Cellular Mobile Communication Systems Lecture 7 Engr. Shahryar Saleem Assistant Professor Department of Telecom Engineering University of Engineering and Technology Taxila TI - 1011 TI - 1011

GSM Architecture GSM is a PLMN (Public Land Mobile Network) Several providers can setup mobile networks following the GSM standard within each country Major components MS (mobile station) BTS (base transceiver station) or BS or cell site BSC (base station controller) MSC (mobile switching centre) LR (location registers): VLR, HLR AUC (Authentication database), EIR (Equipment Identity Register) TI - 1011

GSM Architecture Subsystems RSS (radio subsystem): covers all radio aspects NSS (network and switching subsystem): call forwarding, handoff, switching, location tracking, etc. OSS (operation support subsystem): management of the network Standardized interfaces Allows provider to mix and match vendor equipment TI - 1011

GSM System Architecture TI - 1011

GSM Functional Architecture

GSM System Architecture TI - 1011

Mobile Station An MS consists of the physical equipment used by a user to access a PLMN A mobile station (MS) comprises several functional groups MT (Mobile Terminal) End-point of the radio interface (Um) TA (Terminal Adaptor) Terminal adaptation, hides radio specific characteristics TE (Terminal Equipment) Peripheral device of the MS, offers services to a user SIM (Subscriber Identity Module) Stores user parameters such as subscriber number, authentication Key, PIN etc An Ms has a number of identities IMEI, IMSI, TMSI etc TI - 1011

Radio Station Subsystem Components MS (Mobile Station) BSS (Base Station Subsystem) Physical equipment to provide radio coverage to cells BSS consists of two functional units BTS and BSC BTS (Base Transceiver Station): Antenna + digital radio equipment BSC (Base Station Controller): Controlling several transceivers, map radio channels (Um) onto terrestrial channels A TI - 1011

RSS Interfaces Interfaces Um : radio interface (From MS to BTS) Abis : standardized, open interface with 16 kbit/s user channels (from BTS to BSC) A: standardized, open interface with 64 kbit/s user channels as in wired telephone network (from BSC to MSC) TI - 1011

System Architecture Network and Switching Subsystems Components MSC (Mobile Services Switching Center): IWF (Interworking Functions) ISDN (Integrated Services Digital Network) PSTN (Public Switched Telephone Network) PSPDN (Packet Switched Public Data Net.) CSPDN (Circuit Switched Public Data Net.) Databases HLR (Home Location Register) VLR (Visitor Location Register) EIR (Equipment Identity Register) TI - 1011

NSS NSS includes the main switching function of GSM Databases for users Mobility management Most important role is to manage the communication between GSM and other network users Components Mobile Switching Center (MSC) Performs the necessary switching functions Monitors the mobility of its users Manages Handoff functions Involved in inter-working functions to communicate with other networks such as PSTN and ISDN etc. The inter-working functions depends on the types of networks and the types of services to be performed TI - 1011

NSS Components Home Locator Register (HLR) HLR contains Functional unit for management of mobile subscriber HLR stores two types of information: Subscriber information and part of mobile information (to allow incoming calls to be routed to the MSC for a particular MS) HLR contains Subscriber ID (IMSI and MSISDN) Current Subscriber VLR (Current Location) Supplementary Services Authentication KEY and AUC functionality MSRN TI - 1011

NSS Components Visitor Locator Register (VLR) The VLR is linked to one or more MSCs Functional unit which dynamically stores subscriber information when the subscriber is located in the area covered by the VLR TI - 1011

VLR When a roaming MS enters an MSC area, the MSC infoems the associated VLR about the MS The MS goes through a registration process which includes The VLR recognizes that the MS is from another PLMN If roaming is allowed, the VLR finds the MS’s HLR in home PLMN VLR constructs a Global Title (GT) from the IMSI to allow signalling form the VLR to the MS’s HLR via PSTN/ISDN networks VLR generates a Mobile Subscriber Roaming Number (MSRN) that is used to route incoming calls to the MS The MSRN is sent to the MS’s HLR The information in the VLR includes MSRN, IMSI, TMSI, HLR address or GT, the location area where the MS has registered, local MS identity TI - 1011

Subscriber Identities IMSI: This number identifies the MS subscriber. It is only transmitted over the air during initialization. TMSI: VLR controls the allocation of Temporary Mobile Subscriber Identity TMSI is updated frequently Makes it very difficult for the call to be traced and therefore provides a high degree of security for the subscriber. TMSI may be updated in any of the following situations Call Setup On entry to a new LAI On entry to a new VLR TI - 1011

Operation Subsystem Authentication Center (AUC) OSS (Operation Subsystem) enables centralized operation, management, and maintenance Components Authentication Center (AUC) Generates user specific authentication parameters on the request of VLR Normally co-located with the HLR as it is required to continuously access and update subscriber records Authentication parameters used for Mobile authentication User data encryption TI - 1011

