gsm mobility management.pdf
TRANSCRIPT
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GSM Mobility ManagementGSM Mobility Management
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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.
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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
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GSM Location Area Hierarchy
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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
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Inter-LA Registration Message FlowMessage Flow
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Inter-LA Registration Message Flow
VLR2
VLR1 HLR
VLR2
LA1 LA2
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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
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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.
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Inter-MSC Registration Message FlowMessage Flow
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Inter-MSC Registration Message Flow
VLR2
VLR1 HLR
A1
VLR2
LA1 LA2
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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
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Part III. The VLR1 derives the HLR address of the MS from the MSs IMSI.
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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.
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Inter-VLR Registration Message FlowMessage Flow
4
1
5
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VLR1
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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.
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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
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deleted.
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Call Origination Operation
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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.
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standard PSTN call setup procedure.
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Call Termination Message Flow
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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.
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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.
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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.
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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
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supplementary services
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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
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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.
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p
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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
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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.
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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.
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Restoration - Call TerminationM FlMessage Flow
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Restoration - Call TerminationM FlMessage Flow
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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
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HLR Restoration ProcedureMessage Flow
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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
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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
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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*.
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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
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_in VLR_List*.
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VIA Data Structure
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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
;
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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;
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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
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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
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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
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Overflow Registration O (O )Operation (O-I)
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Cancellation Operation withO f (O )Overflow VLR (O-II)
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Call Origination with Overflow VLR (O-III)Overflow VLR (O-III)
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Call Termination with Overflow VLR (O-IV)
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Call Termination with Overflow VLR (O-IV)
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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.
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