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GSM Mobility Management

GSM Mobility Management. Rashmi Nigalye Mouloud Rahmani Aruna Vegesana Garima Mittal December 5, 2001. EL604: Wireless & Mobile Networking. Outline. GSM architecture overview Network layout Protocols Addresses & identifiers Handover management Talk and Move Location management

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GSM Mobility Management

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  1. GSM Mobility Management Rashmi Nigalye Mouloud Rahmani Aruna Vegesana Garima Mittal December 5, 2001 EL604: Wireless & Mobile Networking

  2. Outline • GSM architecture overview • Network layout • Protocols • Addresses & identifiers • Handover management • Talk and Move • Location management • Move only, without conversation

  3. GSM Network (PLMN) MSC region MSC region Location area Location area BSC BSC MSC region BTS BTS GSM NETWORK LAYOUT

  4. GSM NETWORK LAYOUT PLMN INTERNATIONAL ISC OMC PSTN ISDN BSC MSC GMSC E Abis BSC A B,C BTS HLR EIR BTS VLR AUC Um BTS

  5. Interfaces in GSM MSC or HLR or VLR BSC BTS MSC MS A- interface Radio interface SS7 GSM MAP A-bis

  6. GSM Protocol layers(GSM has MAP just like IS41) • To perform network control operations unique to personal communications, GSM adds a Mobile Application Part (MAP) to SS7. • MAP makes use of the Transactions Capabilities Part (TCAP) transport protocol. • MAP functions: • Updating of residence information in VLR • Storage of routing information in HLR • Updating and supplementing of user profiles in HLR • Handoff of connections between MSCs

  7. 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, which results in a waste of radio bandwidth • Other extreme is to have a Mobile notify the system via location updating messages of its current location at the individual cell level. This requires paging messages to be sent to exactly one cell, but this is wasteful due to the large number of location updating messages. • Hence, in GSM, we group cells into Location Areas (Neighborhoods).

  8. CC MNC MSIN 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) -CC: Country Code -MNC: Mobile Network Code -MSIN: Mobile Subscriber Identification Number (ID in home net.) When subscribing for service with a network, subscriber receives (IMSI) and stores it in the SIM (Subscriber Identity Module) card. The IMSI has the unique subscriber id that identifies the HLR of the MS. It is never made public

  9. Addresses and Identifiers • Mobile Subscriber ISDN (MSISDN) -NDC: National Destination Code, SN:Subscriber Number,CC: Country Code -The “real telephone number”, assigned to the SIM -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. CC NDC SN CC NDC SN

  10. Addresses and Identifiers CC MNC LAC • 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 • 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. • MSCs and location registers (HLR,VLR) are addressed with ISDN numbers. In addition, they may have a Signaling Point Code (SPC) within a PLMN to address them uniquely in SS#7.

  11. TMSI vs IMSI vs MSRN vs MSISDN • TMSI is used during location update and registration for find, paging and call routing. Instead of using IMSI, the MS sends the TMSI to the BSS, which forwards it to the MSC. • MSRN is the routing number that identifies the current location of the called MS. • MSRN is temporary network identity assigned during the call establishment to a mobile subscriber. • MSRN is the address to the serving MSC/VLR. • MSRN is used during call termination (Incoming call to the MS). • MSISDN is dialed during call termination. • MSISDN points to the subscribers records in the HLR that contains information to locate the MSC where the subscriber is currently located. • MSISDN is the telephone number. • There is an association between IMSI and MSISDN in the HLR.

  12. Types of handover (same as “handoff”) • There are four different types of handover in the GSM system. Handover involves transferring a call between: • Channels (time slots) in the same cell • Cells (Base Transceiver Stations) under the control of the same Base Station Controller (BSC), • Cells under the control of different BSCs, but belonging to the same Mobile services Switching Center (MSC), and • Cells under the control of different MSCs.

  13. Attributes of radio-link handover • Hard handover • MAHO • Backward • Handover messages • Channel allocation schemes • COS selection scheme: static • Cross-over switch: anchor switch • Does GSM cross-over fit the generic scheme? • Yes • Circuit switching • No buffering

  14. Handover (MAHO) • Handovers are initiated by the BSS/MSC (as a means of traffic load balancing). • During its idle time slots, the mobile scans the Broadcast Control Channel of up to 16 neighboring cells, and forms a list of the six best candidates for possible handover, based on the received signal strength. • This information is passed to the BSC and MSC, at least once per second, and is used by the handover algorithm.

  15. Integrated/Overlay Handover • Integrated • ISUP messages used for selecting inter-switch channel

  16. Messages Exchanged in Mobile Assisted Handover Mobile Base Logical station Station Channel Conversation Conversation MEASUREMENT REPORT Conversation MEASUREMENT REPORT Conversation HANDOVER COMMAND TCH TCH SACCH TCH SACCH TCH FACCH

  17. Cont’d Logical Channel Mobile Station Base Station HANDOVER ACCESS New TCH HANDOVER ACCESS TCH HANDOVER ACCESS TCH HANDOVER ACCESS TCH PHYSICAL INFORMATION FACCH HANDOVER COMPLETE FACCH Conversation TCH Conversation TCH

  18. Handover procedures in GSM 8 Connection route 9 MSC-A MSC-B MSC-C 1 6 8 BSC 4 3 BSC BTS 1 BSC BTS 2 2 BTS 3 BTS 3 5 7

  19. Inter MSC basic handover VLR-B MS/BSS 1 MSC-A MSC-B Handover required Perform Handover Allocate Handover number Handover report Radio chan. Ack IAM MS/BSS 2 ACM HA Indication HB Indication HB Confirm Send End Signal ANS End of Call REL RLC End Signal Handover report

  20. Subsequent handover from MSC-B to MSC-A MS/BSS 1 MSC-A MSC-B MS/BSS 2 HA Required Perform subsequent Handover Subseq. Handover Acknowledge HB Indication HB Confirm HA Indication End Signal VLR-B Handover report End of Call REL RLC

  21. Subsequent handover from MSC-B to MSC-C MSC-A MSC-B MS HA Request Perform subsequent Handover MSC-C VLR-C Perform Handover Allocate Handover Number Send Handover report Radio chan. Ack. IAM ACM HB Indication (Contd…)

  22. (…contd) Subsequent handover from MSC-B to MSC-C MSC-A MSC-B MS Perform subsequent Acknowledge HA Indication MSC-C HB Confirm Send End Signal ANS MSC-B VLR-B End Signal Handoff Report REL RLC

  23. 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.

