CSF645 Mobile Computing 行動計算 Chapter 3 GSM and Similar Architectures 吳俊興 國立高雄大學資訊工程學系
Chapter 3 GSM and Similar Architectures 3.1 GSM Services and System Architecture 3.2 Radio Interfaces 3.3 Protocols 3.4 Localization 3.5 Calling 3.6 Handover 3.7 Security 3.8 New Data Services 3.9 General Packet Radio Service 3.10 High-speed Circuit Switched Data 3.11 DECT 2
3.1 GSM Services and System Architecture GSM: Global system for mobile communications a second generation (2G) communication standard Tri-band (operable in GSM 900/1800/1900) phones enable easy international roaming in GSM networks Three types of integrated services for voice and data Teleservices Supplementary services Bearer services (of data transmitted from or received) 3
Integration of Services in a GSM System MS (Mobile Station) including MT (Mobile Terminal): interface with the communication network TE (Terminal): used by a caller to connect and talk TE of PSTN has no MT Two Data transfer Modes Transparent: Higher rate, FEC supported to recover error Non-transparent: smaller rate, but reliable Three (Circular-Switched) Data Transmission Modes Synchronous between sender and receiver SMS Asynchronous between sender and receiver Synchronous packet between two ends 4
GSM Network Architecture 3 subsystems 5
Subsystems of GSM Network Architecture Radio subsystem (RSS): consists of BSCs Base station controller (BSC) - connects to BTSs Base transceiver stations (BTS) provide radio interface for mobile devices Network subsystem (NSS): consists of Mobile services switching centres (MSC): interfaces to BSCs Provide call switching and mobility management Gateway MSC (GMSC) Determines which visited MSC the subscriber who is being called is currently located at Interfaces with the PSTN Home location registers (HLR) For obtaining data about the SIM and mobile services ISDN (phone) number Visitor location registers (VLR) For determining where other mobile subscribers are located Operating subsystem (OSS): consists of Authentication centre (AuC) Equipment identity register (EIR) Operation and maintenance centre (OMC) 6
Interfacing Between Different Subsystems When a mobile station MS x communicates to another mobile station MS y, a switching center MSC i establishes (switches) a connection (channel) between (i) MS x interfaced to the BTS p, then to the BSC q, then to MSC r and (ii) MS y interfaced to the BTS u, BSC V, and MSC w RSS and NSS for communication MSCs must have location registries to enable the NSS to discover a path (route or channel) between MSx and MSy The OSS facilitates the operations of MSCs 7
RSS Architecture and Interfacing to NSS 8
MS s Subscriber Identity Module (SIM) An inserted card provided by the service provider Uniquely identifies the user to the service Enables the MS to connect to the GSM network When the MS connects to the GSM subsystems, the SIM saves A temporary mobile (dynamic) cipher key for encryption Temporary mobile subscriber identity (TMSI), and used to identify an MS during a connection for protecting the user ID from hackers Location area identification (LAI) Information which does not change when the MS moves into another location International mobile subscriber identity (IMSI) Card serial number and type Contains a PIN (personal identification number) The user can use the PIN to lock or unlock the MS Using the PIN, MS is unlocked when it seeks connection to another MS 9
SIM Functions Stores the PUK (PIN unblocking key) which enables the subscriber to unlock the SIM if it is accidentally locked Stores a 128-bit authentication key provided by service provider The MS authenticates by a switching centre through an algorithm using this key and a 128-bit random number dynamically sent by authentication centre If the MS is not authenticated, the service to that number is blocked Stores the international mobile subscriber identity (IMSI) IMSI: a unique 15 digit number allocated to each mobile user, three parts IMSI three parts a three digit mobile country code (MCC), a mobile network code (MNC) consisting of two digits, and the mobile subscriber identity number (MSIN) with up to 10 digits Same IMSI all over the globe Identical coding scheme Helps service providers in identifying and locating an MS Helps the MS in obtaining the cipher key, TMSI, and LAI from the mobile service provider during connection setup 10
