GSM 1

2 GSM Mobility Management Originals by: Rashmi Nigalye, Mouloud Rahmani, Aruna Vegesana, Garima Mittal, Fall 2001 Prof. M. Veeraraghavan, Polytechnic University, New York GSM architecture overview –Network layout –Protocols –Addresses & identifiers Location management –Call delivery + location update –Security Handover management

GSM characteristics previous standard in cellular communication were restrictive GSM – global digital standard for cellular phones that offered roaming facility first named Groupe Special Mobile and used in Europe; then usage extended to other continents GSM operate in frequency bands: 900MHz, 1800 MHz, 1900 MHz GSM provides voice and data services

GSM The European TDMA Digital Cellular Standard Services Provided By GSM 1. Telephony - Basic Teleservice 2. Other Services -Emergency calling -Voice Messaging

GSM The European TDMA Digital Cellular Standard Services Provided By GSM (Cont..) 3. Bearer Services - Low Speed data transfer (upto 9.6 Kbps) -Group 3 Fax and Sms 4. Suplementary Services - call offering, call forwarding, call restriction, call waiting, call hold. - Multiparty teleconferencing, special schemes

Subscriber Identity Module (SIM) card SIM – a memory card (integrated circuit) holding identity information, phone book etc. GSM system support SIM cards other systems, like CDMA do not support SIM cards, but have something similar called Re-Usable Identification Module (RUIM)

International Mobile Equipment Identity (IMEI) key IMEI – a unique 15 digit number identifying each phone, is incorporated in the cellular phone by the manufacturer IMEI ex.: when a phone tries to access a network, the service provider verifies its IMEI with a database of stolen phone numbers; if it is found in the database, the service provider denies the connection the IMEI is located on a white sticker/label under the battery, but it can also be displayed by typing *#06# on the phone

International Mobile Subscriber Identity (IMSI) key IMSI – a 15-digit unique number provided by the service provider and incorporated in the SIM card which identifies the subscriber IMSI enables a service provider to link a phone number with a subscriber first 3 digits of the IMSI are the country code

Temporary Mobile Subscriber Identity (TMSI) key TMSI – is a temporary number, shorter than the IMSI, assigned by the service provider to the phone on a temporary basis TMSI key identifies the phone and its owner in the cell it is located; when the phone moves to a different cell it gets a new TMSI key as TMSI keys are shorter than IMSI keys they are more efficient to send TMSI key are used for securing GSM networks

Architecture of the GSM system GSM is a PLMN (Public Land Mobile Network) –several providers setup mobile networks following the GSM standard within each country –components MS (mobile station) BS (base station) MSC (mobile switching center) LR (location register) –subsystems RSS (radio subsystem): covers all radio aspects, NSS (network and switching subsystem): call forwarding, handover, switching OSS (operation subsystem): management of the network

GSM: elements and interfaces NSS MS BTS BSC GMSC IWF OMC BTS BSC MSC A bis UmUm EIR HLR VLR A BSS PDN ISDN, PSTN RSS radio cell MS AUC OSS signaling O

12 GSM network layout BSC MSC BSC BTS EIR HLR AUC VLR BTS Um A Abis E B,C OMC GMSC PSTN ISDN

Radio Subsystem Base Station Subsystem(BSS) Base Transceiver Station(BTS) Base Station Controller(BSC) Mobile Station(MS):- SIM,IMSI, IMEI, PIN, PUK, IMSI

Networking and switching subsystem NSS is the main component of the public mobile network GSM –switching, mobility management, interconnection to other networks, system control, accounting, roaming, handovers between different BSSs. Components –Mobile Services Switching Center (MSC) controls all connections via a separated network to/from a mobile terminal within the domain of the MSC - several BSC can belong to a MSC –Databases (important: scalability, high capacity, low delay) Home Location Register (HLR) central master database containing user data, permanent and semi- permanent data of all subscribers assigned to the HLR (one provider can have several HLRs) Visitor Location Register (VLR) local database for a subset of user data, including data about all user currently in the domain of the VLR

Operation Subsystem The OSS (Operation Subsystem) enables centralized operation, management, and maintenance of all GSM subsystems Components –Authentication Center (AUC) generates user specific authentication parameters on request of a VLR authentication parameters used for authentication of mobile terminals and encryption of user data on the air interface within the GSM system –Equipment Identity Register (EIR) registers GSM mobile stations and user rights stolen or malfunctioning mobile stations can be locked and sometimes even localized –Operation and Maintenance Center (OMC) different control capabilities for the radio subsystem and the network subsystem. Functions are traffic monitoring, status reports, security management.

16 GSM Mobility Management GSM architecture overview –Network layout –Protocols –Addresses & identifiers Location management –Call delivery + location update –Security Handover management

17 What is a location area (LA)? A powered-on mobile is informed of an incoming call by a paging message sent over the PAGCH channel of a cell One extreme is to page every cell in the network for each call - a waste of radio bandwidth Other extreme is to have a mobile send location updates at the cell level. Paging cut to 1 cell, but large number of location updating messages. Hence, in GSM, cells are grouped into Location Areas – updates sent only when LA is changed; paging message sent to all cells in last known LA

18 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) MCCMNCMSIN MCC: Country Code MNC: Mobile Network Code MSIN: Mobile Subscriber Identification Number When subscribing for service with a network, subscriber receives (IMSI) and stores it in the SIM (Subscriber Identity Module) card. The HLR can be identified by a VLR/MSC from the IMSI.

