gsm mobility management.pdf

GSM Mobility ManagementGSM Mobility Management

Introduction Location area (LA) is the basic unit for location

tracking.E LA i t f BTS th t i t ith Every LA consists of BTSs that communicate with the MSs over radio linksM j t k f M bilit M t i t d t Major task of Mobility Management is to update the location of an MS when it moves from one LA to anotherto another.

February 20, 2013 Girish Kumar Patnaik 2

Introduction Location update procedure is referred to as

registration. BTSs periodically broadcasts the

corresponding LA addresses to the MSs When an MS receives an LA address

different from the one stored in its memory, y,it sends a registration message to the network


GSM Location Area Hierarchy


GSM Basic Location UpdatepProcedure

In GSM, registration or location update occurs when an MS moves from one LA to another.

Three cases of location update:Three cases of location update: Case 1. Inter-LA Movement

Case 2 Inter MSC Movement Case 2. Inter-MSC Movement Case 3. Inter-VLR Movement


Inter-LA Registration Message FlowMessage Flow


Inter-LA Registration Message Flow

VLR2

VLR1 HLR

VLR2

LA1 LA2


Inter-LA Movement Two LAs belong to the same MSC. Four major steps:j p

Step 1. MS sends a location update request message (MSBTSMSC) .

P t i l d d P i LA i MSC d i Parameters included: Previous LA, previous MSC and previous VLR.

IMSI (International Mobile Subscriber Identity) is used to id tif MSidentify MS.

However, the MS identifies itself by the Temporary Mobile Subscriber Identity (TMSI).

TMSI is used to avoid sending the IMSI on the radio path. TMSI is temporary identity is allocated to an MS by the VLR at

inter-VLR registration


Inter-LA Movement Step 2. The MSC forwards the location update

request to the VLR by a TCAP message, MAP_UPDATE_LOCATION_AREA.

Parameter includes: Address of the MSC, TMSI, previous Location Area Identification (LAI) targetprevious Location Area Identification (LAI), target LAI, Other related information

Steps 3 and 4.Steps 3 a d . Part I. The VLR finds that both LA1 and LA2 belong

to the same MSC. Part II. The VLR updates the LAI field of the MS. Part III. The VLR replies an ACK to the MS through

the MSCFebruary 20, 2013 Girish Kumar Patnaik 9

the MSC.

Inter-MSC Registration Message FlowMessage Flow


Inter-MSC Registration Message Flow

VLR2

VLR1 HLR

A1

VLR2

LA1 LA2


Inter-MSC Movement The two LAs belong to different MSCs of

the same VLR. The process is:

Steps 1 and 2. MS sends a location update request message (MSBTSMSC) .

Step 3.P I VLR1 fi d h h LA1 d LA2 b l Part I. VLR1 finds that the LA1 and LA2 belong to MSC1 and MSC2, respectively. Two MSCs are connected to VLR1.

Part II. VLR1 updates the LAI and MSC fields of MS.

Part III. The VLR1 derives the HLR address of the


Part III. The VLR1 derives the HLR address of the MS from the MSs IMSI.

Inter-MSC Movement Step 3.

Part IV. The VLR1 sends the MAP_UPDATE_LOCATION to the HLR.

Parameter includes: IMSI, target MSC Address VLR Address other relatedAddress, VLR Address, other related information

Step 4 HLR updates the MSC number Step 4. HLR updates the MSC number field of the MS. An acknowledgement is sent to VLR1.

Steps 5 and 6. The acknowledgement is forwarded to the MS.


Inter-VLR Registration Message FlowMessage Flow

4

1

5

61

VLR1


Inter-VLR MovementT LA b l MSC d Two LAs belong to MSCs connected to different VLRs.Th i The process is: Step 1. MS sends a location update request.

MSC2 sendsMSC2 sends MAP_UPDATE_LOCATION_AREA to VLR 2 with MSs TMSI.

Steps 2 and 3. VLR2 does not have the record of MS. VLR2 identifies the address the VLR1 and sends

MAP_SEND_IDENTIFICATION (with TMSI) to VLR1.

February 20, 2013 Girish Kumar Patnaik 15 VLR1 sends IMSI to VLR2.

Inter-VLR MovementSt 4 d 5 Steps 4 and 5. VLR2 creates a VLR record for the MS.

VLR2 d i i HLR VLR2 sends a registration message to HLR. HLR updates the record of the MS.

HLR d k l d b k t VLR2 HLR sends an acknowledge back to VLR2. Step 6.

VLR2 t TMSI d d it t VLR2 generates a new TMSI and sends it to the MS.

