-defined intervals. If it is not connected to the MSC, then the MSC will save the billing data on some tape drive or any other storage device, which will be then fed to the BC, which then generates the billing informatio. In this case the billing operation has manual job, as to remove the stored data and then feed to the BC.
OMC is th operations and maintenance network. Every entity requires some care and maintenance, so are GSM network elements. GSM is a huge network made up of more than 100 BTS's , several BSC's and MSC, so there will frequent failure with these elements. It will be very difficult to man all these elements, hence some remote operation is required . OMC is the network element which provides remote access to all the network elements and monitors the performance of each network element.
As seen in the previous diagram, there is one OMC system for both MSC and the radio betwork. In case where the MSC and the Radion network are brought from different suppliers, or otherwise also it can be two types of OMC,. OMC -S : which deals with the switch i.e. MSC OMC - R : which deals with the radio network
The figure shows the structure.Heres the network comprises of BTS,BSC and a remote Transcoder.As per GSM recommendations the OMC signalling for every BSC and transcoder is done over 64 kbps channel.This link is known as OML ( Operations and Maintenance Link) and it uses the X.25 Signalling protocal.All OML's from different entities are muliplexed at the MUX and the then switched over by the X.25 switch and fed to the various OMC terminals connected to the server.
OMC Fucntions can be in general dived into three categories. Alarm Monitoring : This is the prime function, which continously monitors the alarm reported by all BTS's, BSC's, MSC and other elements.It gives an audible alarm for pre-set alarm conditions. Alarms for field sites ( BTS's and BSC's ) also include environmental alarms, which are must for field located sites as these are in cabins and rooms which have supporitng accessories like power , airocns, and fire hazards , security problems, since it is not possibel to man hundreds of fields locates sites for these type sof issues, so these alarms are also reported to at the OMCR. Configuartion Changes : All the GSM network elements are highly digital systems, with enormous software control, so there is almost continous requirement of software changes in the existing systems, and also with expansion all the changes are to be done form OMC. Performance Analysis : The OMC also gives the data for network performance which is an indication of quality of service provided to your customer. OMC gives statiscal analysis of traffic like total calls made, no og outging and incoming calls , incoming calls, MS to MS, MS ot PSTN calls etc, no calls dropped , traffic loading on the loading etc. With this data the network quality ia observed, with which further network expansion and other functions could be planned effectively.
The equipment related alarms are generally total failure of the equipment,where the site is totally down but the link to the site is up. The Link Failures are the failure of connectivity between BTS to BSC or BSC to MSC etc.These generally contain some information by which it can be a diagonised that the link at which end has failed. The Module failure is generally failure of any hardware module in the equipment which could be processor,a transceiver,combiner etc. These alarms are generally generated by the equipments and send across to the OMC where the OMC catergorises these and displays it in a systematic manner. These OMC has a database where these alarms are logged and could be reviewed later for analysis.
The above graph represents a performance statistics display for TCH traffic summary report for a 12 hrs period. A1,A2,A3 are the sectorial cells for cell site A.
link and a downlink. It's interesting to note that while the TCH uses a frequency channel in both the uplink and downlink, the BCH occupies a channel in the downlink band only. The corresponding channel in the uplink is effectively left clear. This can be used by the mobile for unscheduled or random access channels (RACH). When the mobile wants to grab the attention of the base station (perhaps to make a call), it can ask for attention by using this clear frequency channel to send a RACH. Since more than one mobile may want to grab attention at the same time, colliding RACHs are possible, and mobiles may need to make repeated attempt to get heard.
GSM uses TDMA (Time Division Multiple Access) and FDMA (Frequency Division Multiple Access). The frequencies available are divided into two bands. The uplink is for mobile transmission, while the downlink is for base station transmission. The slide shows part of one of these bands. Each band is divided into 200kHz slots called ARFCN (Absolute Radio Frequency Channel Number). As well as slicing up frequency, we also slice up time. Each ARFCN is shared between 8 mobiles, each using it in turn. Each mobile uses the ARFCN for one TS (Timeslot) and then waits for its turn to come round again. Mobiles get the use of the ARFCN once per TDMA frame. The slide illustrates 4 TCH (Traffic CHannels). Each one of the TCH uses a particular ARFCN and Timeslot. Three of the TCH are on the same ARFCN, using different timeslots. The fourth TCH is on a different ARFCN. The combination of a TS number and ARFCN is called a physical channel. There's not much space between timeslots and ARFCN's. It's important for the mobile or base-station to transmit their TDMA bursts at exactly the right time and with exactly the right frequency and amplitude. Too early or too late and a burst may collide with an adjacent burst. Poorly controlled modulation spectrum or spurious will cause interference with adjacent ARFCN.
Most modern digital communications systems use some sort of voice compression. GSM is no exception. It uses a voice coder to model the tone and noise generation in the human throat and the acoustic filtering of the mouth and tongue. These characteristics are used to produce coefficients which are sent via the TCH. The speech coder is based on a residually excited linear predictive coder (RELP), this is enhanced by including a long term predictor (LTP). The LTP improves speech quality by removing the structure from vowel sounds prior to coding the residual data
Speech is captured in blocks of 20ms and coded into 260 bits, which are ordered accordingly into Type 1a - 50 bits,Type 1b - 132 bits and Type 2 - 78 bits.These 260 bits gives out a data rate of 13kbps which is the GSM speech rate. These bits are then channel coded for error correction and converted to 456 bits which yields out a data rate of 22.8kbps which is data rate on Air interface.These 456 bits/20ms is deoded down to 260 bits at the BTS which again gives the original speech rate of 13Kbps. These 260 bits speech is again bit stuffed with 60 bits by the transcoder handler which gives out a rate of 16kbps which is mapped on Abis.
The conection between BTS to BSC is on Abis interface. This is done on 2.048 physical layer which has 32 timeslots of 64kbps each. Time slot 0 is for sybchronization whereas one timeslot for signalling and remaining for speech. The speech timeslot of 64kbps each contains 4 sub channels of 16 kbps each of 4 different circuits. Each 16 kbps subchannel consists of 50 TRAU frames ,with each frame consisting of 260 + 60 bits = 320bits derived from 20ms blocks of speech. These 60 bits contains the TRAU information which has 35 bits for synchronisation between Transcoder unit and TRX, 4 bits for Time Alighnment and 21 control bits which has information about the frame like the frame is of speech,data,idle,O &M,fullrate channel and half rate channel.
In the timeslot structures we saw, there was 26 bits referred to as either a mid-amble or a training sequence. For a normal burst this mid-amble will consist of 8 base station colour codes and these are numbered 0 through 7. They are 26 bits long. Another mid-amble or training sequence is used in the random access channel (RACH) and it is 41 bits long. There is also a 64 bit long sequence that is used on the SCH or synchronisation channel. Mid-ambles are placed in the centre of the burst to minimise the time difference from to any bit in the burst. The mid-amble has a number of different uses, the most important is equalisation to improve bit error rate. The mobile knows the mid-amble it should be receiving (part of the information the MS gets when assigned to a BS). This is a pre-defined sequence is 26 bits in the case of a traffic channel. It receives the mid-amble and compares the it to what it should have been. From the difference it can estimate the impulse response of the transmission path at that instant in time. Once it knows the impulse response it can mathematically calculate an inverse filter, it can apply this filter to the data bits on each side of the mid-amble and clean them up, reducing the chance of detecting a bit wrong. This is referred to as equalisation or the equaliser within the radio. Equaliser mechanisms are a closely guarded design feature of most mobiles. It's a key area of competition between mobile manufacturers.
Since GSM is a TDMA system and there are 8 users on a frequency pair, each user must only turn his transmitter on at the allowed time and, have his transmitter off in time so that he does not interfere with other users in the adjacent timeslots. Because of this need, GSM has specified an amplitude envelope for the RF burst of the timeslots. There's also a demanding flatness specification over the active part of the useful bits in the timeslot. The amplitude envelope has greater than 70dB of dynamic range yet needs to measure less than +/-1dB flatness over the active part of the timeslot. All of this is happening over the 577µs period of a timeslot.
ft are tied together by a factor of 4, filtering can not affect the average phase relationships. The filtering does slow down the rate of change of phase velocity (the acceleration of the phase). When Gaussian filtering is applied, the phase makes slower direction changes, but may reach higher peak velocities to catch up again. Without Gaussian filtering, the phase makes instantaneous direction changes, but moves at a constant velocity. The exact phase trajectory is very tightly controlled. GSM radios need to use digital filters and I/Q or digital FM modulators to accurately generate the correct trajectory. The GSM specifications allow no more than 5 degrees rms and 20 degrees peak deviation from the ideal trajectory.
