GSM Mobility Management.pdf

GSM Mobility Management

Introduction • Location area (LA) is the basic unit for location tracking. • Every E LA consists i t off BTSs BTS that th t communicate i t with ith the MSs over radio links • Major M j ttaskk off Mobility M bilit Management M t is i to t update d t the location of an MS when it moves from one LA to another. another

February 20, 2013

Girish Kumar Patnaik

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

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GSM Location Area Hierarchy

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GSM Basic Location Update p Procedure • In GSM, registration or location update occurs when an MS moves from one LA to another. • Three cases of location update: – Case 1. Inter-LA Movement – Case 2. 2 Inter-MSC Inter MSC Movement – Case 3. Inter-VLR Movement February 20, 2013


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Inter-LA Registration Message Flow

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Inter-LA Registration Message Flow VLR1

HLR VLR2

LA1

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LA2


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Inter-LA Movement • Two LAs belong to the same MSC. j steps: p • Four major – Step 1. MS sends a location update request message (MS→BTS→MSC) . • P Parameters t iincluded: l d d Previous P i LA, LA previous i MSC andd previous i VLR. • IMSI (International Mobile Subscriber Identity) is used to id tif MS. identify 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 February 20, 2013


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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), (LAI) target LAI, Other related information

– Steps 3 aand d 4.. • 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 MSC MSC. February 20, 2013


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Inter-MSC Registration Message Flow

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Inter-MSC Registration Message Flow VLR1

HLR VLR2

LA1 A1

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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 (MS→BTS→MSC) . – Step 3. •P Part II. VLR1 finds fi d that h the h LA1 andd LA2 belong b l 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 MS from the MS’s IMSI. February 20, 2013


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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 Address, Address, other related information

– Step 4. 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. February 20, 2013


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Inter-VLR Registration Message Flow 4 5 6 1

VLR1

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Inter-VLR Movement • T Two LAs LA belong b l to MSCs MSC connectedd to different VLRs. • The Th process is: i – Step 1. MS sends a location update request. MSC2 sends MAP_UPDATE_LOCATION_AREA to VLR 2 with MS’s 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. • VLR1 sends IMSI to VLR2. February 20, 2013


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Inter-VLR Movement – Steps St 4 and d 55. • VLR2 creates a VLR record for the MS. • VLR2 sends d a registration i i message to HLR. HLR • HLR updates the record of the MS. • HLR sends d an acknowledge k l d bback k tto VLR2. VLR2

– Step 6. • VLR2 generates t a new TMSI andd sends d it to t the MS.

– Steps 7 and 88. • The obsolete record of the MS in VLR1 is deleted. February 20, 2013


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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 MSC. MSC – Step 2. MSC forwards the request to VLR with message MAP_SEND_INFO_FOR_OUTGOING_CAL L. – Step 3. VLR checks MS’s profile and sends MAP_SEND_INFO_FOR_OUTGOING_CAL L ack to MSC to grant the call request. L_ack request – Step 4. MSC sets up the trunk according to the standard PSTN call setup procedure. February 20, 2013


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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. • The basic call termination process: – Step 11. A MS’ MS’s ISDN (MSISDN) number b iis dialed by a PSTN user. The call is routed to a gateway MSC by an SS7 ISUP IAM message message. – Step 2. GMSC sends MAP_S SEND N _ROUTING OU NG_INFORMATION N O ON with 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 MSC number stored in the VLR record. • MSRN is sent back to the ggateway y MSC through g HLR. • MSRN provides the address of the target MSC where the MS resides.

– Step 66. An SS7 ISUP IAM message is directed from the gateway MSC to the target MSC to setup the voice trunk. February 20, 2013


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Mobility Databases • The hierarchical databases used in GSM. – The home location register (HLR) is a database used for MS information management. – 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 (the ISDN number b - “Phone “Ph N Number” b ” off th the MS)

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

– Service information • service subscription • service restrictions • supplementary l services i February 20, 2013


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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 February 20, 2013


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Two Issues of GSM Mobility Databases • Fault Tolerance. – If the database fail, the loss or corruption of location information will seriously degrade the service. service

• Database Overflow. – VLR may overflow fl if too t many users move into the VLR-controlled area in a short period. – If VLR is full, full a new arrival user fails to register in VLR and thus cannot receive service. – This pphenomenon is called VLR overflow. February 20, 2013


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VLR Failure Restoration

• After Aft a VLR ffailure, il VLR’ VLR’s iinformation: f ti

– Mobile Station Information MS • Recovered 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.