19 Addresses and Identifiers Mobile Subscriber ISDN (MSISDN) –The “real telephone number”: assigned to the SIM –The SIM can have several MSISDN numbers for selection of different services like voice, data, fax CCNDCSN NDC: National Destination Code (NDC identifies operator); SN: Subscriber Number; CC: Country Code; Digits following NDC identifies the HLR

20 Addresses and Identifiers Mobile Station Roaming Number (MSRN) –It is temporary location dependent ISDN number –It is assigned by local VLR to each MS in its area. CCNDCSN

21 Addresses and identifiers Temporary Mobile Subscriber Identity (TMSI) –It is an alias of the IMSI and is used in its place for privacy. –It is used to avoid sending IMSI on the radio path. –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.

22 TMSI, IMSI, MSRN and MSISDN Unlike MSISDN, IMSI is not known to the GSM user. The CC of MSISDN translates to an MCC of IMSI as follows, e.g, Denmark CC: 45 MCC: 238 TMSI is used instead of IMSI during location update to protect privacy. As user moves, TMSI is used to send location update. Thus a third party snooping on the wireless link cannot track a user as he/she moves. MSRN is the routing number that identifies the current location of the called MS. –MSRN is temporary network identity assigned to a mobile subscriber. –MSRN identifies the serving MSC/VLR. –MSRN is used for call delivery (calls incoming to an MS). MSISDN is the dialed number to reach a GSM user

23 Addresses and Identifiers Location Area ID (LAI) –CC: Country Code, MNC:Mobile Network Code, LAC: Location Area Code –LAI is broadcast regularly by Base Station on BCCH –Each cell is identified uniquely as belonging to an LA by its LAI CCMNCLAC

24 Location management Set of procedures to: –track a mobile user –find the mobile user to deliver it calls Current location of MS maintained by 2-level hierarchical strategy with HLRs and VLRs.

25 Ways to obtain MSRN 1.Obtaining at location update – MSRN for the MS is assigned at the time of each location update, and is stored in the HLR. This way the HLR is in a position to immediately supply the routing info (MSRN) needed to switch a call through to the local MSC. 2.Obtaining on a per call basis – This case requires that the HLR has at least an identification for the currently responsible VLR. When routing info is requested from the HLR, it first has to obtain the MSRN from the VLR. This MSRN is assigned on a per call basis, i.e. each call involves a new MSRN assignment

26 Routing information: case when MSRN is selected per call by VLR/MSC If MSRN is allocated to each subscriber visiting at an MSC, then the number of MSRNs required is large. If instead, an MSRN is allocated only when a call is to be established, then the number of MSRNs is roughly equal to number of circuits at MSC – a much smaller number – hence MSRNs typically allocated per call by VLR/MSC MSISDN GMSC HLR MSISDN  MSC/VLR  MSRN  IMSI MSRN  MSISDN  IMSI, VLR number MSRN

27 Call routing to a mobile station: case when HLR returns MSRN GMSC BSC EIR HLR AUC VLR MSC BTS LA 1 LA 2 ISDN 1 MS 1 MSISDN 6 TMSI 4 MSRN 3 MSRN 2 MSISDN 7 TMSI 7 TMSI 7 TMSI 8 TMSI 5 MSRN MSC

28 Messages exchanged: call delivery PSTN GMSC HLR VLR Target MSC Originating Switch GMSC HLR VLR Target MSC 1. ISUP IAM 2. MAP_SEND_ROUTING_INFO 3. MAP_PROVIDE_ROAMING_NUMBER 4. MAP_PROVIDE_ROAMING_NUMBER_ack 5. MAP_SEND_ROUTING_INFO_ack 6. ISUP IAM

Mobile Terminated Call PSTN calling station GMSC HLR VLR BSS MSC MS : calling a GSM subscriber 2: forwarding call to GMSC 3: signal call setup to HLR 4, 5: request MSRN from VLR 6: forward responsible MSC to GMSC 7: forward call to current MSC 8, 9: get current status of MS 10, 11: paging of MS 12, 13: MS answers 14, 15: security checks 16, 17: set up connection

Mobile Originated Call 1, 2: connection request 3, 4: security check 5-8: check resources (free circuit) 9-10: set up call PSTN GMSC VLR BSS MSC MS

31 Find operation in GSM ISDN switch recognizes from the MSISDN that the call subscriber is a mobile subscriber. Therefore, forward the call to the GMSC of the home PLMN (Public Land Mobile Network) GMSC requests the current routing address (MSRN) from the HLR using MAP By way of MSRN the call is forwarded to the local MSC Local MSC determines the TMSI of the MS (by querying VLR) and initiates the paging procedure in the relevant LA After MS responds to the page the connection can be switched through.