Base Transceiver Station (BTS) Each MS establishes connection through Um to BTS, which connected to a BSC at through the Abis interface Transmits and receives data with four multiplexed channels of 16 kbps or with a 64 kbps channel Main functions Formation of cells using appropriately directed antennae Processing of signals and amplification of signals to acceptable strength Channel coding and decoding coding voice into bits to be transmitted at 13 kbps and decoding received coded signals back to voice Frequency hopping Encryption and decryption of data Paging Adapting to the rate of data synchronous data transmission The receiver clock of the transceiver at one end of an interface adapts itself according to transmitter clock of the transceiver at the other end 11
Base Station Controller (BSC) Manages a number of BTSs connected with Abis interface Connected to an MSC in the networking and switching layer using an interface A Main functions Reserve radio frequencies for communication and manage handovers between BTSs within a BSS Control signals to the connected BTSs Control and handover of the signals from BSC to MSC Mapping the signals of a channel Receives signals from a BTS at 16 kbps through Abis and interfaces them to an MSC at 16 kbps Interface to a PSTN switching centre at 64 kbps through a fixed line network - mapped by assigning a 16 kbps channel for 64 kbps signals and vice versa Traffic control by continuous measurement of the frequency channel spectrum being used at a given instant Authentication, encryption, and decryption of data Updating location registry for the MSs Paging 12
Network Subsystem (NSS) Acts as an interface between wireless and fixed networks Consists of switches and databases l mobile services switching centres (MSC) m home location registers n visitor location registers Gateway MSCs (GMSC): to connect to PSTN, ISDN, or PSPDN Inter-working functions (IWF) with the MSC Manages functions such as handovers between BSSs worldwide user localization maintenance of user accounts and call charges, and management of roaming 13
Mobile Services Switching Centre (MSC) Consists mainly of high-performance digital ISDN switches Has a HLR and a VLR Connects to a number of BSCs over the A interface Connects to other MSCs or fixed-line networks through GMSCs Main functions Manage BSCs in a geographical area Processing of signals Establishing and terminating the connection between various mobile stations via BSCs The mobile stations to be connected may fall in a given MSCs own area or in the area assigned to another MSC, in which case the communication path has to be via the other MSC Establishing and terminating the connection between an MS and a fixed line phone via a GMSC or IWF Monitoring of calls made to and from an MS Call charging, multi-way calling, call forwarding, and other supplementary services 14
HLR and VLR HLR Has the MT databases Each mobile user has only one HLR record worldwide Each MS must register at a specific HLR of a specific MSC The HLR contacts AuC in the OSS for authentication Stores all the relevant subscriber data including mobile subscriber ISDN number (MSISDN), details of subscription permissions such as call forwarding, roaming, etc., subscriber s ISMI, user s location area, user s current VLR and MSC status Each HLR is associated to an MSC so that when an MS registered at a certain HLR moves to another location area (LA), serviced by another MSC, the user s home MSC update the user s current VLR VLR A dynamic real-time database that stores both permanent and temporary subscriber data which is required for communication between the MSs in the coverage area of the MSC associated with that VLR. The VLR is an integral part of the MSC 15
Operation Subsystem (OSS) Administers the operation and maintenance of the entire network Each AuC associates with an HLR in the NSS and each EIR connects to an MSC An OMC at OSS can connect to an MSC or a GMSC in the NSS and to a BSC at RSS 16
Operation and Maintenance Centre (OMC) Operation and Maintenance Centre (OMC) Monitors and controls all other network entities through the O interface Main functions: management of status reports, traffic monitoring, subscriber security management, accounting and billing Authentication Centre (AuC) Used by the HLR to authenticate a user Calculates authentication parameters and then conveying to the HLR AuC database stores subscriber authentication keys equipment identity register (EIR) Stores international mobile equipment identity (IMEI) numbers IMEI enables the MSC in identifying the type of terminal, mobile equipment manufacturer, and model helps the network in locating the device in case it is stolen or misplaced Three special list A black list that includes mobile stations which have been reported stolen or are currently locked due to some reason A white list which records all MSs that are valid and operating A grey list including all those MSs that may not be functioning properly 17