09 UMTS System

8/9/2019 09 UMTS System

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PCS & Wire less In t ernet LAB

Speaker: Phone Lin

Email: [email protected]

Mobi l i ty Managem ent Mobi l i ty Managem ent (GPRS to UMTS)(GPRS to UMTS)

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I n t roduc t ion

z Universal Mobile Telecommunication System(UMTS) is a third

generation mobile network evolved from the second generation

systems such as GSM and General Packet Radio Service(GPRS).

z In this talk, we consider the core network evolution path from

GSM/GPRS to UMTS.

z The Core Network (CN) consists of two service domains, a circuit-

switched(CS) service domain (i.e., PSTN/ISDN) and a packet-

switched(PS) service domain ( i.e., IP).

z In the CS domain, an Mobile Station(MS) is identified by IMSI and

TMSI. In the PS domain, an MS is identified by IMSI and P-TMSI.


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GPRS Netw ork

z To be simplified (but may be misleading), GPRS is evolved fromGSM by introducing two new core network nodes Serving GPRSSupport Node(SGSN) and Gateway GPRS Support Node(GGSN).

z Existing GSM nodes (BSS, MSC/VLR, and HLR) are upgraded.

z GPRS BSS consists of Base Transceiver Station (BTS) and BaseStation Controller(BSC) where the BSC is connected to the SGSNthrough frame relay link.

z The BTS communicates with the MS through the radio interface Um

based on the TDMA technology.

z Three operation modes are defined for GPRS MS:

q Class A MS allows simultaneous CS and PS connections.

q Class BMS provides automatic choice of CS or PS connection, but onlyone at a time.

q Class CMS only supports PS connection.

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UMTS Netw ork

z UMTS is evolved from GPRS by replacing the radio access network.

z The UTRAN (UMTS Terrestrial Radio Access Network) consists ofNodeBs (the 3G term for BTS) and Radio Network Controllers(RNCs)connected by an ATM network.

z The RNC and the Node B serving an MS are called the Serving RadioNetwork System(SRNS).

z In UMTS, every Node B is connected to an RNC through the Iubinterface.

z Every RNC is connected to an SGSN through the IuPSinterface, andto an MSC through the IuCSinterface.

z An RNC may connect to several RNCs through the Iurinterface.

z Unlike RNCs in UMTS, the BSCs in GPRS/GSM do not connect to eachother.

z The IuCS, IuPS, Iub, and Iur interfaces are implemented on the ATMnetwork.


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Cont rol Plan es fo r UMT S and GPRS (2/2)

z Unlike GPRS, the LLC layer is not supported in UMTS. In GPRS, re liable

communication between MS and SGSN is guaranteed by LLC.

z In UMTS, Radio Resource Control(RRC) protocol is responsible for reliable

connection between MS and UTRAN, and Signaling Connection Control Part

(SCCP) is responsible for reliable connection between UTRAN and SGSN.

z Specifically, radio resources are managed by RRC exercised between the MS

and the UTRAN.

z On top of SCCP, the Radio Access Network Application Part(RANAP) protocol

supports transparent mobility management signaling transfer between the MS

and the CN which are not interpreted by the UTRAN.

z RANAP is also responsible for serving RNC relocation, radio access bearer (

RAB) management, and so on.

z In both GPRS and UMTS theGPRS Mobility Management(GMM) protocol

supports mobility management functionality, which is the focus of this paper.

In 3GPP 23.121, GMM for UMTS is also referred to asUMTS MM(UMM).

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Mobi l i ty Managem ent (MM) Messaging

z The mobility management (MM) messages areexchanged among GPRS/UMTS nodes through variousinterfaces described in two aspects:

q Between the MS and the SGSN

q Between the SGSN and Core Network Nodes (HLR, VLR, GGSN)


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MM Messaging be t w een MS and SGSN

z In GPRS, the mobility management (MM) messages aredelivered through the Gb and the Um interfaces.

q Specifically, an LLC link provides signaling connection between

the MS and the SGSN in GPRS.

z In UMTS, the MM message transmission is performed

through the Iu and the Uu interfaces.

q In UMTS, the signaling connection consists of an RRC connection

between the MS and UTRAN, and an Iu connection ( one RANAPinstance) between the UTRAN and the SGSN.

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MM Messaging bet w een SGSN and

Ot her CN Nodes

z In both GPRS and UMTS, GSM Mobile Application Part(MAP) is used to interface SGSN and the GSM nodes.

q For example, Gr for HLR and Gs (the BSSAP+ protocol or

BSS Application Protocol+ ) for MSC/VLR. SGSNs and

GGSNs communicate by using the GPRS TunnelingProtocol (GTP) through the Gn interface.

q Specifically, a GTP tunnelis established between two GPRS

nodes to deliver the packets. This tunnel is identified by a

tunnel endpoint identifier(TEID), an IP address and a UDP

port number.


