1、TI - 1011,1,Cellular Mobile Communication Systems Lecture 7,Engr. Shahryar Saleem Assistant Professor Department of Telecom Engineering University of Engineering and Technology Taxila TI - 1011,TI - 1011,2,GSM Architecture,GSM is a PLMN (Public Land Mobile Network)Several providers can setup mobile
2、networks following the GSM standard within each countryMajor 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)
3、,TI - 1011,3,GSM Architecture,SubsystemsRSS (radio subsystem): covers all radio aspectsNSS (network and switching subsystem): call forwarding, handoff, switching, location tracking, etc.OSS (operation support subsystem): management of the networkStandardized interfacesAllows provider to mix and matc
4、h vendor equipment,TI - 1011,4,GSM System Architecture,TI - 1011,5,GSM Functional Architecture,TI - 1011,6,GSM System Architecture,TI - 1011,7,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
5、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, authentica
6、tion Key, PIN etc An Ms has a number of identities IMEI, IMSI, TMSI etc,TI - 1011,8,Radio Station Subsystem,ComponentsMS (Mobile Station)BSS (Base Station Subsystem) Physical equipment to provide radio coverage to cells BSS consists of two functional units BTS and BSCBTS (Base Transceiver Station):
7、Antenna + digital radio equipmentBSC (Base Station Controller): Controlling several transceivers, map radio channels (Um) onto terrestrial channels A,TI - 1011,9,RSS Interfaces,InterfacesUm : radio interface (From MS to BTS)Abis : standardized, open interface with 16 kbit/s user channels (from BTS t
8、o BSC)A: standardized, open interface with 64 kbit/s user channels as in wired telephone network (from BSC to MSC),TI - 1011,10,System Architecture Network and Switching Subsystems,Components MSC (Mobile Services Switching Center): IWF (Interworking Functions) ISDN (Integrated Services Digital Netwo
9、rk) PSTN (Public Switched Telephone Network) PSPDN (Packet Switched Public Data Net.) CSPDN (Circuit Switched Public Data Net.) DatabasesHLR (Home Location Register)VLR (Visitor Location Register)EIR (Equipment Identity Register),TI - 1011,11,NSS,NSS includes the main switching function of GSM Datab
10、ases 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 functio
11、ns 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 - 1011,12,NSS Components,Home Locator Register (HLR) Functional unit for management of mobile subscriber HLR stores two types of
12、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 - 10
13、11,13,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 - 1011,14,VLR,When a roaming MS enters an MSC area, the MSC infoems the associate
14、d 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 MSs HLR in home PLMN VLR constructs a Global Title (GT) from the IMSI to allow signalling form the VLR to the MSs HLR via PSTN/ISD
15、N 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 MSs HLRThe information in the VLR includes MSRN, IMSI, TMSI, HLR address or GT, the location area where the MS has registered, local MS identity,TI - 1011,15,Subs
16、criber 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
17、 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 - 1011,16,Operation Subsystem,OSS (Operation Subsystem) enables centralized operation, management, and maintenance Components Authentication C
18、enter (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 - 1011,17,Operation Subsystem,E
19、quipment 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
20、 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
21、”.,TI - 1011,18,EIR,Call Processing Functions,TI - 1011,19,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 netw
22、ork engineering and planning tools. An OMC manages a certain area of the PLMN thus giving regionalized network management.,TI - 1011,20,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
23、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
24、 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 - 1011,21,GSM TDMA / FDMA,45 MHz,BS Transmiss
25、ion Band : 935 960 MHZ,MS Transmission Band : 890 915 MHZ,TI - 1011,22,GSM TDMA / FDMA,TI - 1011,23,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
26、 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 s
27、tandard,TI - 1011,24,GSM Protocol Stack,Three Layers specified in the protocolNetwork layer has three sub layers1. Call Management Establishment, maintenance, and termination of circuit-switched calls2. Mobility ManagementRegistration, authentication, and location tracking3. Radio Resource Managemen
28、tEstablishment, maintenance, and termination of radio channel connections Link LayerUses 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 - 1011,25,GSM Protoco
29、l Stack,TI - 1011,26,GSM Channels,Physical Channel 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) F
30、ull 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/sControl Channels1. BCH (broadcast channels): point-to-multipoint downlink only2. CCCH (common control channel): for paging and access3. DCCH (dedicated control
31、channel): bi-directional point-to-point signalling,TI - 1011,27,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-sys
32、tem and allow an MS accurately turn to a BSSynchronization 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
33、 Channel (BCCH): control channel structure,TI - 1011,28,Control Channels (cont.),Common Control Channel (CCCH): Point-to-multipoint bidirectional channels Paging Control Channel (PCH): DownlinkUsed to page MS Random Access Channel (RACH): Uplink Used to request assignment of DCCH Access Grant Channe
34、l (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 - 1011,29,Control Channels (cont.),Slow Associated Control Channel (SACCH): Used to carry general control informationFast Associated Control Channel (FACCH): Used to transmit handoff orders.,TI - 1011,30,GSM Channels,TI - 1011,31,Framing Scheme in GSM (Traffic Channels),TI - 1011,32,END,
