Chapter 2: Global System for Mobile Communication

Chapter 2: Global System for Mobile Communication (22 Marks) Introduction- GSM services and features, GSM architecture, GSM channel types, Example of GSM Call: GSM to PSTN call, PSTN to GSM call. GSM frame structure, Signal processing in GSM, Location tracking and call setup GSM Services and Features SMS - Short Message Service - Allows you to send text messages too and from phones Multi Party Calling - Talk to five other parties as well as yourself at the same time Call Holding - Place a call on Hold Call Waiting - Notifies you of another call whilst on a call Mobile Data Services - Allows handsets to communicate with computers Mobile Fax Service - Allows handsets to send, retrieve and receive faxes Calling Line Identity Service - This facility allows you to see the telephone number of the incoming caller on our handset before answering Advice of Charge - Allows you to keep track of call costs Cell Broadcast - Allows you to subscribe to local news channels Mobile Terminating Fax - Another number you are issued with that receives faxes that you can then download to the nearest fax machine. Prepared By: Ms. M. S. Karande Mobile Computing (17632) Page 1 of 14

GSM System Architecture Interfaces in GSM GSM Architecture GSM system consists of three major components: Base Station System (BSS). Operation and Maintenance Center (OMC). Network and Switching Subsystem (NSS). Prepared By: Ms. M. S. Karande Mobile Computing (17632) Page 2 of 14

(1) Base Station System (BSS): This system consists of Mobile Station (MS), Base Station Controller (BSC), Base Transreceiver Station (BTS). As shown in Fig. the BSS and NSS connected to each other via A interface (solid lines) and the connection to OMC via O interface (dashed lines). Base Station Subsystem (BSS): GSM system consists of many BSS, each one is controlled by Base Station Controller (BSC). BSS performs all the functions which are required to maintain connection to MS, coding/decoding of voice etc. BSS also contains Base Transreceiver Stations (BTS). Base Transreceiver Station (BTS): BTS is responsible for handling radio interface to the mobile station. It is connected to MS via Um interface and it is also connected to BSC via the Abis interface. The Um interface contains all mechanism for wireless interface (TDMA, FDMA etc.). The BTS is a radio equipment (Transreceiver or antenna) needed to service each cell in the network. Base Station Controller (BSC): BSC provides all the control functions and physical link between MSC and BTS. BSC is connected to BTS and MSC (Mobile Switching Centre). o The BSC manages the radio resources for one or more BTS. o It handles radio channel setup, frequency hopping and handovers. o The BSC is the connection between the mobile and the MSC. o It assigns and releases frequencies and time-slots for the MS. o The BSC also handles inter-cell handover. It controls the power transmission of the BSS and MS in its area. (2) Operation and Maintenance Center (OMC): OMC is connected to all equipments in switching system and to the BSC. It maintains operation of the GSM network by observing the handovers, system load, blocking rates etc. OMC provides network overview and allow network engineers to monitor, diagnose and troubleshoot every aspect of GSM network (3) Network and Switching Subsystem (NSS): NSS is responsible for performing call processing and subscriber related functions. It also includes Mobile Switching Center (MSC), Home Location Register (HLR), Visitor Location Register (VLR), Authentication Center (AUC), Equipment Identity Register (EIR) etc. Mobile Switching Centre (MSC): It is used to handle communication between different MS connected to different BSCs. MSC performs the switching of calls between the mobile and other fixed or mobile network users as well as the management of mobile services such as registration, authentication, location updating, handovers and call routing to a roaming subscriber. It also performs such functions as network interfacing, common channel signalling and Prepared By: Ms. M. S. Karande Mobile Computing (17632) Page 3 of 14