To see how information is transmitted let's look at an example. We have been assigned timeslot 2 and we're in a traffic mode, receiving and transmitting information to the base station. The downlink, on which we receive information, will be in the frequency range of 935 to 960MHz. The uplink, the frequency which the mobile will transmit information to the base station, will be in the frequency range of 890 to 915MHz. The uplink and the downlink make up a frequency pair, which for GSM900, is always separated by 45MHz. We can see that the timeslots are offset by 3 between the downlink and the uplink. We receive information in timeslot two in the downlink we have two timeslots in which to switch to the uplink frequency and be ready to transmit information. Then, we have to get ready to receive our next time slot of information in the next frame.
As the mobile moves around the cell, it's transmitter power needs to be varied. When it's close to the base station, power levels are set low to reduce the interference to other users. When the mobile is further from the base station, it's power level needs to increase to overcome the increased path loss. All GSM mobiles are able to control their output power in 2dB steps. The base station commands the mobile to a particular MS Tx Level (Power level). GSM900 mobile have a maximum power of 8W (the specifications allow 20W, but so far, no 20W mobiles exist). DCS1800 mobiles have a maximum power of 1W. Consequently DCS1800 cells need to be smaller.
Besides receiving and transmitting information, the mobile must switch frequency and get ready to receive and measure the level of the adjacent cell's broadcast channels. It then reports this (RXLev) information to its own base station in order to establish when a handover is appropriate between cells. Again, information is received on timeslot 2, we switch 45MHz to transmit information and then, need to switch back 45MHz +/- a few MHz to monitor and measure the level of the adjacent cell's broadcast channels. This information will be reported back to the base station at least every 30 seconds so that the base station can determine the appropriate time to do a handoff. The RxLev information is reported back to the base-station on the uplink SACCH (Slow Associated Control CHannel). The mobile uses a list of ARFCN in the BA (Base Allocation) table to know which BCH frequencies to go out and measure. The BA table is coded onto the BCH, and also the downlink SACCH. This is the primary (or non-hopped) mode of operation in the GSM system. If there is an area which has bad multipath, such as urban areas with lots of reflections from buildings, the cell may need to be defined as a hopping cell.
Timing advance is required in GSM because it uses TDMA with cells up to 35 km radius. Since a radio signal take a finite period of time to travel from the mobile to the base-station, there must be some way to make sure the signal arrives at the base-station at the correct time. Without timing advance, the transmitted burst from a user at the edge of a cell would arrive late and overlap (and corrupt) the signal from a user right next to the base station (unless a guard time, between timeslots, greater than the longest signal travel time was used). By advancing the timing of the mobiles, their transmissions arrive at the base station at the correct time. As a mobile (MS) moves, the Base Station (BTS) will signal the MS to reduce its timing advance as it gets closer to centre of the cell, and increase its timing advance as it away from the centre of the cell. Mobile's in idle mode (not on a call, but still camped to the network) receive and decode the BCH (Broadcast CHannel) from the base station. One element of the BCH, the SCH (Synchronisation CHannel) allows the mobile to adjust it's internal timing. When the mobile is receiving the SCH, it doesn't know how far it is from the base station. A distance of 30km will cause the mobile to set it's internal timing 100µs behind the base-station. When the mobile sends it's first RACH burst, it will leave 100µs late, after a 100µs transit delay, it will arrive 200µs late, colliding with the bursts from mobile's closer to the base station. For this reason, the RACH, and other types of access burst are shorter than normal. The mobile only sends normal length bursts once it's received timing advance information from the base-station. The mobile in our example would need to advance it's timing by 200µs. We'll see later how the base station commands the mobile to change it's timing advance or transmitter power using the SACCH (Slow Associated Control CHannel)
In GSM , concept of channel needs to be understood very thoroughly. There are several types of channesls defined in GSM . If the context is not clear, it could be vry confusing . To introduce you to the concept of time shared channels , we will use an analogy of a Transport Vehicle. Though the vehicle is a common resource, we use it for various purposes in a day on the basis of time sharing. At times of emergency , we allocate a high priority task to the vehicle to achieve the needs of the hour. . In a similar way several channels in GSM are carried by a few common carriers. The timelines for the usage are very accurately defined. A vehicle that is brand new and is quite strong, is used for moving around in the city. If you put your companies name on the front , people in a city would idntify the vehicle with the company.. This is similar to BCH that carries Network Identity and Base station a Identity. Similarly , in GSM , RF Channels and Time slots define total number of available channels . What information is to be carried on these channels and what time periods is defined by GSM stsandards. There is enough flexibility in the standard to allow operators to configure the channels based upon their current needs. To use the analogy again, If the company is small , you may have just a couple of such vehicles. ( Two channels ) . When company grows bigger, you may dedicate one or more vehicles just to carry employees and drop ( Dedicated channels for TCH ).
eeling flags allow the TCH and FACCH to be distinguished. The remainder of the burst carries data (speech for example) and tail/guard bits to fill the gaps between bursts. It's easy to get confused about the number of bits in a timeslot, are there 148 bits in a timeslot or 147 bits in a timeslot? There are 148 ACTIVE bits in a timeslot, consisting of the mid-amble, the control bits, the data and the tail-bits. There are 147 USEFUL bits from the middle of the first bit to the middle of the last. Effectively 1/2 a bit off each end is lost.
The concept of a BCH is very simple, but the details can get a little complicated. In simple terms, the BCH acts like a beacon, or lighthouse. It's on all the time and is the first thing the mobile looks for when it's trying to find service. The BCH ARFCN has to be active in all timeslots to allow mobiles synchronised to other cells to measure it's power. The useful BCH information is always carried in timeslot 0. The other timeslots are filled with dummy bursts, or are available for TCH. There are a number of interesting parts to the BCH: The FCH (Frequency correction CHannel) uses a special burst which repeats on the BCH, it has a special fixed bit sequence to allow the mobile to tune it's internal frequency reference when it first turns on. The SCH (Synchronisation CHannel) has a burst with extended midamble. It's used by the mobile after the FCH to adjust it's internal timing and get synchronised to the multiframe sequence. The BCCH (Broadcast Control CHannel) has information encoded on it which identifies the network. It also carries lists of the channels in use in the cell (BA and CA tables)
The CCCH (Common Control CHannel) is like a message board. Just like the FCH, SCH and BCCH, it can be received by any mobile. Sub-channels like PCH (Paging CHannel) are posted on the CCCH. When the mobile sees its number on the PCH it recognises that it should respond by requesting service with a RACH. Another CCCH sub-channel is the AGCH (Access Grant CHannel). Once a mobile has sent a RACH, the base station responds by putting an AGCH on the CCCH, bearing the mobiles random number (read from the RACH). The AGCH instructs the mobile to go to an SDCCH or TCH.
When the mobile has become synchronised to the frequency and frame timing of the cell, and looked at the other information on the BCH it is ready to make and receive calls. Once the mobile is in this state it is 'camped' to the base station. If the mobile is near the base station their timing will be closely aligned. If the mobile is on the edge of the cell, maybe 30km from the base station, the SCH will have a propagation delay of 100µs. The mobile's timing will be 100µs in error. When the mobile sends out a RACH, to start a call, the RACH is transmitted 100µs late, with another 100µs transit time to the base station, it arrives 200µs late. To avoid collisions with bursts in adjacent TS, RACH busts are shorter than normal. The RACH is not the only type of short access burst. When a mobile is handed over to another cell, there will be a short period of time before it receives timing advance information on the downlink SACCH from the new cell. During this period, there's a risk of the mobiles bursts colliding with bursts in the new cell. Until it gets timing advance information from the new cell it sends short access bursts.
The SDCCH is sometimes configured as a logical channel on the BCH, and sometimes on it's own physical channel. The SDCCH has a different multiframe structure to the TCH. SDCCH bursts repeat less frequently than once per frame. For this reason, more than 8 SDCCH can share a physical channel. As a consequence, the data rate on the SDCCH is lower than on the TCH. The SDCCH is used like a stepping stone. During the call set-up process, there can be a lot of time between the mobile sending a RACH and getting service, to the start of conversation. Time is taken up while the phone is ringing and waiting to be answered. During this period, there's a need to exchange control information between the mobile and base station. Alerting messages are sent, and authentication takes place, but there's no need to send speech information. The SDCCH, by using less of the cells resource of physical channels, improves efficiency, and provides a useful holding channel for the mobile until speech data needs to be exchanged. Just like the TCH, the SDCCH has an SACCH associated with it.
