GSM Mobility Management GSM architecture overview

GSM Mobility Management • GSM architecture overview – Network layout – Protocols – Addresses & identifiers • Location management – Call delivery + location update – Security • Handover management Originals by: Rashmi Nigalye, Mouloud Rahmani, Aruna Vegesana, Garima Mittal, Fall 2001 Prof. M. Veeraraghavan, Polytechnic University, New York 1

GSM network layout GSM Network (PLMN) PLMN: Public Land Mobile Network MSC: Mobile Switching Center BTS: Base Transceiver Station BSC: Base Station Controller MSC region Location area BSC BTS BSC Location area MSC region 2

GSM network layout PSTN ISDN OMC BSC BTS Abis MSC E BSC A B, C EIR BTS GMSC Um HLR AUC VLR 3

GSM MAP protocol • GSM MAP similar to IS 41 MAP • MAP uses Transactions Capabilities Part (TCAP) of the SS 7 stack • MAP functions: – Updating of location information in VLRs – Storing routing information in HLRs – Updating and supplementing user profiles in HLRs – Handoff of connections between MSCs 4

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 5

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) MCC MNC MSIN 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. 6

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 CC NDC SN NDC: National Destination Code (NDC identifies operator); SN: Subscriber Number; CC: Country Code; Digits following NDC identifies the HLR 7

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. CC NDC SN 8

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. 9

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 10

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 CC MNC LAC 11

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. 12

Ways to obtain MSRN 1. 2. 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. 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 13

Routing information: case when MSRN is selected per call by VLR/MSC HLR RN MS MS GMSC • RN MSRN SI ISD N IM MSISDN MS MSISDN IMSI, VLR number MSC/VLR 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 14 VLR/MSC

Call routing to a mobile station: case when HLR returns MSRN 1 MSISDN GMSC LA 1 1 4 MSRN 2 3 BSC MSISDN MSRN MSC BTS MSC HLR 7 TMSI 5 7 MSRN TMSI LA 2 BSC EIR BTS 8 TMSI MS 7 TMSI BTS 6 TMSI AUC VLR 15

Messages exchanged: call delivery PSTN 1 GMSC 5 2 4 HLR VLR 3 6 Target MSC HLR GMSC Originating 1. ISUP IAM Switch 2. MAP_SEND_ROUTING_INFO VLR Target MSC 3. MAP_PROVIDE_ROAMING_NUMBER 4. MAP_PROVIDE_ROAMING_NUMBER_ack 5. MAP_SEND_ROUTING_INFO_ack 6. ISUP IAM 16

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. 17

GSM security • Authentication • What signed response (SRES) are you able to derive from the input challenge RAND by applying the A 3 algorithm with your personal key Ki (Ki is per subscriber)? Ki RAND (128 bit) RAND A 3 algorithm SRES Ki MS network SRES equal? 18

GSM security • Encryption • Digital technology – easy to encrypt voice data • A 5 derives a ciphering sequence of 114 bits for each burst independently • XOR 114 bits of a radio burst with 114 bits of a ciphering sequence generated by A 5 BTS MS Kc (64 bits) frame number Kc (22 bits) A 5 algorithm S 1(114) deciphering S 2(114) ciphering S 1 ciphering S 2 deciphering 19

Key management • Ciphering key Kc is generated using algorithm A 8 in the same manner as SRES (from RAND and Ki) • Each time a mobile station is authenticated the MS and network compute the ciphering key Kc by running algorithm A 8 with the same inputs RAND and Ki as for SRES • Ciphering with Kc applies only when the network knows the identity of the subscriber it is talking to. – Bootstrap period during which network does not know who the subscriber is • Up to and including the first message carrying the nonambiguous subscriber identity is carried in the clear (unencrypted) – Protection: use TMSI instead of IMSI when possible – TMSI should be exchanged during protected signaling (ciphered) procedures 20

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. 21

MS BSS/MSC VLR Loc. Upd. Req (IMSI, LAI) Upd Loc. Area Aut. Par. Req (IMSI, LAI) (IMSI) Aut. Info. (IMSI, Kc, RAND, SRES) Authenticate Authentic. Req (RAND) Auth. Info. Req (IMSI) Auth. Info (IMSI, Kc, RAND, SRES) RAND SRES A 3 & A 8 Kc AUC Location registration IMSI Ki Ki HLR SRES Auth. Resp. (SRES) Auth. Resp (SRES) = Update Location (IMSI, MSRN) Generate TMSI Contd. . . 22

(…contd) Location registration. MS VLR BSS/MSC HLR AUC Generate TMSI Start Ciph. (Kc) Forw. New TMSI (TMSI) Ciph. Mod. Com. Kc Ins. Subsc. Data (IMSI) Subs. Dat. Ins. Ack Loc. Upd. Accept Message M (IMSI) A 5 Kc(M) Ciph. Mod. Kc(M) Kc A 5 TMSI Realloc. Cmd. Loc. Upd. Accept TMSI Realloc. Ack New TMSI is received by MS (TMSI Reallocation) in ciphering mode. M can be combined TMSI. Ack 23

MS BSS/MSC VLR HLR Location update AUC IMSI, TMSI Ki, Kc, LAI Loc. Upd. Req (TMSI, LAI) Update Loc. Area (TMSI, LAI) Authentication Update Location (IMSI, MSRN) Generate TMSI Start ciphering (Kc) Start ciphering. Insert Subscriber. data IMSI Subs. Data Insert Ack (contd. . ) 24

(. . contd) Location update. MS BSS/MSC VLR HLR AUC Start ciphering. Forward new TMSI (TMSI) Loc. Upd. Acept (IMSI) Loc. Upd. Acept TMSI Realloc. Cmd. Auth. Para. Req (IMSI) Auth. Info. Loc. Upd. Acept TMSI Reallocation Complete TMSI Ack (IMSI, Kc, RAND, SRES) Auth. Info. Req (IMSI) Auth. Info (IMSI, Kc, RAND, SRES) 25

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. 26

Attributes of radio-link handover • • Hard handover MAHO Backward COS selection scheme: static – Cross-over switch: anchor switch 27

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. 28

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 5 BTS 3 7 29

Inter MSC basic handover MS/BSS 1 MSC-A Handover required VLR-B MSC-B Perform Handover Allocate Handover number Handover report Radio chan. Ack MS/BSS 2 IAM ACM HA Indication HB Indication Send End Signal HB Confirm ANS End of Call REL RLC End Signal Handover report 30

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 HB Indication Acknowledge HB Confirm HA Indication VLR-B End Signal Handover report End of Call REL RLC 31

Subsequent handover from MSC-B to MSC-C MSC-B MSC-A MS HA Request Perform subsequent Handover MSC-C Perform Handover VLR-C Allocate Handover Number Radio chan. Ack. Send Handover report IAM ACM HB Indication (Contd…) 32

(…contd) Subsequent handover from MSC-B to MSC-C MSC-B MSC-A MS Perform subsequent HA Indication Acknowledge MSC-C HB Confirm Send End Signal ANS MSC-B VLR-B End Signal Handoff Report REL RLC 33

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 34

References • The GSM Sytem for Mobile communications by Mouly & Pautet • Wireless and Mobile Network Architectures by Yi-Bing Lin & Imrich Chlamtac • Wireless Personal Communications Systems by Dr. Goodman • GSM Switching, Services and Protocols by Jorg Eberspacher and Hans-Jorg Vogel 35