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GSM Mobility Management

Rashmi NigalyeMouloud RahmaniAruna VegesanaGarima Mittal

December 5, 2001

EL604: Wireless & Mobile Networking

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Outline

• GSM architecture overview– Network layout– Protocols– Addresses & identifiers

• Handover management– Talk and Move

• Location management– Move only, without conversation

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GSM NETWORK LAYOUT

GSM Network (PLMN)

MSC region

MSC region

MSC region

Location area

BSC

BSCLocation area

BTS BTS

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GSM NETWORK LAYOUT

ISC

BSC

OMC

BSC

EIRHLR

AUCVLR

MSC GMSC

BTS

BTS

BTS

PSTN

ISDN

PLMN

INTERNATIONAL

AAbis

Um

EB,C

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Interfaces in GSM

MS BTS BSC MSC

MSC or HLR or VLR

Radio interface A-bis

A- interfaceSS7 GSM MAP

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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

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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).

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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

CC MNC MSIN

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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

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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

• 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.

CC MNC LAC

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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.

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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.

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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

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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.

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Integrated/Overlay Handover

• Integrated– ISUP messages used for selecting inter-switch

channel

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Messages Exchanged in Mobile Assisted Handover

Mobile Base Logicalstation Station Channel

ConversationConversation

MEASUREMENT REPORT

Conversation

MEASUREMENT REPORTConversation

HANDOVER COMMAND

TCH

TCH

SACCH

TCH

SACCH

TCH

FACCH

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Cont’d

HANDOVER ACCESS

HANDOVER ACCESS

HANDOVER ACCESS

HANDOVER ACCESS

PHYSICAL INFORMATION

HANDOVER COMPLETE

Conversation

Conversation

Mobile Station Base Station Logical Channel

New TCH

TCH

TCH

TCH

FACCH

FACCH

TCH

TCH

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Handover procedures in GSM

BSC

MSC-A

BSC

MSC-B

BTS 1

BTS 3

BTS 2

BSC

MSC-C

BTS 3

Connection route

1

2

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5

6

7

8

8

9

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Inter MSC basic handover MS/BSS 1 MSC-A

Handover required

HA Indication

MSC-B VLR-B

Radio chan. AckHandover report

MS/BSS 2

Allocate Handover number

RLC

ANS

REL

End Signal

HB Indication

HB Confirm

Handover report

Perform Handover

IAM

ACM

Send End Signal

End of Call

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Subsequent handover from MSC-B to MSC-A

MS/BSS 1 MSC-A

HB Indication

HB Confirm

MSC-B

VLR-B

Subseq. Handover

Acknowledge

MS/BSS 2

RLC

REL

End Signal

HA Required

HA Indication

Handover report

Perform subsequent

Handover

End of Call

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Subsequent handover from MSC-B to MSC-C

MSC-A

MSC-C

Perform Handover

Radio chan. Ack.

MSC-B

Allocate Handover

Number

MS

IAM

ACM

HA RequestPerform subsequent

Handover

VLR-C

Send Handover report

HB Indication(Contd…)

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(…contd) Subsequent handover from MSC-B to MSC-C

MSC-A

MSC-C

Send End Signal

Handoff Report

MSC-B MS

REL

RLC

HA IndicationPerform subsequent

Acknowledge

HB Confirm

ANS

MSC-B VLR-B

End Signal

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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.

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Location registrationMS BSS/MSC VLR HLR AUC

IMSI Ki

A3 & A8

=

Generate TMSI

Loc.Upd.Req

(IMSI,LAI)Upd Loc.Area

(IMSI,LAI)Aut.Par.Req

(IMSI)

Auth.Info.Req(RAND)

Authenticate(IMSI,Kc,

RAND,SRES)

Aut. Info.

(IMSI)

(RAND)

Authentic. Req

(IMSI,Kc,

RAND,SRES)

Auth.Info

Auth.Resp.

(SRES)(SRES)

Auth.Resp

Update Location

(IMSI,MSRN)

SRES

RANDKi

Kc SRES

Contd...

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(…contd) Location registration.

MS BSS/MSC VLR HLR AUC

A5

Generate TMSI

(Kc)

Start Ciph.

Ciph.Mod.Com.

Message MKc

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

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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.

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Location updateMS BSS/MSC VLR HLR AUC

IMSI, TMSIKi, 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..)

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(..contd) Location update.

MS BSS/MSC VLR HLR AUC

(IMSI)

Auth.Info.Req

(IMSI,Kc, RAND,SRES)

Auth.Info

Start ciphering.

Forward new TMSI

Auth. Para. Req

Loc. Upd. Acept

Loc. Upd. Acept

TMSI AckTMSI Reallocation

Complete

TMSI Realloc. Cmd.

(TMSI)

Auth. Info.

(IMSI,Kc, RAND,SRES)

(IMSI)

(IMSI)

Loc. Upd. Acept

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Ways to obtain MSRN

1. 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.

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

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Call routing to a Mobile Station

GMSC

BSC

BSC

EIR

HLR

AUCVLR

MSCBTS

BTS

BTS

LA 1

LA 2

ISDN1

MS

1

MSISDN

6

TMSI

4

MSRN

3

MSRN

2

MSISDN

7

TMSI

7

TMSI

7

TMSI

8

TMSI

5

MSRN

MSC

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Call Origination

VLR

u1

MSCPSTN

VLRPSTN

MSC

2. MAP_SEND_INFO_FOR_OUTGOING_CALL

3. MAP_SEND_INFO_FOR_OUTGOING_CALL_ack

4. IAM

1.call origination request

1

2

3

4

MS

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Call Termination

PSTNGMSC

HLR VLR

Target

MSC

Originating Switch GMSC HLR VLR

Target MSC

1. ISUP IAM2. MAP_SEND_ROUTING_INFO

3. MAP_PROVIDE_ROAMING_NUMBER

4. MAP_PROVIDE_ROAMING_NUMBER_ack

5. MAP_SEND_ROUTING_INFO_ack

6. ISUP IAM

1

2 3

45

6

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Find Operation

• Inter-LA– Both LA’s belong to same MSC, call/packets

will be routed directly

• Inter-MSC

• Inter-VLR

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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.

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Location update for inter LA, inter VLR

MS BSS/MSC HLR

Loc.Upd.Req

(TMSI,LAI)Update Loc.Area

(TMSI,LAI)

Update Location

(IMSI,MSRN)

Start ciphering

(Kc) IMSI

Insert Subscriber. data

Subs. Data Insert Ack

Authentication

Generate TMSI

IMSI, TMSIKi, Kc, LAI

VLR new

Send para. From VLR new(TMSI, LAI)

IMSI response(IMSI,RAND,SRES,Kc

Cancel Location(IMSI)Cancel location ack(IMSI)

Forward new TMSI(TMSI)

Location update accept

VLR old

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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

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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

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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