One of the two spare frames every 12 TCH frames is used for the SACCH (Slow Associated Control Channel). On the down-link, the SACCH is used to send slowly but regularly changing control information to the mobile. Examples are instructing the mobile to change its transmitter power (MS TX Lev) and burst timing advance (to compensate for RF transit time) as it moves around the cell. It also carries the BA and CA tables. The up-link SACCH carries information about received signal strength (RXLev) and quality (RXQual) of the TCH and the adjacent cell BCH measurement results (also RXLev).
When the SACCH reports coming back to the base station indicate that another cell would offer the mobile better signal quality, a handover is necessary. The SACCH just doesn't have the bandwidth to transfer all the information associated with a handover (like the new ARFCN and timeslot, or the MA table). For a short period of time, the TCH is replaced by an FACCH. The FACCH uses consecutive bursts, so has a much higher data rate that the SACCH, which uses only one burst in 26. The frame stealing flags (the control bits on either side of the midamble) are set to indicate that the data being sent is an FACCH, not the TCH. In other respects, the FACCH looks just the same as the TCH. It uses the same physical channel (ARFCN and timeslot). When the FACCH steals bursts from the TCH, speech data is lost. It's often possible to hear a small speech drop-out when handovers take place.
1 TDMA burst = 577 usec 1 TDMA Frame = 8 bursts = 4.616 ms 1 TCH Multiframe = 26 TDMA frames = 120 ms 1 CCH Multiframe = 51 TDMA frames = 234.6 ms 1 Superframe is over when the TCH and the CCH Frames are alighned. This can only happen after 26 CCH Muliframes(51) or after 51 TCH Multiframes ( 26 ). This means that 1 Superframe = 51 x 26 = 1326 TDMA Frmames = 6.12 sec 1 Hyperframe = 2048 Superframes = 2715648 TDMA frames = 3hrs 28 min 53 sec 760ms The above derivations show that TDMA frames are numbered from 0 to 2715647 . Confused !! What is the significance for this complex frame structure. As we proceed with our further modules ( Advanced GSM ) these concepts will become clear. For the time being, Multiframe are used for distrubution of logical channels. Superframe is used for Mobile synchrnization and Hyperframe is used for Signalling procedures and Ciphering.
BSS calculates the delay from the access burst. Access burst termed as RACH is the GSM burst of 577usec having 88bits of information anf 68.25 bit periods as guard period. This guard period is used
All mobiles must have the capability of hopping. However, not all cells will be hopping cells. Only those cells which have bad multipath problems will be defined as hopping cells. In this example, there are three frequencies pairs to hop among. The mobile still needs to go out and measure the adjacent cells' broadcast channel (BCH). In the first frame, the mobile receives information on channel 1 downlink, then switches to the uplink for channel 1 (45MHz away), transmits it's information, and finally monitors one of the adjacent cells to measure its level. The mobile must move to the downlink for channel 2 and receive information in timeslot 2, switch 45MHz, and transmit on the uplink for channel 2. Then it monitors another cell's broadcast channel and measures its level. This continues through the sequence of frequencies that have been assigned to the cell. The hopping sequence is defined by the CA (Cell Allocation) and MA (Mobile Allocation) tables. The CA table is a master list of all the hop frequencies available in a particular cell. It's sent to the mobile on the BCH and also the downlink SACCH. The MA table is an index into the CA table, and gives a hopping sequence for a particular mobile. The MA table is sent to the mobile as part of the handover or channels assignment process.
Location Area is the area covered by one or more BTS' s where a mobile can move freely without updating the system. One Location Area can be covered by more than one BSC,but only by one MSC.
By dividing a PLMN area into various location areas,paging load reduce since whenever there is page for a mobile it is send by the MSC to those BSC's which have cells with location areas same as that of the current location area of the Mobile. If entire PLMN area is termed as one location area then the MSC has to page for every mobile in every BSC which increases Paging load and as a result signaling load and hence wastage of resources.
Once a mobile turns on power ,it camps to the BCCH .On the BCCH is broadcasted the Location area identity (LAI) of the cell to in which the mobile is.Mobile reads this LAI and compares this LAI with the LAI which it has store d when it switched off.If this new Lai is different form the one stored,it does a normal location update.
The Mobile many a times may enter a non-coverage zone for a very long period.The MSC has no information about it .The MSC goes on sending Paging messages for the Mobile. In order to avoid this a period is set after which the Mobile has to inform MSC that it is still in the attach state.Now,if the mobile remains in the non-coverage zone for this period,it will not be able to send any message,.As a result of this after the expiry of this period the MSC marks this mobile as detached and rejects all incoming calls to this mobile. This type of update which is done everytime after an expiry of a fixed period is known as Periodic Location Update. Period set ranges from 0 to 255 decihours
TMSI : Temporary Mobile Subscriber Identitiy
Handover is a GSM feature by which the control and communication of a mobile is transferred from one cell to another if certain criteria's are met.
A mobile on TCH is evaluating the RXLEV & RXQUAL and passing the measurement taken to the cell on the SACCH. The BSS interprets the measurements and determines what actions to be taken to maintain th e link, whether to increase /decrease power output or go for a handover. BSS continously processes the following information : Measurements reported by the Mobile on SACCH ---- RXLEV on Downlink ---- RXQUAL on Downlink ---- Neighbouring Cells RXLEV on Downlink Measurement performed by the BSS ---- RXLEV on Uplink ---- RXQUAL on Uplink ---- Mobile to BSS Distance ---- Interference levels On every SACCH frame ( 480ms) the BSS performs an averaging function on the measurements collected regarding the radio links supported by that cell plus measurements regarding neighbor cells. These averaged values are then processed by the decision algorithms and compared against thresholds to determine whether any action is necessary ( power control or handover ). The period between averaging and decision is user settable with reference to SACCH frame periods.
Cell barring is a GSM Feature by which certain Mobiles could be barred access to certain cells.
Every Mobile has an access class. This access is programmed on the SIM card. These classes range from 0 - 15,in which Classes 0 - 9 are termed as normal class Classes 11 - 15 are emergency classes. Every Cell has a set parameter which defines which access classes are barred for the particular cell.This parameter is broadcasted on the BCCH.
During any conversation it is found that generally one person speaks at a time,that is both users talk alternately. So on an average it is found that transmission in one direction is only for the half the time. DTX is a feature by which the transmitter of the mobile is switched on only for useful information frames.
Voice Activity Detector determines which 20ms blocks contains speech and it only forwards those frames.For case when the mobile is in car,there is background noise of the car which almost stationary.The VAD detects this constant noise and removes it. VAD is an energy detector.It compares the energy of the filtered speech to a threshold and indicates speech whenever the threshold is exceeded.This threshold must be kept above the noise level so as to avoid noise as speech,but it should not be very high so that some speech is lost in the process of avoiding noise.
During any voice transmission there is a background noise which is transmitted with the speech.This noise when received at the other end gives a comfortable feeling to the listener about the progress of the call. With DTX as the speech burst ends,transmission is cut off and with this the background noise also cuts off which gives an annoying feeling to the listener.To resolve this problem the VAD adds an artificial noise known as comfort noise at the receiving end when speech discontinues.
Short Message Service is a broadcast of message to a particular mobile subsrciber. The short message to a mobile is originated at the Short Message Service Centre, and sent to the MSC with the corresponding MSISDN number. The MSC then checks with VLR about the mobile state,whether it is attached or not.If attached the MSC orders the BSS to page the mobile and the message is received by the mobile.