Operation Subsystem Equipment Identity Register (EIR) Contains a centralized database for validating the IMEI The database is concerned solely with MS equipment and not with the IMSI. The EIR database consists of lists of IMEIs organized as follows White List Contains those IMEIs which are known to have been assigned to valid MS equipment. Black List Contains IMEIs of MS which have been reported stolen or which are to be denied service for some other reason. Grey List Contains IMEIs of MS which have problems (for example, faulty software). These are not, however, sufficiently significant to warrant a ‘‘black listing”. TI - 1011

EIR Call Processing Functions TI - 1011

Operation Subsystem Operation and Maintenance Center (OMC) The Operations and Maintenance Centre (OMC) is a centralized facility that supports the day to day management of a cellular network as well as providing a database for long term network engineering and planning tools. An OMC manages a certain area of the PLMN thus giving regionalized network management. TI - 1011

GSM Interfaces The Um Radio Interface (MS to BTS) Uses Physical FDMA/TDMA/FDD physical In 900 MHz band: 890-915 MHz Uplink band, 935-960 MHz Downlink Radio carrier is a 200kHz channel => 125 pairs of radio channels Called Absolute Radio Frequency Channel Number (ARFCN) ARFCN numbers given by f(n) = 890 +.2n MHz for Uplink band n = 0,…124 Corresponding downlink is f(n) + 45 MHz Channels and ARFCN slightly different in other frequency bands A TDMA frame is defined on the radio carrier (8 users per carrier) Channel rate is 270.833 kbps Two types of logical channels map onto physical channels Control Channels (call setup, power adjustment, etc..) Traffic Channels (voice or data) = 22.8kbps = 1 slot in a TDMA frame TI - 1011

GSM TDMA / FDMA MS Transmission Band : 890 – 915 MHZ BS Transmission Band : 935 – 960 MHZ Year Introduced 1990 Access method TDMA Channel Bandwidth 200 kHz Number of duplex channels 125 Users per channel 8 Speech coding bit rate 13 kbps Data coding bit rate 12 kbps Frame size 4.6 ms 45 MHz TI - 1011

GSM TDMA / FDMA TI - 1011

GSM Interfaces (cont) Abis Interface (BTS to BSC) The inter-connection between BTS and BSC is through a standard Abis Primary functions are traffic channel transmission, terrestrial channel management and radio channel management Supports two types of communication links Traffic channels at 64 Kbps carrying speech or data Signalling channels at 16Kbps carrying information for BSC-BTS and BTS-BSC signalling A Interface (BSC to MSC) Inter-connection between BSc and MSC Physical layer is a 2 Mbps standard TI - 1011

GSM Protocol Stack Three Layers specified in the protocol Network layer has three sub layers 1. Call Management Establishment, maintenance, and termination of circuit-switched calls 2. Mobility Management Registration, authentication, and location tracking 3. Radio Resource Management Establishment, maintenance, and termination of radio channel connections Link Layer Uses variation of ISDN Link Access Protocol on D channel protocol – termed LAPDm Physical layer (already discussed) Time slot on a 200 KHz carrier – absolute radio frequency channel number (ARFCN) TI - 1011

GSM Protocol Stack TI - 1011

GSM Channels Physical Channel Traffic Channels Control Channels 125 radio carriers, 8 slots per carrier => Traffic Channels Full rate (TCH/F) at 22.8 kb/s or half rate (TCH/H) at 11.4 kb/s Physical channel = full rate traffic channel (1 timeslot) or 2 half rate traffic channels (1 timeslot in alternating frames) Full rate channel may carry 13 kb/s speech or data at 12, 6, or 3.6 kb/s Half rate channel may carry 6.5 kb/s speech or data at 6 or 3.6 kb/s Control Channels 1. BCH (broadcast channels): point-to-multipoint downlink only 2. CCCH (common control channel): for paging and access 3. DCCH (dedicated control channel): bi-directional point-to-point signalling TI - 1011

Control Channels Broadcast Channels (BCH): Point-to-multipoint unidirectional channels Frequency Correction Channel (FCCH): correcting mobile frequency in downlink. This channel is required for the correct operation of the radio sub-system and allow an MS accurately turn to a BS Synchronization Channel (SCH): Frame synchronization The SCH has a 64-bit binary sequence known to the MS MS achieves the exact timing synchronization with respect to the GSM frame by correlating the bits with the internally stored 64-bits. Broadcast Control Channel (BCCH): control channel structure TI - 1011

Control Channels (cont.) Common Control Channel (CCCH): Point-to-multipoint bidirectional channels Paging Control Channel (PCH): Downlink Used to page MS Random Access Channel (RACH): Uplink Used to request assignment of DCCH Access Grant Channel (AGCH): Downlink Used to assign an MS to a specific DCCH Dedicated Control Channel (DCCH): Used for signalling and control after call establishment Stand Alone dedicated control channel (SDCCH): Used for authentication of MS, location updates, assignments to TCH TI - 1011

Control Channels (cont.) Slow Associated Control Channel (SACCH): Used to carry general control information Fast Associated Control Channel (FACCH): Used to transmit handoff orders. TI - 1011

GSM Channels TI - 1011

Framing Scheme in GSM (Traffic Channels) TI - 1011

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