others. Every MSC is identified by a unique ID. (3) Network and Switching Subsystem (NSS): Home Location Register (HLR): It is a database for managing the mobile subscriber. HLR stores permanent data of subscriber which include subscribers service profile, location information and its activity. A home subscriber charges are less than the roaming subscriber. Visitor Location Register (VLR): It is a database which consists of temporary information about subscribers which is used by MSC in order to provide services to visiting subscriber. MSC updates the VLR by determining which users are in roaming. Once, the roaming mobile information is updated, then MSC sends necessary information to roaming mobile subscribers so that roaming mobile call can be properly routed. (3) Network and Switching Subsystem (NSS): Authentication Center (AUC): This authentication center is used to provide authentication and encryption method that is used to verify the user identity and ensure the confidentiality and secrecy of each call. Equipment Identity Register (EIR): It contains a list of all valid MS equipment within the network, where each MS is known by it's IMEI. This IMEI is divided into three groups. White IMEI: All known IMEI. Black IMEI: All stolen mobile handset. Gray IMEI: Handset that is uncertain. Addresses and Identifiers International Mobile Station Equipment Identity (IMEI) It is similar to a serial number. It is allocated by equipment manufacturer, registered by network, and stored in EIR International Mobile Subscriber Identity (IMSI) IMSI is stored in MS SIM card and HLR Mobile Subscriber ISDN (MSISDN) The real telephone number : assigned to the SIM Prepared By: Ms. M. S. Karande Mobile Computing (17632) Page 4 of 14

The SIM can have several MSISDN numbers for selection of different services like voice, data, fax Mobile Station Roaming Number (MSRN) It is temporary location dependent ISDN number It is assigned by local VLR to each MS in its area. Temporary Mobile Subscriber Identity (TMSI) o It is an alias of the IMSI and is used in its place for privacy. o It is used to avoid sending IMSI on the radio path. o It is an temporary identity that is allocated to an MS by the VLR at inter-vlr registration, and can be changed by the VLR TMSI is stored in MS SIM card and in VLR. Location Area ID (LAI) LAI is broadcast regularly by Base Station on BCCH Each cell is identified uniquely as belonging to an LA by its LAI GSM Channels Physical Channel: Each timeslot on a carrier is referred to as a physical channel Logical Channel: Variety of information is transmitted between the MS and BTS. Different types of logical channels: o Traffic channel o Control Channel Prepared By: Ms. M. S. Karande Mobile Computing (17632) Page 5 of 14

Traffic Channel (TCH) TCHs are intended to carry user information (speech or data). Full-rate TCH (TCH/F) This channel carries information at rate of 22.8 Kbps Transmission speed: 13 Kbps for speech Transmission speed: 9.6, 4.8 or 2.4 Kbps for data Enhanced full-rate (EFR) speech coders for improving the speech quality Half-rate TCH (TCH/H) Half-rate TCH (TCH/H) This channel carries information at rate of 11.4 Kbps Transmission speed: 6.5 Kbps speech Transmission speed: 4.8 or 2.4 Kbps of data. Control Channels (CCH) CCHs: to carry signalling information Three types of CCHs : a) Broadcast channel (BCH) b) Common control channel (CCCH) c) Dedicated control channel (DCCH) a) Broadcast Channels (BCHs) BTS broadcasts system information to the MSs through BCHs. Two types in BCH: Frequency Correction Channel (FCCH) and Synchronization Channel (SCH) The information allows the MS to acquire and stay synchronized with the BSS. Broadcast Control Channel (BCCH) (downlink) Broadcasts Network and cell specific information required to identify the network and gain access. Broadcasts parameters include Location Area Code( LAC), Mobile Network Code ( MNC), Control Channel Structures, BCCH frequencies of neighboring cells and other access parameters b) Common Control Channel (CCCH) Three types in CCCH: i) Random Access Channel (RACH) (uplink) Used by the MSs for initial access to the network for a dedicated resource/channel. Prepared By: Ms. M. S. Karande Mobile Computing (17632) Page 6 of 14

ii) Access Grant Channel (AGCH) (downlink) In response to request from MS on RACH, the network allocates a dedicated signalling channel (SDCCH) for further communication. This response is sent on AGCH. iii)paging Channel (PCH) (downlink) This channel is used for alerting to mobile subscribers for incoming calls, SMS and other mobility services. Every MS in a cell periodically listen to this channel. Used by the network to page the destination MS in call termination c) Dedicated Control Channel (DCCH) DCCH is for dedicated use by a specific MS. Four types in DCCH: i) Standalone Dedicated Control Channel (SDCCH) (down/uplink) used only for signalling and for location update, voice call setup and SMS. ii) Slow Associated Control Channel (SACCH) (down/uplink) Associated with either a TCH or an SDCCH For non-urgent procedures like Power and time alignment control information (downlink), transmit power control and timing advance Measurement reports from the MS (uplink) iii) Fast Associated Control Channel (FACCH) (down/uplink) Used for time-critical signalling, such as call establishing progress, authentication of subscriber, or handover information. FACCH use TCH during a call. iv) Cell Broadcast Channel (CBCH) (downlink) Carries broadcast messages to network subscribers such as weather, traffic, sports, stocks and other public services and announcements. This channel is assigned with SDCCH and usually occupies the second subslot of the SDCCH. Prepared By: Ms. M. S. Karande Mobile Computing (17632) Page 7 of 14