ObjectivesObjectivesUnderstand Concept of CellularUnderstand the functional blocks in a GSM NetworkUnderstand the Air Interface with ChannelsUnderstand Mobile Call Setup OperationsBasics
AgendaNeed for CellularIntroduction to CellularChannelization ConceptGSM Evolution and MarketsGSM Network ArhcitecureGSM Air Interface ParametersGSM Speech CodingPhysical and Logical ChannelsFrames and MultiFramesGSM Basic Operations
TelephonyCommunication is todays basic needTelephone has become part of life
Fixed Line TelephonesNo MobilityDelay in New ConnectionsSecurity HazardsProne to Failures ( Line Disconnection, etc)Very less value added services
Mobile CommunicationsDrawbacks of Fixed Phones have triggeredwireless communications"Call People , Not Places ""Call People , Not Places "
MCG Spokane Division RTEd Sem, 1000-1268 9/93Company ConfidentialHFirst Wireless Signal3Postcard: Chicago Daily NewsWireless Access Methods
Authoring Division Name File NameSecurity Notice (if required)HWireless Communication ModelSourceDisplayTransmitterReceiverMedia of transmission is Radio Frequency
Authoring Division Name File NameSecurity Notice (if required)HTypes of Wireless CommunicationsSimplexThe direction of transmission is in one direction onlyBEEPEx : Broadcast Services ( AM/FM Radios, Television )Paging Services
Authoring Division Name File NameSecurity Notice (if required)HTypes of Wireless CommunicationsHalf - DuplexThe direction of transmission is alternate in both directionsTxRxTxRx"A""B"A transmits -- B receives, thenB transmits -- A receivesEx : PTT Handsets, Trunked Radios
Authoring Division Name File NameSecurity Notice (if required)HTypes of Wireless CommunicationDuplexThe direction of transmission is simultaneous in both directionsEx : Cordless Telephones, Mobile Phones, Microwave RadiosIs there separate frequency of transmission at both ends ?
Authoring Division Name File NameSecurity Notice (if required)HThe Electromagnetic SpectrumMICROWAVESAMBROADCASTRADIOSHORT-WAVERADIOMOBILERADIOVHFTVFMBROADCASTRADIOMOBILERADIOVHFTVFIBEROPTICSCOMMUNICATIONVISIBLELIGHT1MHz10MHz100MHz1GHz 10GHz100GHz101210141015MFHFVHFUHFSHF EHF1µm1000m100m10m1m10cm1cm 1mmWireless Communication !!! At what frequency ?
Authoring Division Name File NameSecurity Notice (if required)HSelection of Band for WirelessCommunicationMF : 300 KHz - 3 MHz ( Domestic Radios )HF : 3 MHz - 30 MHzVHF : 30 MHz - 300 MHz ( FM, Paging, PTT )UHF : 300 MHz - 3 GHz ( Mobile Radios, Cordless Phones )SHF : 3 GHz - 30 GHz ( Microwave Band )Properties of Radio FrequenciesLow Frequency - Less Loss ,,, High Frequency - More LossTo overcome loss , more power required,More Power - Health hazardsLow Frequency - High Beamwidth - Wider and Deeper CoverageSelection of Band will depend on ApplicationFrequency Band is subdivided into Application BandApplication Band is further divided into Technology Channels
Authoring Division Name File NameSecurity Notice (if required)HChannelizationFrequency Band has several application segmentsCertain blocks of the Band are reserved for certain applications by regulatingauthorities and as well by standard TechnologiesTechnologies have decided on fixed bandwidth Channels within these bandsExample : Mobile CommunicationsAMPS / DAMPS : 824 MHz -- 894 MHz ( 50 MHz + 20 MHz separation)GSM : 890 MHz -- 960 MHz -- ( 50 MHz + 20 MHz separation)These 50 Mhz are bands are further divided into ChannelsChannelization can be done by three methods--- FDMA--- TDMA--- CDMA
Authoring Division Name File NameSecurity Notice (if required)HChannelization MethodsFDMA PowerFrequencyTimeFDMAEx: AMPS / DAMPS band is divided into 30 Khz Channels ( 1666 Freq Chs)GSM band is divided into 200 Khz Channels ( 250 Freq Chs ).Television Channels ( Star, Zee, Sony, MTV, BBC, CNN etc. )
Authoring Division Name File NameSecurity Notice (if required)HChannelization MethodsFrequencyPower TimeFDMA/TDMATDMAEach FDMA Channel is divided into TimeslotsEach Timeslot is of fixed periodThis method increases the number of Channels in a systemEx: DAMPS has 3 timeslots on each 30 Khz Channel ( 4998 Channels)GSM has 8 timeslots on each 200 Khz Channel ( 2000 Channels )
Authoring Division Name File NameSecurity Notice (if required)HChannelization MethodsCDMAFrequencyCDMAPowerTimeFrequency Channel is divided into Code Channels1.25 MHz of FDMA Channel is divided into 64 Code Channels
Authoring Division Name File NameSecurity Notice (if required)HMobile Telephony -- ChannelizationMobile Telephony needs Duplex CommunicationHow many Channels will be required for one call ?What type of Channels ?-- FDMA, TDMA, FDMA/TDMA , FDMA/CDMA or somethingelse
FrequencyReuseGSM uses concept of cellsOne cell covers small part of networkNetwork has many cellsFrequency used in one cell can be usedin another cellsThis is known as Frequency Re-useF=1F=2F=3F=4,8F=5,9F=6,10F=7F=1F=2F=3F=4,8F=5,9F=6,10F=7F=1F=2F=3F=4,8F=5,9F=6,10F=7F= 1,2,3,4,5,6,7,8,9,10ClustersCo-Channel ( Re-use ) Cells
Distance between two cellsusing same frequencies1,13,29DD=R x SQRT( 3 x N )N= Cluster size ( 7 in this case )R=Radius of one cellD=Dist between two cells using same channels1,13,291,13,291,13,29D R
Authoring Division Name File NameSecurity Notice (if required)HGLOBAL System for MobilesGSM900239 licenses in 109 countries now44 million subscribers nowOne New subscriber Every Second !> 200 million subscribers by Year 2000
Authoring Division Name File NameSecurity Notice (if required)HPersonal CommunicationsNetworksUSA: PCS1900for new PCSUK: DCS1800Mercury One-2-OneMicrotel-OrangeGermany:DCS1800E NetFrance: DCS1800For new PCN Australia: DCS1800for new PCNThailand:DCS1800AISDCS1800PCS1900Hong Kong:DCS1800for 6 New PCNs
Authoring Division Name File NameSecurity Notice (if required)HEvolution of GSMGSM RequirementsGood subjective speech qualityEncryption of user informationMust operate in the entire 890 - 960 Mhz frequency bandSpectral efficiencySupport for international roamingMinimize modifications to the existing fixed public networksLow handsets and service costISDN compatibilitySupport for range of new services and facilities
Authoring Division Name File NameSecurity Notice (if required)HThe Global Standard• Full Urban coverage in WesternEurope• WW Roaming available• Subscribers can utilize multiplenetworks• First dualband networksappearing in Europe and Asia• Full Urban coverage in WesternEurope• WW Roaming available• Subscribers can utilize multiplenetworks• First dualband networksappearing in Europe and Asia• Full Urban coverage in WesternEurope• WW Roaming available• Subscribers can utilize multiplenetworks• First dualband networksappearing in Europe and Asia
Authoring Division Name File NameSecurity Notice (if required)HOther GSM standardsGSM 900 and DCS 1800 use the same standardsDCS 1800 specs are defined as a delta standard to GSM specsSame GSM switches can be used for DCS 1800Some software upgrading may be required ( if RR are manged by switch )Dual Mode handsets will be required to support bothPCS 1900DCS 1800ETSI has assisted ANSI T1 andTIA TR-46 committees to formulatespecs for PCS 1900.