• After a VLR failure, the VLR record restoration is initiated by one of the following three events: – MS registration – MS call origination – MS call termination February 20, 2013


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Restoration- MS Registration • After a VLR failure: – No record of MS in VLR – VLR considers the registration as an inter-VLR movement. – VLR ask MS to follow the normal registration pprocedure defined in inter-VLR movement. – The TMSI sent from the MS to the VLR cannot be recognized g – VLR asks MS to send IMSI over the air. February 20, 2013


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Restoration-MS Call Origination • After a VLR failure: – VLR receives the call origination g request q MAP_SEND_INFO_OUTGOING_CALL from the MSC (and MS). – No record of MS in VLR – VLR considers it as a system error: “unidentified subscriber” and rejects j the request. q – VLR asks MS to initiate the registration procedure of inter-VLR movement. – After the registration procedure, the VLR record is recovered. February 20, 2013


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Restoration - Call Termination M Message Fl Flow

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Restoration - Call Termination M Message Fl Flow

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HLR Failure Restoration • In HLR, it is mandatory to save the updates into backup storage. • The service information is immediately transferred from the HLR into the backup. backup • The location information is periodically t transferred f d from f the th HLR into i t the th backup. b k • After an HLR failure, the data in the backup are reloaded into the HLR February 20, 2013


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HLR Restoration Procedure Message Flow

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Questions in HLR Restoration Procedure • The HLR restoration p procedure is not robust. – 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 p problem. February 20, 2013


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VLR Identification Algorithm • VIA identifies the exact VLRs to be contacted by the HLR after an HLR failure. • Extra data structures are needed. • Extra procedures are needed: – Check Check-point point procedure – Registration procedure – Restoration R t ti procedure d

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Data Structure in VLR Identification Algorithm (VIA) • To simplify the description, description we assume that every VLR covers exactly one MSC. • An extra data structure VLR_List* VLR List* is a set of VLRs that have been contacted by HLR during the uncovered period period. • After an HLR failure, the HLR only needs to send d the h MAP_RESET MAP RESET messages to VLRs listed in VLR_List*. February 20, 2013


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Data Structure in VLR Identification Algorithm (VIA) • In HLR, every record includes two extra fields. – tts = the th last l t time ti off location l ti update d t – 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 HLR – TS TS= the th last l t check-pointing h k i ti or backup b k time ti – 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*. February 20, 2013


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VIA Data Structure

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VIA Procedure 1: CheckP i ti Pointing • In VIA,, information of the HLR is periodically p y saved into the backup by this procedure. • Step p 1. For every y entryy p in HLR* do: HLR[p]*.VLR ← HLR[p].VLR • Step p 2. TS ← current time;; • Step 3. For every location entry p in HLR do: HLR[p].ts←TS; [p] ; HLR[p].PVLR [p] ← HLR[p].VLR [p] • Step 4. VLR-Counter ← NULL; VLR-List* ←NULL;; February 20, 2013


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VIA Procedure 2: Registration • Step 1. Update HLR: Vold ← HLR[p].VLR; HLR[p] VLR; • Send message, MAP CANCEL LOCATION to cancel MAP_CANCEL_LOCATION, the VLR entry of p at Vold; HLR[p] VLR ← Vnew; HLR[p].VLR told ← HLR[p].ts; HLR[ ] t ← t; HLR[p].ts t

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VIA Procedure 2: Registration • Step 2. Update the Vnew Count field in VLR_Counter: [p] <> HLR[p].PVLR){ [p] ){ If ((HLR[p].VLR 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 (told > TS and Vold <> HLR[p].PVLR){ HLR[p] PVLR){ VLR_Counter[Vold].Count

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VIA Procedure 3: Restore • Step 1. TS <- current time; • Step 2. for (every location entry p in HLR){ p = HLR[p].VLR p <- HLR[p]*.VLR; p HLR[p].PLVR HLR[p].ts <- TS; } • Step 3. for (every VLR entry V in VLR_List*){ sendd an SS7 TCAP MAP MAP_RESET RESET message tto V V; } February 20, 2013


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VLR Overflow Control • VLR may overflow if too many mobile users move into i the h LA in i a short h period. i d • When a VLR is full, a new arrival user can not register i t andd gett service. i • If we want to let the new arrival user can get service all of the following procedures need to be service, modified: – registration registration, cancellation, cancellation call origination, origination call termination

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

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

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Call Origination with 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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Acknowledgement • Slides obtained from home page of Prof.Phone Lin • Slides obtained from home page of Prof Gerald Q. Prof.Gerald Q Maguire Jr Jr.

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GSM Mobility Management.pdf

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