Steps 7 and 8 Steps 7 and 8. The obsolete record of the MS in VLR1 is

deleted.February 20, 2013 Girish Kumar Patnaik 16

deleted.

Call Origination Operation

4


GSM Basic Call Origination The process is

Step 1. MS sends the call origination request to MSCMSC.

Step 2. MSC forwards the request to VLR with messagemessage MAP_SEND_INFO_FOR_OUTGOING_CALL.

Step 3. VLR checks MSs profile and sends MAP_SEND_INFO_FOR_OUTGOING_CALL ack to MSC to grant the call requestL_ack to MSC to grant the call request.

Step 4. MSC sets up the trunk according to the standard PSTN call setup procedure.


standard PSTN call setup procedure.

Call Termination Message Flow


Call Termination Routing information for call termination can

be obtained form the serving VLR.be obtained form the serving VLR. The basic call termination process:

Step 1 A MS ISDN (MSISDN) b i Step 1. A MSs ISDN (MSISDN) number is dialed by a PSTN user. The call is routed to a gateway MSC by an SS7 ISUP IAM messagegateway MSC by an SS7 ISUP IAM message.

Step 2. GMSC sends MAP SEND ROUTING INFORMATION _S N _ OU NG_ N O ONwith the MSISDN to HLR.


Call Termination Step 3. HLR sends a

MAP_PROVIDE_ROAMING_NUMBER to VLR. Parameter included: IMSI of the MS, the MSC number.

Steps 4 and 5. VLR creates Mobile Subscriber Roaming Number (MSRN) by using the MSCRoaming Number (MSRN) by using the MSC number stored in the VLR record.

MSRN is sent back to the gateway MSC through HLR.g y g MSRN provides the address of the target MSC where the

MS resides.Step 6 An SS7 ISUP IAM message is directed from Step 6. An SS7 ISUP IAM message is directed from the gateway MSC to the target MSC to setup the voice trunk.


Mobility Databases The hierarchical databases used in GSM.

The home location register (HLR) is a database The home location register (HLR) is a database used for MS information management.The visitor location register (VLR) is the The visitor location register (VLR) is the database of the service area visited by an MS.


Home Location Register (HLR) An HLR record consists of 3 types of

information: Mobile station information

IMSI (used by the MS to access the network)MSISDN (th ISDN b Ph N b f th MS) MSISDN (the ISDN number - Phone Number of the MS)

Location information ISDN number of the VLR (where the MS resides) ISDN number of the VLR (where the MS resides) ISDN number of the MSC (where the MS resides)

Service information service subscription service restrictions

l iFebruary 20, 2013 Girish Kumar Patnaik 23

supplementary services

Visitor Location Register (VLR) The VLR information consists of three parts:

Mobile Station Information IMSI MSISDN TMSI

Location Information MSC Number Location Area ID (LAI)

Service Information A subset of the service Information stored in HLR


Two Issues of GSM MobilityDatabasesDatabases

Fault Tolerance. If the database fail, the loss or corruption of

location information will seriously degrade the serviceservice.

Database Overflow.VLR fl if t VLR may overflow if too many users move into the VLR-controlled area in a short period.

If VLR is full a new arrival user fails toIf VLR is full, a new arrival user fails to register in VLR and thus cannot receive service.

This phenomenon is called VLR overflow.


p

VLR Failure RestorationAft VLR f il VLR i f ti After a VLR failure, VLRs information: Mobile Station Information

Recovered either by the first contact with HLR or MSRecovered either by the first contact with HLR or MS. Location Information

Recovered by the first radio contact with MS. Service Information

Recovered by the first contact with HLR or the corresponding MS.corresponding MS.

After a VLR failure, the VLR record restoration is initiated by one of the following three events: MS registration MS call origination

MS call terminationFebruary 20, 2013 Girish Kumar Patnaik 26

MS call termination

Restoration- MS Registration After a VLR failure:

No record of MS in VLRNo record of MS in VLR VLR considers the registration as an inter-VLR

movement.movement. VLR ask MS to follow the normal registration

procedure defined in inter-VLR movement.p The TMSI sent from the MS to the VLR cannot

be recognizedg VLR asks MS to send IMSI over the air.


Restoration-MS Call Origination After a VLR failure:

VLR receives the call origination request g qMAP_SEND_INFO_OUTGOING_CALL from the MSC (and MS).No record of MS in VLR No record of MS in VLR

VLR considers it as a system error: unidentified subscriber and rejects the request.j q

VLR asks MS to initiate the registration procedure of inter-VLR movement.

After the registration procedure, the VLR record is recovered.