Authoring Division Name File NameSecurity Notice (if required)HEvolution of GSM1982 : Group Special Mobile formed within CEPT1986 : A permenent Nucleus formed1987 : Radio transmission Techniques are chosen.Field trialscompleted1987 : GSM becomes ETSI technical committee1987 : 13 Operators sign a memorandum of uderstanding1989 : Prototype ( validation ) systems are on the air1990 : GSM Phase I specifications are finalised1991 : UK,France,Germany andItaly introduce GSM services1992 : Motorola cuts over the first commercial system built forCOMVIQ on Sept 11994 : GSM Phase 2 specifications released1996 : GSM Phase 2+ specs are now definedGSM Phase I specification document has 5230 pages !!!• Full Urban coverage in WesteEurope• WWRoaming available• Subscribers can utilize multipnetworks• First dualband networksappearing in Europe and AsiaThe History
Authoring Division Name File NameSecurity Notice (if required)HGrowth• Full Urban coverage in WesternEurope• WW Roaming available• Subscribers can utilize multiplenetworks• First dualband networksappearing in Europe and Asia• Full Urban coverage in WesternEurope• WW Roaming available• Subscribers can utilize multiplenetworks• First dualband networksappearing in Europe and Asia• Full Urban coverage in WesternEurope• WW Roaming available• Subscribers can utilize multiplenetworks• First dualband networksappearing in Europe and Asia
Authoring Division Name File NameSecurity Notice (if required)HTechnology Evolution• Full Urban coverage in WesternEurope• WW Roaming available• Subscribers can utilize multiplenetworks• First dualband networksappearing in Europe and Asia• Ful l Ur ban cover age i n Wester nEur ope• WW Roami ng avai l abl e• Subscr i ber s can uti l i ze mul t i pl enet w or k s• Fi r st dual band netw or k sappear i ng i n Eur ope and Asi a
Authoring Division Name File NameSecurity Notice (if required)HGSM 2G+• HSCSD - High Speed Circuit Switched Data– Multiple TDMA timeslots allocated for data transmission– Bandwidth on demand, up to 64kBit/s– Compatible with existing GSM network infrastructure• HSCSD - High Speed Circuit Switched Data– Multiple TDMA timeslots allocated for data transmission– Bandwidth on demand, up to 64kBit/s– Compatible with existing GSM network infrastructure• HSCSD - High Speed Circuit Switched Data– Multiple TDMA timeslots allocated for data transmission– Bandwidth on demand, up to 64kBit/s– Compatible with existing GSM network infrastructure
Authoring Division Name File NameSecurity Notice (if required)HIMT-2000 / 3G+(International Mobile Telecommunications -2000)• HSCSD-HighSpeedCircuitSwitchedData–MultipleTDMAtimeslotsallocatedfordatatransmission–Bandwidthondemand,upto64kBit/s–CompatiblewithexistingGSMnetworkinfrastructure• HSCSD-HighSpeedCircuitSwitchedData– MultipleTDMAtimeslotsallocatedfordatatransmission– Bandwidthondemand,upto64kBit/s– CompatiblewithexistingGSMnetworkinfrastructure
Authoring Division Name File NameSecurity Notice (if required)HGSM Standards structure01 Series : General02 Series : Service aspects03 Series : Network aspects04 Series : MS-BS interface andprotocols ( air interface layer 2 & 3)05 Series : Physical layer on the Radiopath ( air interface layer 1)06 Series : Speech coding specs.
Authoring Division Name File NameSecurity Notice (if required)HGSM STANDARDS - Continued07 Series : Terminal adaptation formobile stations08 Series : BSS - MSC interfaces ( A &Abis)09 Series : Network interworking10 Series : Empty - For future use11 Series : Equipment and Type approvalspecifications12 Series : Operation & Maintenance
BTSBTSBTSMSMSEIRAUCHLRVLRMS - Mobile StationMobile station provides user access to GSM network for Voice & Data.All GSM mobiles comply to the GSM standards.Subscriber data is read from a SIM card that plugs into MS.Each MS has a unique number called as IMEI number, which is stored in EIR for authenticationpurposes.Mobile camps on to the GSM network through a BTS serving the cell.Mobile also scans neighbouring cells and reports signal strength.Network knows whereabouts of mobiles from HLR & VLR databases.Mobile Transmit and Receive voice at 13 KB/s over air interface.SIM
H GSM CourseMobile Station Output PowerCLASS 1 20 watts Vehicle and PortableCLASS 2 8 watts Portable and VehicleCLASS 3 5 watts Hand-heldCLASS 4 2 watts Hand-heldCLASS 5 0.8 watts Hand-heldOutput Power determines:---- Accessibility in areas of coverage---- Talk time and Standby TimeOutput Power on call is varied as commanded by BTSMS - Mobile Station
H GSM CourseMobile Station IdentitiesMSISDN : Human Identity used to call a Mobile StationCC NDC SN98 XXX 12345IMEI: Serial number unique to every Mobile StationTAC FAC SNR SP6 digits 2 digits 6 digits 1digit
H GSM CourseSIM - Subscriber Identity ModuleGSMRemovable Module inserted when the subscriber wants to use the MSIMSI : Network Identity unique to a SIMMCC MNC MSIN404 XX 123453 digits 2 digits 10 digitsTwo SizesCredit CardStamp Size4-8 digits PIN code3 false entries - blocks8 digit PUK10 false entries - disabledMSROM = 6kb to 16kbRAM = 128 byte to 256 byteEEPROM = 3 kb to 8 kb
H GSM CourseSIM - Subscriber Identity ModuleContents of SIMSerial NumberIMSI, Subscriber Key ( Ki )Algorithms for Authentication, CipheringNetwork CodePIN, PUKCharging InformationAbbreviated DiallingSupplementary Features ( e.g. call barring )SIM features and contents are personalized by the Service ActivatorMS also stores some temporary data on SIM during operation
H GSM CourseBase Station Cell Site ArchitectureMainsPowerPanelDCPowerSupplyUnit BatteryBackupAbisBTSDMRGSM Antenna SystemBTS Cabin/Shelter/RoomAirCon
H GSM CourseBTS - Base Transceiver StationBTSMSBSC MSCCELLBTS has a set of Transceivers to communicate with mobiles in its areaOne BTS covers one or more than one cellThe capacity of a cell depends upon number of tranceivers in a cell.BTS is connected to the BSC through Abis Interface, which is a 2Mb/sBTS transmit and receive voice at 13 kbps over air interface to the mobiles.BTS commands mobiles to set Tx. power, timing advance and HandoversRF ChannelsAbis - 2 MBits/s
H GSM CourseBTS Architecture 1BSCTxRx A Rx BSplitterProcessor ModulesTSBP PPCMInterfaceAlarmInterfaceA1TRX TRX1 2B1A2B2TRAUBTSCOMRFUBPFFrRef
H GSM CourseBTS Architecture 2TRX LogicTRX RFPowerAmplifierTRX UnitPSU & Climate ControlAntennaABISover G703BackplaneBTS Cabinet. 3 Channel. Single SectorI QRFI QCombiner / Distribution UnitRx FilterLNASplitterCombinerDuplexerCouplerCouplerInterfaceFrequency ReferenceUnitCPUBTSAlarmsNEMProprietaryInterface
H GSM CourseBSC - Base Station ControllerSeveral BTSs are connected to one BSCBSC manages channel allocation,handovers and release of channels at connected BTSsBSC connects to each BTS on an Abis interface & to the MSC on A interfaceBSC has the entire database for all cell parameters associated with the BTSs.MSCAbisA
H GSM CourseBase Station ControllerBTSProcessor ModulesPCMPCMPCMTSBP PTMGTRAUMSCBSC
H GSM CourseAbis InterfaceE1 / T1Abis is a G.703 interface. It could be E1 or T1Abis carrries Traffic information of all the mobiles in the cells controlled bythe BTS.Abis also carriers signalling information between BTS and BSCSignaling over Abis is done by LAPD protocolsLAPD has several modes of implementation--- LAPD--- LAPD Concentrated--- LAPD Multiplexed
H GSM CourseAbis InterfaceLAPD ModesLAPDSignaling for each TRX is on a dedicated 64 Kbps circuitMaximum Signalling for 10 Transceivers on 1 E1 link64 kbps 0 Sync64 kbps 1 TRX Signaling64 kbps 2 4 Traffic Channels64 kbps 3 4 Traffic Channels64 kbps 4 TRX Signaling64 kbps 5 4 Traffic Channels64 kbps 6 4 Traffic Channels64 kbps 7 TRX Signaling64 kbps 8 4 Traffic Channels64 kbps 9 4 Traffic Channels} 1 TRX} 1 TRX} 1 TRX
H GSM CourseAbis InterfaceLAPD ModesLAPD Concentrated mode 1Signaling for 4 TRXs is on a dedicated 64 Kbps ciruitMaximum Signalling for 13 Transceivers on 1 E1 link64 kbps 0 Sync64 kbps 1 4 x TRX Signaling64 kbps 2 4 Traffic Channels64 kbps 3 4 Traffic Channels64 kbps 4 4 Traffic Channels64 kbps 5 4 Traffic Channels64 kbps 6 4 Traffic Channels64 kbps 7 4 Traffic Channels64 kbps 8 4 Traffic Channels64 kbps 9 4 Traffic Channels64 kbps 10 4 x TRX Signaling} 1 TRX} 1 TRX} 1 TRX} 1 TRX
H GSM CourseLAPD ModesLAPD Concentrated mode 2Signaling for All TRXs is on a dedicated 64 Kbps ciruitMaximum Signalling for 15 Transceivers on 1 E1 link64 kbps 0 Sync64 kbps 1 ALL TRX Signaling64 kbps 2 4 Traffic Channels64 kbps 3 4 Traffic Channels64 kbps 4 4 Traffic Channels64 kbps 5 4 Traffic Channels64 kbps 6 4 Traffic Channels64 kbps 7 4 Traffic Channels64 kbps 8 4 Traffic Channels64 kbps 9 4 Traffic Channels64 kbps 10 4 Traffic Channels} 1 TRX} 1 TRX} 1 TRX} 1 TRXAbis Interface
TRAU - Transcoder / Rate Adaptation UnitThe MSC is based on ISDN switching. The Fixed Network is also ISDN basedISDN has speech rate of 64kbps. Mobile communicates at 13 KbpsTRAU converts the data rates between 13 KB/s GSM rate to 64 Kbits /s Standard ISDN rateTRAU can be colocated with the BTS,BSC or MSC or it can be a separate unit.MSMSRF ChannelsBTS BSCMSCTRAUAbisinterface2 MBits/sAinterface2 MBits/s13 KBits/secPSTN16 KBits/sec 16 KBits/sec 64 KBits/secVOICE
H GSM CourseLOCATION OF TRANSCODERColocated with MSC,BSC,BTSSeparate Unit16 kbps64 kbpsBSCTranscoderMSC
H GSM CourseMSC - Mobile Switching CentreExchange where calls are established,maintained and released.Database for all subcribers and their associated features.Communicates with BSCs on MS side and with PSTN on fixed line side.MSC is weighted on the number of subcribers it can supportBSCsBTSsHLRVLRMSC
H GSM CourseMSC - Mobile Switching CentreMultiple MSCsBSCsBSCsMSCMSCGMSCMore subscribers ? More MSCs !
