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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 l.jpg

GSM Mobility Management

Rashmi Nigalye

Mouloud Rahmani

Aruna Vegesana

Garima Mittal

December 5, 2001

EL604: Wireless & Mobile Networking


Outline l.jpg

Outline

  • GSM architecture overview

    • Network layout

    • Protocols

    • Addresses & identifiers

  • Handover management

    • Talk and Move

  • Location management

    • Move only, without conversation


Gsm network layout l.jpg

GSM Network (PLMN)

MSC region

MSC region

Location area

Location area

BSC

BSC

MSC region

BTS

BTS

GSM NETWORK LAYOUT


Gsm network layout4 l.jpg

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


Interfaces in gsm l.jpg

Interfaces in GSM

MSC or HLR or VLR

BSC

BTS

MSC

MS

A- interface

Radio interface

SS7 GSM MAP

A-bis


Gsm protocol layers gsm has map just like is41 l.jpg

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


What is a location area la l.jpg

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).


Addresses and identifiers l.jpg

CCMNCMSIN

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


Addresses and identifiers9 l.jpg

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.

CCNDCSN

CCNDCSN


Addresses and identifiers10 l.jpg

Addresses and Identifiers

CCMNCLAC

  • 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.


Tmsi vs imsi vs msrn vs msisdn l.jpg

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.


Types of handover same as handoff l.jpg

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.


Attributes of radio link handover l.jpg

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


Handover maho l.jpg

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.


Integrated overlay handover l.jpg

Integrated/Overlay Handover

  • Integrated

    • ISUP messages used for selecting inter-switch channel


Messages exchanged in mobile assisted handover l.jpg

Messages Exchanged in Mobile Assisted Handover

Mobile BaseLogical

stationStationChannel

ConversationConversation

MEASUREMENT REPORT

Conversation

MEASUREMENT REPORT

Conversation

HANDOVER COMMAND

TCH

TCH

SACCH

TCH

SACCH

TCH

FACCH


Cont d l.jpg

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


Handover procedures in gsm l.jpg

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


Inter msc basic handover l.jpg

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


Subsequent handover from msc b to msc a l.jpg

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


Subsequent handover from msc b to msc c l.jpg

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…)


Contd subsequent handover from msc b to msc c l.jpg

(…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


Location management l.jpg

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.


Location registration l.jpg

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...


Contd location registration l.jpg

(…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


Location registration26 l.jpg

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.


Location update l.jpg

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..)


Contd location update l.jpg

(..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)


Ways to obtain msrn l.jpg

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


Call routing to a mobile station l.jpg

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


Call origination l.jpg

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


Call termination l.jpg

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


Find operation l.jpg

Find Operation

  • Inter-LA

    • Both LA’s belong to same MSC, call/packets will be routed directly

  • Inter-MSC

  • Inter-VLR


Find operation in gsm l.jpg

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.


Location update for inter la inter vlr l.jpg

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


Vlr overflow l.jpg

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


Abbreviations l.jpg

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


Reference material l.jpg

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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