GSM Frame Structure The length of GSM frame in a frequency channel is 4.615 ms. The frame is divided into 8 bursts of length of 0.577ms The timeslots in the uplink are derived from downlink by a time delay of time slots This arrangement prevents an MS from transmitting and receiving at the same time. However, due to propagation delay (when MS is far away from BTS) the 3 TS delay cannot be maintained accurately. GSM Burst structure Each burst contains 148 bits (0.546ms) followed by 0.031ms guard time (8.25bits) The burst begins with 3 head bits and 3 tail bits (logical Zeroes) Two groups are data bits are separated by an equalizer Training sequence of 26 bits Each data group consists of 57 bit information bits and 1 flag that indicates whether the information bits are for user speech/ data or signaling. Mobile Phone (GSM) to Public Switched Telephone Network (PSTN) Procedure: When a mobile subscriber makes a call to a PSTN telephone subscriber, the following sequence of events takes place: The MSC/VLR receives the message of a call request. The MSC/VLR checks if the mobile station is authorized to access the network. If so, the mobile station is activated. If the mobile station is not authorized, then the service will be denied. MSC/VLR analyses the number and initiates a call setup with the PSTN. MSC/VLR asks the corresponding BSC to allocate a traffic channel (a radio channel Prepared By: Ms. M. S. Karande Mobile Computing (17632) Page 8 of 14

and a time slot). The BSC allocates the traffic channel and passes the information to the mobile station. The called party answers the call and the conversation takes place. The mobile station keeps on taking measurements of the radio channels in the present cell and the neighbouring cells and passes the information to the BSC. The BSC decides if a handover is required. If so, a new traffic channel is allocated to the mobile station and the handover takes place. If handover is not required, the mobile station continues to transmit in the same frequency. Public Switched Telephone Network (PSTN) to Mobile Phone (GSM) When a PSTN subscriber calls a mobile station, the following sequence of events takes place: The Gateway MSC (GMSC) receives the c all and queries the HLR f or the information needed to route the call to the serving MSC/VLR. The GMSC routes the call to the MSC/VLR. The MSC checks the VLR f or the location area of the MS. The MSC contacts the MS via the BSC through a broadcast message, that is, through a paging request. The MS responds to the page request. The BSC allocates a traffic channel and sends a message to the MS to tune to the channel. The MS generates a ringing signal and, after the subscriber answers, the speech connection is established. Prepared By: Ms. M. S. Karande Mobile Computing (17632) Page 9 of 14

GSM Signal Processing 1. Speech coding: The GSM speech coder is based on the Residually Excited Linear Predictive Coder (RELP). The coder provides 260 bits for each 20 ms blocks of speech, which yields a bit rate of 13 kbps. 2. Channel Coding (TCH/FS, SACCH, FACCH) The output bits of speech coder are grouped for error protection, out of the total 260 bits in a frame, the most important 50 bits, called type Ia bits, have 3 parity check (CRC) bits added to them. This facilitates the detection of non-correctable error at the receiver. The next 132 bits along with first 53 (50 types Ia bits + 3 parity bits) appended by four zero bits, thus, providing a data block of 189 bits. This block is then encoded for error protection. It provides a sequence of 378 bits. The least important 78 bits do not have any error protection and are concatenated to the existing sequence of block of 456 bits in 20 ms frame error protection coding increases the gross data rate of GSM speech signal, with channel coding to 22.8 kbps. Prepared By: Ms. M. S. Karande Mobile Computing (17632) Page 10 of 14