H GSM CourseHLR - Home Location RegisterHLRMSC has all subscriber database stored in HLRHLR has all permanent subscriber databaseMSC communicates with HLR to get data for subscribers on callHLR will have the series of all subscriber numbers, which may not be activated or issued .
H GSM CourseVLR - Visiting Location RegisterVLRHLRA subscribtion when activated is registered in VLRVLR has all the subscriber nos which are activatedVLR also has temporary database of all activated subscribers ( on/off, location )MSC communicates with HLR for susbcribers coming from different MSCsand if found valid, then registers them in its VLR
H GSM CourseAUC - Authentication CentreHLR AUCMSCMSAuthentification is a process by which a SIM is verifiedSecret data and the verification process alogorithm are stored at AUCAUC is the element which carries out the verification of SIMAUC is associated with the HLR
H GSM CourseEIR : Equipment Identity RegisterEIR is the Mobile Equipment Database which has a series of IMEIsMSC asks the Mobile to send its IMEIMSC then checks the validity of IMEI with the EIRAll IMEI are stored in EIR with relevant classficationsEIRMSCClassifications of IMEI( Mobile Stations )White List Grey ListBlack List
H GSM CourseBC - Billing CentreBCBC Generates the Billing Statement for each SubscriberBC may be directly connected to the MSCMSC sents the billing information ( duration of call ) to BCBC then produces the billing amount based on the units set
H GSM CourseOMC - Operations & Maintenance CentreIt is central monitoring and remote maintenance centre for all network elementsOMC has links to BSCs and MSCOMC TerminalsBTSsBTSsBTSsOMC SystemBSCsMSCasasasaaaaaaqwtttsssdfaaqwrqrncnceasasasasaaaaaaqwtttsssdfaaqwrqrncnceasasasasaaaaaaqwtttsssdfaaqwrqrncnceas
H GSM CourseOMC - Operations & Maintenance CentreBTSsBTSsOMC SystemBSCsMSCasasasaaaaaaqwtttsssdfaaqwrqrncnceasasasasaaaaaaqwtttsssdfaaqwrqrncnceasOMC - ROMC - S
H GSM CourseOMC - FunctionsE nvironm ental Alarm sP ow er , F ire, S ecurity,Aircons, etc.E quipm ent F ailure Alarm sB TS , B S C site F ailuresM S C and pheripheral failuresA larm M on ito ringAdd new hardw areM odify control param etersother softw are changes.C on figuratio n C hang esC ell Traffic AnalysisN o of calls, o/g ,i/cP S TN , C all drops etc.P erfo rm ance A nalysisO M C Fu nctions
H GSM CourseEquipment AlarmsBTS , BSC , Transcoder FailuresLink FailuresModule Failures ( Transceiver,Processors)NetworkBTSATransceiver 1 Fail17:35hrs Site ATransceiver 1 Fail
H GSM CourseCell Traffic012345678A1 A2 A3 B1 B2 B3 C1 C2 C3 D1 D2 D3cell namesTCH TRAFFIC IN ERLANGS FROM 09:00 to 21:00hrs1
H GSM CourseSMSC - Short Message Service CentreMessage is sent to a particular mobile.Message transfer takes place through SMSCMessages are be sent through a Manual Terminal connected to SMSCSMSCMSC
H GSM CourseVMSC : Voice Mail Service centreVMSCMSCIt has a database for all Voice Mail SubscribersIt also stores all the Voice Mail - Voice Messages
A GSM CellBroadcastCHannelTrafficCHannelUPLINK890-915 MHzDOWNLINK935-960 MHzBTSAbisInterfaceTo BSCBCHTCH
Separate Bands for Uplink and DownlinkDown link : 935 - 960 MHz ( E-GSM 925 - 960 MHz )Uplink : 890 - 915 MHz( E-GSM 880 - 915 MHz)TDMA and FDMA Multiplex–124 Frequency Channels (ARFCN) for GSM900– 1 to 124 for current band– 975 to 1023 for E-GSM–200kHz Channels–8 Mobiles share ARFCN by TDMA0.3 GMSK Modulation–270.833 kbits/sec. rateGSM Air Interface
TDMA and FDMA1 2 3456734 5 67012TimeFrequencyAmplitudeARFCNTimeslotPhysical Channel is anARFCN and Timeslot
The GSM BurstGuardPeriodTimeFrequencyAmplitudeMidamble8.25bits357 bits126bits157 bits3DataTailbitsDataControlbitTailbitControlbit
RPE and LTP Coder ( Regular PulseExcited - Long Term Prediction )RPE-LTP is a combination of RELPand MPE-LTP codecs.Coverts Speech to Low Data Rate20ms Speech makes 260 BitsOutput 13 kbit/s20 ms BlocksSpeech CoderBits Ordered260 Bits260 Bits132 78ImportantBitsOtherBits50Very ImportantBitsSpeech Coder - Defined under GSM TS 6.10
Block Code132 7850Type Ia Type Ib Type IIRe-orderingHalf rate convolutional codeCRC50 132 783Type Ia Type Ib Type II378 78Type II25 25 78466 663TailCRC Type Ib Type IaType Ia Type Ib Type II262 Bits in456 Bits O456Bits from 20ms of SpeechError Correction
Diagonal Interleaving57 57 57 57 57 57 57 57 57 57 57 57 57 57 57 57 57456Bits from 20ms of Speech456 Bits from 20ms of Speech57 57 57 57 57 57 57 57 57 57 57 57 57 57Traffic Channel (TCH) Bursts Carry Two 57 Bit Blocks (114) Each120ms of Speech = 456x6 = 2736 bits2736 / 114 = 24 bursts i.e. 24 frames (mobile Tx once per frame)Multiframe has 26 frames in 120msThere are 2 spare frames ......One SACCH, One IdleTCH
Convolutional Coding &InterleavingHELLO FOLKSHHEELLLLOO FFOOLLKKSSELSOLHLOFK LEOLSHOLKFEL SOL HLOFK LEOLSHOLKFHHEELLL -OO FFO -LLKK -SHELLO FOLKSBits to be Txed:ConvolutionallyEncoded:Interleaved:Bits Rxed:De-Interleaved:Viterbi Decoded:ConvolutionalEncoderInterleaverDe-InterleaverDecoderHello.....Example:
TRAU Frame260 bits info + 60 TRAU bits = 320 bits/ 20ms = TRAU Frame60 bits contains Frame Information data which indicatesspeech,data,idle,O & M , full-rate/half-rate.16 KbpsT =T T T TSynch SignT0 T1 T2 T3 T30 T31Abis60 bits = 35 synchronization + 21control + 4 timing
8 Midamble Patterns (Colour Codes) of 26 bitsRACH and SCH have Longer 41 and 64 bit MidamblesEqualizer Estimates Channel Impulse Response FromMidambleMathematically Construct Inverse FilterUses Inverse to Decode Data BitsMidamble or Training BitsTimeslot(normal burst)Midamble8.25bits357bits126bits157bits3GuardPeriodDataTailbits DataControlbitTailbitControlbit
Uplink Lags Downlink by 3 Timeslot periodsUplink and Downlink use same Timeslot NumberUplink and Downlink use same Channel Number (ARFCNUplink and Downlink use different bands (45MHz apart fGSM900)Downlink and Uplink0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 0 1250 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 0 1DownlinkUplink45 MHz
Measurements by MS and BTSUplink RXLEV ( - 48 to -110 dbm )Uplink RXQUAL ( 0 - 7 )Downlink RXLEV ( - 48 to -110 dbm )Donwlink RXQUAL ( 0 - 7 )RXLEV is the received power levelRXQUAL is the received quality.It is the bit errorr measured on the M idamble01 < 0.2 %2 0.2 - 0.4 %3 0.4 - 0.8 %4 0.8 - 1.6 %5 1.6 - 3.2 %6 3.2 - 6.4 %7 6.4 - 12.8 %RXQUAL
Mobile PowerControlMobile is commanded to change its Transmit PowerChange in Power is proportionate to the Path LossChange is Power is done in steps of 2 dbsTx Level567.1415Power dBm333129.1513Path LossLow RXLEVPwr Command
TDMA approach requires signals to arrive at BTS atthe correct time. They must not overlap.BTSTiming Advance
Concept of Channels in GSM1. To pick up employees in the morning2. To receive company guests from airport3. To carry material to the site / stores4. To Collect mail/courier5. To drop the employees back home6. To get a doctor in case of emergency7. To carry company gusts for a dinnerIf there were two vehicles, we can allocate a set of tasksto one , and rest to the other. However, since both areidenticle vehicles, there is greater flexibility in usage.Concept of Channels in GSMA company vehicle is used for several purposes in a day..