Restoration - Call TerminationM FlMessage Flow


HLR Failure Restoration In HLR, it is mandatory to save the updates

into backup storage.into backup storage. The service information is immediately

transferred from the HLR into the backuptransferred from the HLR into the backup. The location information is periodically

t f d f th HLR i t th b ktransferred from the HLR into the backup. After an HLR failure, the data in the backup

are reloaded into the HLR


HLR Restoration ProcedureMessage Flow


Questions in HLR RestorationProcedureProcedure

The HLR restoration procedure is not probust. HLR does not know a VLR at checkpoint.p An MS move into the VLR during the

uncovered period. HLR will not ask the VLR to send location

information. VLR Identification Algorithm is to solve

the problem.February 20, 2013 Girish Kumar Patnaik 33

p

VLR Identification AlgorithmVLR Identification Algorithm VIA identifies the exact VLRs to be

contacted by the HLR after an HLR failure. Extra data structures are needed.Extra data structures are needed. Extra procedures are needed:

Check point procedure Check-point procedure Registration procedure

R t ti d Restoration procedure


Data Structure in VLR Identification Algorithm (VIA)Identification Algorithm (VIA) To simplify the description we assume thatTo simplify the description, we assume that

every VLR covers exactly one MSC. An extra data structure VLR List* is a set An extra data structure VLR_List* is a set

of VLRs that have been contacted by HLR during the uncovered periodduring the uncovered period.

After an HLR failure, the HLR only needs d h MAP RESETto send the MAP_RESET messages to

VLRs listed in VLR_List*.


Data Structure in VLR Identification Algorithm (VIA)Identification Algorithm (VIA)

In HLR, every record includes two extra fields.t th l t ti f l ti d t ts = the last time of location update

PVLR = the address of VLR where the MS resided at the last check-pointing time. Thus, for any MS p, we have

HLR*[p].VLR = HLR[p].PVLR

Two extra data structures in the HLRTS th l t h k i ti b k ti TS= the last check-pointing or backup time

VLR_Counter = {(VLR1,Count 1), (VLR2,Count 2), , (VLRn,Count n)} where Count n represents the effective number of MSs entering the VLRn during the uncovered period.

Note that the VLRs recorded in VLR Counter are the VLRs


_in VLR_List*.

VIA Data Structure


VIA Procedure 1: Check-P i tiPointing

In VIA, information of the HLR is periodically , p ysaved into the backup by this procedure.

Step 1. For every entry p in HLR* do: p y y pHLR[p]*.VLR HLR[p].VLR

Step 2. TS current time;p ; Step 3. For every location entry p in HLR do:

HLR[p].tsTS; HLR[p].PVLR HLR[p].VLR[p] ; [p] [p] Step 4. VLR-Counter NULL; VLR-List* NULL;February 20, 2013 Girish Kumar Patnaik 38

;

VIA Procedure 2: Registration Step 1. Update HLR:

V HLR[p] VLR;Vold HLR[p].VLR; Send message,

MAP CANCEL LOCATION to cancelMAP_CANCEL_LOCATION, to cancel the VLR entry of p at Vold;

HLR[p] VLR V ;HLR[p].VLR Vnew;told HLR[p].ts;HLR[ ] t tHLR[p].ts t;


VIA Procedure 2: Registration Step 2. Update the Vnew Count field in VLR_Counter:

If (HLR[p].VLR HLR[p].PVLR){( [p] [p] ){If (VLR_Counter[Vnew] exists){

VLR_Counter[Vnew].Count

VIA Procedure 2: Registration Step 3. Update the Vold counter entry:If (t > TS and V HLR[p] PVLR){If (told > TS and Vold HLR[p].PVLR){

VLR_Counter[Vold].Count

VIA Procedure 3: Restore Step 1. TS

VLR Overflow Control

VLR may overflow if too many mobile users i h LA i h i dmove into the LA in a short period.

When a VLR is full, a new arrival user can not i t d t iregister and get service.

If we want to let the new arrival user can get service all of the following procedures need to beservice, all of the following procedures need to be modified: registration cancellation call origination callregistration, cancellation, call origination, call

termination


Overflow Registration O (O )Operation (O-I)


Cancellation Operation withO f (O )Overflow VLR (O-II)


Call Origination with Overflow VLR (O-III)Overflow VLR (O-III)


Call Termination with Overflow VLR (O-IV)


AcknowledgementAcknowledgement

Slides obtained from home page of Prof.Phone Lin

Slides obtained from home page of Prof Gerald Q Maguire JrProf.Gerald Q. Maguire Jr.


gsm mobility management.pdf

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