3. Interleaving: In order to reduce the effect of sudden fades on the received data, the total 456 encoded bits within each 20 ms speech frame or control message frame are broken into eight 57 bits subblocks. These eight sub-blocks which make up a single speech frame are spread over eight consecutive TCH time slots. 4. Burst formatting: Burst formatting adds binary data to ciphered block, in order to help synchronization of the received signal. 5. Ciphering: Ciphering modifies the contents of the eight interleaved blocks through the use of encryption techniques known only to the particular mobile station and base transceiver station 6. Modulation: The modulation scheme used by GSM is 0.3 GMSK where 0.3 describes the 3 db bandwidth of the Gaussian pulse shaping filter with relation to the bit rate (e.g., BT = 0.3). GMSK is a special type of digital FM modulation. Frequency hopping: Under normal condition, data belong to particular physical channel is transmitted using same frequency. Some time user in a particular cell have served with multipath problem, then the cell can be called as hopping cell by the network operator, in that case slow frequency hopping is carried out to cope up with multipath. Frequency hopping is carried out frame by frame. Frequency hopping is specified by the service provider. Equalization: Equalization is performed at receiver end with the help of training sequences transmitted in midamble of every time slots. Type of equalization is not fixed in GSM, it depends upon manufacturer. Prepared By: Ms. M. S. Karande Mobile Computing (17632) Page 11 of 14

Demodulation: At receiver's end, appropriate TS is demodulated with the aid of synchronization data provided by the burst formatting. After demodulation the binary data is deciphered, deinterleaved, channel decoded and speech decoded. Location Tracking The current location of an MS is maintained by a two-level hierarchical strategy with the HLR and the VLRs Location area (LA) is the basic unit for location tracking Location Tracking: A GSM network is divided into cells. A group of cells is considered a location area. A mobile phone in motion keeps the network informed about changes in the location area. If the mobile moves from a cell in one location area to a cell in another location area, the mobile phone should perform a location area update to inform the network about the exact location of the mobile phone. Home Location Register (HLR) The HLR maintains a database for the mobile subscribers. At any point of time, the HLR knows the address of the MSC VLR that controls the current location area of the mobile. The HLR is informed about a location area update only if the location area change has resulted in a change of the MSC VLR. Mobile Switching Center - Visitor Location Register (MSC-VLR) The MSC VLR is responsible to switching voice calls and it also keeps track of the exact location area where the mobile user is present. Prepared By: Ms. M. S. Karande Mobile Computing (17632) Page 12 of 14

Call Setup in GSM: Mobile Originating Call (MOC): Call setup, which are initiated by an MS 1. Channel Request: The MS requests for the allocation of a dedicated signalling channel to perform the call setup. 2. After allocation of a signalling channel the request for MOC call setup, included the TMSI (IMSI) and the last LAI, is forwarded to the VLR 3. The VLR requests the AC via HLR for Triples [(random number (RAND), Signed Response (SRES) and a Cipher Key (Kc)] (if necessary). 4. The VLR initiates Authentication, Cipher start, IMEI check (optional) and TMSI Re-allocation (optional). 5. If all this procedures have been successful, MS sends the Setup information (number of requested subscriber and detailed service description) to the MSC. 6. The MSC requests the VLR to check from the subscriber data whether the requested service an number can be handled (or if there are restrictions which do not allow further proceeding of the call setup) 7. If the VLR indicates that the call should be proceeded, the MSC commands the BSC to assign a Traffic Channel (i.e. resources for speech data transmission) to the MS 8. The BSC assigns a Traffic Channel TCH to the MS 9. The MSC sets up the connection to requested number (called party) Prepared By: Ms. M. S. Karande Mobile Computing (17632) Page 13 of 14

Questions 1 Describe the services provided by the GSM (any 3). 2 With neat diagram describe the GSM Frame structure 3 Describe the process of GSM to PSTN call. 4 With neat diagram describe the GSM Frame structure 5 Mention two types of GSM Channel along with its subtypes. State the characteristics of its subtype. 6 Explain GSM channel types (4M S-15) 7 Explain GSM architecture in detail with neat sketch (8M S-15) 8 How a signal is processed in GSM? (Diagram 2M, Explanation 2M) GSM. (4M S-15). 9 State any four features of GSM. (4M W-15) 10 With neat diagram describe GSM frame structure. (4M W-15). 11 Describe basic call originating procedure. (4M W-15). 12 Describe GSM Traffic Channels with its type. (4M W-15). 13 Explain TCH Channel Coding in GSM signal Processing (4M W-15). 14 With neat Labelled diagram explain GSM architecture. (6M W-15). 15 State Various GSM control channel in brief. (8M W-15). 16 With neat diagram explain any two data bus in GSM frame. (4M W-15). Prepared By: Ms. M. S. Karande Mobile Computing (17632) Page 14 of 14