0 156742 3 40123FRAME NUMBERTIMESLOTTime Sharing by ChannelsFCCHSCHBCCHBCCHBCCHTCHTCHTCHTCHTCHTCHTCHTCHTCHTCH
Concept of FramesC T T T T T T TC T T T T T T TC T T T T T T TFrame No. 1Frame No. 2Frame No. 3Time slot Number..0 1 2 3 4 5 6 70 1 2 3 4 5 6 70 1 2 3 4 5 6 7C T T T T T T T Frame No. 4
CHANNEL ALLOCATIONS IN A CELL01234567U1U1 U1 U1U5 U5 U5 U5 U5 U5 U5 U5 U5U2 U2 U2 U2 U2U3 U3 U3 U3 U3 U3 U3U4 U4 U4 U4 U4 U4 U4 U4U6 U6 U6 U6 U6 U6 U6 U6FRAME NumberU7 U7 U7 U7 U7U8 U8 U8 U8C CCCC C C C CC C C
Channels in GSM Air InterfacePHYSICAL CHANNELSARFCNs - 1 to 124 ( D & U )Timeslots - 0 to 7 ( D & U )Total of 124 x 8 = 992 ChannelsLOGICAL CHANNELSARFCN - X , Timeslot - YFrame Number - N1 2 34534 5 6012BCH -BROADCASTCHANNELSDCCH -DEDICATEDCONTROLCHANNELSCCCH -COMMONCONTROLCHANNELSCONTROL CHANNELS TRAFFIC CHANNELSPCH -PAGINGCHANNELFCCH -FrequencyCorrectionChannelSCH -SynchronisationChannelBCCH -BroadcastControl ChannelBCHSDCCH -StandaloneDedicatedControlChannnelSACCH -SlowAssociatedControlChannnelFACCH -FASTASSOCIATEDCONTROLAGCH -ACCESSGRANTCHANNELRACH -RANDOMACCESSCHANNELTCH - FFULL RATETRAFFICCHANNELTCH -TRAFFICCHANNELTCH - HHALF RATETRAFFICCHANNELTCH
One ARFCN, On all the time, in every cellUses Timeslot 0 on a channel, inDownlink.Allows Mobiles to tune to BTS freq. -FCCH This channel carries a 142 bitzero sequence and repeats once in every10 frames on BCHAllows Mobile to Synchronise - SCHThis channel carries the Framenumber and BSIC in encrypted dataformat. Amidamble of 64 bits helpsmobiles to synchronize. SCH also repeatsonce every 10 Frames.Allows Mobiles to identify Network -BCH - Broadcast CHannelBCH
CCCH shares Timeslot 0 with BCH on aMultiframeCCCH consists of PCH , RACH & AGCH.PCH - Paging Channel is used to alert mobileson incomming calls. PCH carries IMSI to pagefor Mobiles in the cell. PCH is Downlinkchannel.RACH - Random Access Channel - is a shortburst sent by mobile to BTS , to initiate a callrequest . RACH uses Timeslot 0 on reverseBCH channel on Uplink.AGCH - Access Grant Channel - When mobilesends a RACH to BTS, BTS responds byCCCH - Common Control CHannelBCH
Used by the MOBILE to get attentionfrom BASE STATION in the Uplink.Several mobiles might originate RACHsimultaneously.RACH uses a Slotted ALOHA accessscheme.Mobile doesnt know path delay–So RACH has to be a special SHORTBURST–Mobile sends normal burst only afterRACH - Random Access CHannel8 3 68.2541 36ExtendedGuard PeriodStopBitsStartBitsSynchronisationBitsEncryptedData Bits88 bits
BCH & CCCH - 51 Frame structure -DOWNLINKFrame number ( DOWN LINK )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 25FCCH - Frequency correction ChannelSCH - Synchronisation ChannelBCCH - Broadcast Control ChannelAGCH - Access Grant ChanPCH - Paging Channel26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50F S B B B B A A A A F S P P P P P P P P F SF S F S I01230123Timeslots
BCH & CCCH - 51 Frame structure - UPLINKRACHRACHRACHRACHRACHRACHRACHRACHRACHRACHRACHRACHRACHRACHFrame number ( UP LINK )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 25012RACHRACHRACHRACHRACHRACHRACHRACHRACHRACHRACHRACHRACH26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50012RACH - Random Access Channel( A short burst is transmitted by mobile towards BTS on RACH)Timeslots
DCCH - Dedicated Control ChannelDedicated Control Channels have a TCH like allocation.DCCH have three Sub Channels.SDCCH - Standalone Dedicated Control Channel Thisis used as an interim channel before final assignment ofTCH. SDCCH is used for signalling and Authenticationmessage transfers.FACCH - Fast Associated Control Channel . FACCH isused by BTS to command a handoff to the mobile. ATCH frame is used up by FACCH , since handoff has totake place on priority.SACCH - Slow Associated Control Channel - SACCHflows at a slower rate on Uplink & Downlink along withTCH or SDCCH. During a call, SACCH flows once forevery 24 Frames of TCH .