  24. MS BSS/MSC VLR HLR AUC Location registration IMSI Ki Loc.Upd.Req Upd Loc.Area Aut.Par.Req (IMSI,LAI) (IMSI,LAI) (IMSI) Aut. Info. Authenticate (IMSI,Kc, RAND,SRES) Authentic. Req (RAND) (RAND) Auth.Info.Req (IMSI) Ki RAND Auth.Info SRES A3 & A8 (IMSI,Kc, RAND,SRES) SRES Kc Auth.Resp. Auth.Resp = (SRES) (SRES) Update Location (IMSI,MSRN) Generate TMSI Contd...

  25. (…contd) Location registration. VLR MS BSS/MSC HLR AUC Generate TMSI Start Ciph. Ins.Subsc.Data (Kc) (IMSI) Forw. New TMSI Subs.Dat.Ins.Ack (TMSI) Ciph.Mod.Com. Loc.Upd.Accept Kc Loc.Upd.Accept Message M (IMSI) A5 Ciph.Mod. Kc(M) Kc(M) Kc(M) Kc A5 M TMSI Realloc.Cmd. Loc.Upd.Accept TMSI Realloc.Ack TMSI.Ack

  26. 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.

  27. MS BSS/MSC VLR HLR AUC Location update IMSI, TMSI Ki, Kc, LAI Loc.Upd.Req Update Loc.Area (TMSI,LAI) (TMSI,LAI) Authentication Update Location (IMSI,MSRN) Generate TMSI Start ciphering Insert Subscriber. data (Kc) IMSI Start ciphering. Subs. Data Insert Ack (contd..)

  28. (..contd) Location update. VLR MS BSS/MSC HLR AUC Start ciphering. Forward new TMSI (TMSI) Loc. Upd. Acept (IMSI) Loc. Upd. Acept TMSI Realloc. Cmd. Auth. Para. Req (IMSI) Loc. Upd. Acept Auth. Info. Auth.Info.Req (IMSI,Kc, RAND,SRES) TMSI Reallocation Complete TMSI Ack (IMSI) Auth.Info (IMSI,Kc, RAND,SRES)

  29. Ways to obtain MSRN • Obtaining at location update– MSRN for the MS is assigned at the time of each location update which is stored in the HLR. This way the HLR is in a position to supply immediately the routing info (MSRN) needed to switch a call through to the local MSC. • 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

  30. Call routing to a Mobile Station 1 MSISDN GMSC ISDN LA 1 1 4 MSRN 2 MSISDN 3 MSRN BSC MSC MSC HLR BTS 7 TMSI 5 MSRN 7 TMSI LA 2 BSC EIR BTS VLR 8 TMSI 7 TMSI AUC 6 TMSI BTS MS

  31. Call Origination VLR 2 u1 3 PSTN MSC 1 4 PSTN VLR MSC MS 1.call origination request 2. MAP_SEND_INFO_FOR_OUTGOING_CALL 3. MAP_SEND_INFO_FOR_OUTGOING_CALL_ack 4. IAM

  32. Call Termination 5 GMSC 1 4 PSTN HLR 2 VLR 3 6 Target MSC Target MSC Originating Switch VLR HLR GMSC 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

  33. Find Operation • Inter-LA • Both LA’s belong to same MSC, call/packets will be routed directly • Inter-MSC • Inter-VLR

  34. 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.

  35. Location update for inter LA, inter VLR MS BSS/MSC VLR new HLR VLR old IMSI, TMSI Ki, Kc, LAI Loc.Upd.Req Update Loc.Area (TMSI,LAI) Send para. From VLR new (TMSI, LAI) (TMSI,LAI) IMSI response (IMSI,RAND,SRES,Kc Authentication Update Location (IMSI,MSRN) Cancel Location (IMSI) Generate TMSI Cancel location ack (IMSI) Start ciphering Insert Subscriber. data (Kc) IMSI Forward new TMSI (TMSI) Subs. Data Insert Ack Location update accept

  36. VLR Overflow • Too many mobile users move into the LA in a short period • If VLR is full when mobile arrives: • User fails to “register” in the database • It cannot receive cellular services • VLR Overflow Resolutions

  37. Abbreviations • ISC: International switching center • OMC: Operations and maintenance center • GMSC: Gateway switching center • MSC: Mobile switching center • VLR: Visitor location register • HLR: Home Location register • EIR: Equipment Identification register • AUC: Authentication center • BSC: Base station controller • BTS: Base transceiver station • MS: Mobile subscriber • TMSI: Temporary Mobile Subscriber Identity • IMSI: International Mobile Subscriber Identity

  38. Reference Material • Wireless and Mobile Network Architectures by Yi-Bing Lin & Imrich Chlamtac • The GSM Sytem for Mobile communications by Mouly & Pautet • Wireless Personal Communications Systems by Dr. Goodman • GSM Switching, Services and Protocols by Jorg Eberspacher and Hans-Jorg Vogel

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