Security in GSM Security services –access control/authentication user  SIM (Subscriber Identity Module): secret PIN (personal identification number) SIM  network: challenge response method –confidentiality voice and signaling encrypted on the wireless link (after successful authentication) –anonymity temporary identity TMSI (Temporary Mobile Subscriber Identity) newly assigned at each new location update (LUP) encrypted transmission 3 algorithms specified in GSM –A3 for authentication (“secret”, open interface) –A5 for encryption (standardized) –A8 for key generation (“secret”, open interface) “secret”: A3 and A8 available via the Internet network providers can (and do) use stronger mechanisms

Security in GSM GSM offers several security services using confidential information stored in the AuC and in the individual SIM. The security services offered by GSM are explained below: Access control and authentication Confidentiality Anonymity Three algorithm have been specified to provide security services in GSM. Algorithm A3 is used for authentication, A5 for encryption and A8 for the generation of cipher key

Authentication Before a subscriber can use any service from the GSM network, he or she must be authenticated. Authentication is based on the SIM, which stores the individual authentication key K i, the user identification IMSI and the algorithm used for authentication A3.

GSM - authentication A3 RANDKiKi 128 bit SRES* 32 bit A3 RANDKiKi 128 bit SRES 32 bit SRES* =? SRES SRES RAND SRES 32 bit mobile network SIM AC MSC SIM K i : individual subscriber authentication keySRES: signed response

Encryption To ensure privacy, all messages containing user- related information are encrypted in GSM over the air interface. After authentication, MS and BSS can start using encryption by applying the cipher key k c. K c is generated using the individual key K i and a random value by applying the algorithm A8. The SIM in the MS and the network both calculate the same K c based on the random value RAND. MS and BTS can now encrypt and decrypt data using the algorithm A5 and the cipher key K c.

GSM - key generation and encryption A8 RANDKiKi 128 bit K c 64 bit A8 RANDKiKi 128 bit SRES RAND encrypted data mobile network (BTS) MS with SIM AC BSS SIM A5 K c 64 bit A5 MS data cipher key

38 Location registration MS has to register with the PLMN to get communication services Registration is required for a change of PLMN MS has to report to current PLMN with its IMSI and receive new TMSI by executing Location Registration process. The TMSI is stored in SIM, so that even after power on or off, there is only normal Location Update. If the MS recognizes by reading the LAI broadcast on BCCH that it is in new LA, it performs Location Update to update the HLR records. Location update procedure could also be performed periodically, independent of the MS movement. The difference in Location Registration and Location Update is that in location update the MS has already been assigned a TMSI.

39 Location registration MSBSS/MSCVLRHLRAUC IMSI Ki A3 & A8 = Generate TMSI Loc.Upd.Req (IMSI,LAI) Upd Loc.Area (IMSI,LAI) Aut.Par.Req Auth.Info.Req (IMSI) (RAND) Authenticate (IMSI,Kc, RAND,SRES) Aut. Info. (IMSI) (RAND) Authentic. Req(IMSI,Kc, RAND,SRES) Auth.Info Auth.Resp. (SRES) Auth.Resp Update Location (IMSI,MSRN) SRES RANDKi Kc SRES Contd...

40 (…contd) Location registration. MSBSS/MSC VLR HLRAUC A5 Generate TMSI (Kc) Start Ciph. Ciph.Mod.Com. Message M Kc Kc(M) Ins.Subsc.Data (IMSI) Forw. New TMSI (TMSI) Subs.Dat.Ins.Ack Loc.Upd.Accept (IMSI) Loc.Upd.Accept Ciph.Mod. Kc(M) A5 Kc(M) Kc M TMSI Realloc.Ack TMSI Realloc.Cmd. TMSI.Ack Loc.Upd.Accept can be combined New TMSI is received by MS (TMSI Reallocation) in ciphering mode.

41 Location update MSBSS/MSCVLRHLRAUC IMSI, TMSI Ki, Kc, LAI Start ciphering. Authentication Loc.Upd.Req (TMSI,LAI) Update Loc.Area (TMSI,LAI) Update Location (IMSI,MSRN) Generate TMSI Start ciphering (Kc) IMSI Insert Subscriber. data Subs. Data Insert Ack (contd..)

42 (..contd) Location update. MSBSS/MSC VLR HLRAUC (IMSI) Auth.Info.Req (IMSI,Kc, RAND,SRES) Auth.Info Start ciphering. Forward new TMSI Auth. Para. Req Loc. Upd. Acept TMSI Ack TMSI Reallocation Complete TMSI Realloc. Cmd. (TMSI) Auth. Info. (IMSI,Kc, RAND,SRES) (IMSI) Loc. Upd. Acept

GSM The European TDMA Digital Cellular Standard Handoff BSC MSC GMSC Handoff is of 3 types 1. Intra BSC 2 Inter BSC 3. Inter MSC

44 Handover procedures in GSM BSC MSC-A BSC MSC-B BTS 1 BTS 3 BTS 2 BSC MSC-C BTS 3 Connection route

4 types of handover MSC BSC BTS MS 1 234