SDCCH - Stand-alone Dedicated Control CHannelUSED DURING CALL SET-UPStepping Stone between BCH and TCHUsed for Authentication Etc.SDCCHSDCCHTCHBCH BCH BCH
DOWNLINK ( BTS - MS )–Mobile Tx Power Commands–Mobile Timing Advance–Cells Channel ConfigurationUPLINK ( MS - BTS )–Received signal quality report(RXQual)–Received signal level report(RXLev)–Adjacent BCH power measurements–Mobiles statusSACCH - Slow Associated Control CHannel
INTERRUPTS TCH ON UPLINK ANDDOWNLINKRapid message exchange for handoversControl Bits either side of midamble:–Indicate TCH ( 0 ) or FACCH ( 1 )FACCH - Fast Associated Control CHannelMiidamble8.25bits357bits126bits157bits3GuardPeriodDataTailbits DataControlbitTailbitControlbit
SDCCH - Combined Channel Config( Shares Time slot 0 with BCH and CCCH )Frame number ( DOWN LINK )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 25Timeslots26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50F S B B B B A A A A F S P P P P P P P P F S D D D DD D D D F S D D D D D D D D F S H H H H H H H H ISDCCH ( D ) : Standalone Dedicated Control ChannelSACCH ( H ) : Slow Associated Control Channel01230123
BCH & CCCH - 51 Frame structure - UPLINKSDCCHSDCCHSDCCHSDCCHRACHRACHSACCHSACCHSACCHSACCHSACCHSACCHSACCHSACCHRACHRACHRACHRACHRACHRACHRACHRACHRACHRACHRACHRACHFrame number ( UP LINK )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 25012RACHRACHRACHRACHRACHRACHRACHRACHRACHRACHRACHSDCCHSDCCHSDCCHSDCCHSDCCHSDCCHSDCCHSDCCHRACHRACHSDCCHSDCCHSDCCHSDCCH26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50012RACH - Random Access Channel( A short burst is transmitted by mobile towards BTS on RACH)Timeslots
8.25bits357bits126bits157bits3GuardPeriodDataTailbits DataControlbitMidambleTailbitControlbitTCH - Traffic ChannelOne time burstTraffic Channel carries the Voice data.Two blocks of 57 bits contain voice data .One TCH is allocated for every active call. While call is inprogress if there is degradation in quality of current channel,BTS may shift the communication to another TCH on a differentCarrier and/or Time slot .A Full rate TCH carries 13 KB/s voice data , and Half rate TCHcarries a 6.5 KB/s voice data.156.25 bits or 576.92 uS
TCH Multiframe - TCH Full rate26 Frames - 120 ms24 Carry Speech, 1 Idle, 1 SACCH0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25T T A -T T T T T T T T T T T T T T T T T T T T T TSACCH Idle
TCH Multiframe - TCH Halfrate26 Frames - 120 ms24 Carry Speech , 2 SACCH( shared by two mobiles - a & b )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 25Ta Tb Aa AbTa Tb Ta Tb Ta Tb Ta Tb Ta Tb Ta Tb Ta Tb Ta Tb Ta Tb Ta Tb Ta TbSACCH-a SACCH-b
Click here to type bulleted textF S B B B B A A A A A A A A F S P P P P D D D D D DT T T T T T T T T T T T SA T T T T T T T T T T T T IdleT T T T T T T T T T T T SA T T T T T T T T T T T T IdleT T T T T T T T T T T T SA T T T T T T T T T T T T IdleT T T T T T T T T T T T SA T T T T T T T T T T T IdleT T T T T T T T T T T T SA T T T T T T T T T T T T IdleT T T T T T T T T T T T SA T T T T T T T T T T T T IdleT T T T T T T T T T T T SA T T T T T T T T T T T T IdleFrame number0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 2501234567T - TCH SA - SACCH I - IdleA delay of (12 * 8)+1 Timeslotsis kept between two successiveSACCH .TCH and SACCH - FRAMESTimeSlots
TDMA approach requires signals to arrive at BTSatthe correct time. They must not overlap.BTSTiming Advance
HTiming AdvanceTBSynch Seq41 bitsEncryptedbits 36TBGuard Period68.25 bp63 bitsTiming Adv5.25 GPBSS calculates access delay from RACH in terms of bitsInforms Mobile to delay its timing in terms of bitsMaximum Timing Advance of 63 bitsRACH Burst
HMobile Maximum RangeRange = Timing Advance x bit period x velocity2Range = Distance between Mobile to Base StationTiming Advance = Delay of Bits ( 0 -- 63 )Bit period = 577 / 156.25 = 3.693 usecs = 3.693 x 10e-6 secsVelocity = 3 x 10e5Range = ( 63 ) x ( 3.693 x 10e-6 ) x (3 x 10e5)2= 34.9 kms
IREG• PURPOSE: To test various call scenarios to provide reliableRoaming services• Various Call Scenarios•Mobile to Mobile•PSTN to Mobile•Checking Announcements•Call Forwarding•SMS Test•Combined Test
HMobile Searches for Broadcast Channels(BCH)Synchronises Frequency and TimingDecodes BCH sub-channels (BCCH)Checks if Network Allowed by SIMLocation UpdateAuthenticationMobile Turn-On
BSCMSCLocation Area 1BTSBTSBTSBTSBTSBTSBTSBTSBTSBTSBSCBSCBSCLocation Area 3Location Area 2Location AreaMCC MNC LACLocation Area Identity
HWhat is Location UpdateMSC should know the Location of the Mobile for paging.Mobile is continously changing Location Area.Mobile when changes Location Area informs the MSC about its new L.AProcess of informing MSC about new Location Area is Location UpdateL.Area = 1 L.Area = 2 L.Area = 3Location Update
HIMSI ATTACHMobile turns off and sends an IMSI Detach to MSC.Mobile turns on again and compares LAI.If same,sends an IMSI attach to MSC.1. Normal Location Update.2. IMSI Attach.3. Periodic Location Update.Types of Location Update
HNORMAL LOCATION UPDATEMobile turns on power.Reads the new LAI.If different,does a Location Update.
HPERIODIC LOCATION UPDATEMobile enters non-coverage zone.MSC goes on sending PagesMobile has to inform MSC after a set period.
HLocation UpdateMobile Changes Location AreaReads the new Location Area from BCCHSends a RACH ( request for channel )Gets a SDCCH on AGCHSends its IMSI and new & old LAI in a Location Update Request to MSC on SDCCHMSC starts AuthenticationIf successful, Updates the new Location area for the Mobile in the VLRSends a confirmation to the MobileMobiles leaves SDCCH , and comes to idle mode
HSecurity FeaturesAuthentication--- Process to verify the Authenticity of SIM--- Mobile is asked to perform an operation using anidentity unique to SIM.Ciphering--- Process of coding speech for secrecy--- The speech bits are EXORed with bit stream unique to MS
HHANDOVERCell 1 Cell 2--- Handover is a process by which the control/communicationof a Mobile is transferred from one cell to another
HCRITERIA FOR HANDOVERReceive Quality (RXQUAL) on Uplink & Downlink.Receive Signal Strength (RXLEV) on Uplink & Downlink.Distance ( Timing Advance ).Interference Level.Power Budget.
HHandover DecisionBSC processes the measurement reported by Mobileand the BTS.BSS performs averaging function on these measurementsevery SACCH frame ( 480ms).Handover Decision algorithm is activated after a set number ofSACCH frame periods by comparison againstThresholds.
HINTRA - CELL HANDOVER- Handover between channels / timeslots of same cellBTS
HINTER - CELL HANDOVER--- Handover between cells of same BTSC0C0BTS
HINTRA - BSC HANDOVERMSC BSCBTSBTS--- This type of Handover takes place if the cell to whichwhich handover is to be done belongs to the sameBSC.--- In this the BSC handles everything without involving MSC.--- The MSC will be informed by the BSC after Handover.
HINTER BSC HANDOVERMSCBSCBSCBTSBTS--- In this type of Handover,the Mobile is handed overto a cell which belongs to another BSC.--- The MSC is completely involved in this Handover
HGMSCMSC BSC BTSMSC BSC BTS--- If the cell belongs to another MSC,then it isInter-MSC handover.--- In this case the handover takes place through theinterconnecting element (PSTN) between the MSCsINTER - MSC HANDOVER
HEvery Mobile has an access class .Every cell defines the Mobile classes which are barredaccess.Cell BarringUSE OF CELL BARRING--- Reserving Cells for Handovers.--- Reserving Cells for a certain Mobile Class.
HBoth users talk alternately.Each direction of Transmission is only 50 %Transmitter is switched ON for useful information frames.What isDTX ?
HNeed for DTX---- To increase Battery Life---- To reduce the average interference levelDTX is done by DTX Handlers whichhave the following functions.
HVoice Activity Detector ( VAD )Senses for speech in 20ms blocksRemoves stationary noise.VAD is an energy detector.Compares Energy of filtered speech threshold
HEvaluation of Background noiseBackground noise is always present with speech.DTX cuts off this noise with speech.Gives an uncomfortable feeling to the listener.VAD takes care by inserting comfort noise.
HCELL BROADCASTMessage is continously broadcasted in cell/or cellsBroadcast is done on SDCCHBCCH informs the mobile the details of SDCCH for CBCHMobile tunes to SDCCH at certain intervals and reads messagesAll Mobiles dont support this feature
HShort Message ServiceSMSCentreMSC/VLRBSC BTSShort Message is sent to a particular Mobile Station
HEmergency Calls-- GSM Specs define 112 as emergency number-- 112 is accessible with or without SIM-- Without SIM it is sent on the best channel-- Mobile on sensing 112 sets the establishmentcause to emergency call in the RACH-- Routing of this call can be done to a desiredlocation defined in the Switch.
HGSM Phase 2 featuresExtended Frequency Band ( 50 more channels )Multiple and Alternate Ciphering AlgorithmHalf - Rate CodingCompatibility with DCS 1800 SpecificationsEnhancement of SMS and SIM functionsAdditional functions for bearer services
HGSM Phase 2 + featuresData transmission at 64 Kbps and aboveDECT access to GSMPMR/ Public Access Mobile Radio ( PAMR ) - like capabilitiesGSM in the local loopPacket RadioSIM enhancementsPremium rate services ( e.g. Stock prices sent to your phone )
HAutomatic National RoamingPLMN "A"PLMN "B"PLMN "D"PLMN "C"STPRoamer from "A"
HAutomatic International RoamingPLMN "A"PLMN "Z"CountryInterntlGatewayInterntlGatewayCountryInterntlGateway