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User Plane

z Protocols for user data transmission are defined in theuser plane.

q In GPRS, the SubNetwork Dependent Convergence Protocol

(SNDCP) carries out transmission of NPDUs ( Network Packet DataUnits) on top of the LLC link between the MS and the SGSN.

q In UMTS, the Packet Data Convergence Protocol(PDCP) carriesout NPDU transmission on top of the RLC connection between the

MS and the UTRAN, and the GTP-U (GTP for the user plane)protocol carries out transmission of NPDUs on top of the UDP/IPlink (Iu link).

q Packets of user data transmission may be lost when some MM

signaling procedures are executed.

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Summ ary: GPRS and UMTS

Arch i tec tu re

z In both GPRS and UMTS, IMSI is used as the commonuser Identity, and common MAP signaling is applied toboth systems as well as GSM.

z Unlike GPRS, UMTS Radio network parameters andradio resources are managed in the UTRAN.

z Link GPRS BSS, the UTRAN does not coordinate MMprocedures that are logically between the MS and CN.


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Conc ept s o f MM

z In order to track the MSs, the cells (i.e., BTSs/Node Bs) in GPRS/UMTS service area are partitioned into several groups. To deliverservices to an MS, the cells in the group covering the MS will page theMS to establish the radio link.

z In the CS domain, cells are partitioned into location areas(LAs). TheLA of an MS is tracked by the VLR.

z In the PS domain, the cells are partitioned into routing areas( RAs ).An RA is typically a subset of an LA. The RA of an MS is tracked by

the SGSN.

z In GPRS, the SGSN also tracks the cell of an MS in PS connection(i.e., when packets are delivered between the MS and the SGSN).

z In UMTS, the cells in an RA are further partitioned into UTRAN RAs(URAs). The URA and the cell of an MS are tracked by the UTRAN.

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MM Func t ions (1 /4)

z The MM functions for PS-based services are

(1) PS attach procedure allows an MS to be known by thePS service domain of the network.

q For example, after the MS is powered on, the PS attach procedure

must be executed before the MS can obtain access to the PSservices.

q Note that the term PS attach is used in UMTS and the termGPRS attach is used in GPRS.

q Similarly, we have the term CS attach for UMTS and IMSIattach for GPRS.

(2) PS detach procedure allows the MS or the network to informeach other that the MS will not access the SGSN-based services.


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(3) Security procedures include authentication, user identity

confidentiality (e.g., P-TMSI reallocation and P-TMSI signature) and

ciphering. Here, we elaborate more on P-TMSI signature.

q When the SGSN allocates the P-TMSI to an MS, it may also send

the P-TMSI signature to the MS.q Then when the next MS identity checking is performed, e.g., in the

attach procedure, the MS sends the P-TMSI signature to the SGSN

for comparison.

q If the comparison fails, the authentication procedure must be usedby the SGSN to authenticate the MS.

q GPRS Ciphering is performed between the MS and the SGSN.UMTS ciphering is performed between the UTRAN and MS.

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(4) Tunneling of non-GSM signaling messageprocedures support communication between

GPRS/UMTS and non-GSM systems (e.g.,

EIA/TIA IS-136).q The SGSN forwards the signaling messages to the non-

GSM MSC/VLR using the BSSAP+ protocol in the Gs

interface.


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(5) Subscriber management procedures are used by theHLR to inform the SGSN about changes of the PSsubscriber data.

(6) Service request procedure (UMTS only) is used by theMS to establish a secure connection to the SGSN, sothat the MS can send uplink signaling message or userdata.

q This procedure is used, for example, when the MS replies a pagefrom the UMTS network or when the MS attempts to requestresource reservation.

q In GPRS, LLC link is always established between the MS andSGSN after the attach procedure. Therefore, the service requestprocedure is not needed and is not defined in GPRS.

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MM St a t es

z In GPRS and UMTS, an MM finite state machine is exercised inboth SGSN and MS to characterize the mobility managementactivities for the MS.

z In GPRS, the states in the machine are IDLE, STANDBY andREADY.

z For UMTS PS service domain, these states are renamed asPMM-DETACHED, PMM-IDLE and PMM-CONNECTED.

z The MM state machines for both GPRS and UMTS are basicallythe same.

z The MM states are stored in the MM contexts maintained by theMS and the SGSN.


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IDLE or PMM -DETACHED

zThe MS is not known (i.e., not attached) toGPRS (UMTS/PS). That is, the MS is not

reachable by the network.

zIn this state, the MS may perform attach

procedure.

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STAN DYBY or PMM -IDLE

zThe MS is attached to GPRS (UMTS/PS); thatis, both the MS and SGSN have established MM

contexts.

zIn this state, the MS may perform the detachand location update procedures.

zThe SGSN may perform paging procedure.

zThe MS is tracked by the SGSN at the RA level.


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READY or PMM -CONNECTED

z PDUs can only be delivered in this state.

z In GPRS, the SGSN tracks the MS at the cell level.

z In UMTS, a PS signaling connection is establishedbetween the MS and the SGSN (that is, the MS is inRRC Connected mode).

z The SGSN tracks the MS with accuracy of the RA level,

and the serving RNC is responsible for cell leveltracking.

z In UMTS, serving RNC relocation is executed in thisstate.

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MM St a t e Trans i t ions

zIDLE to READY (PMM-DETACHED PMM-CONNECTED): This transition is triggered by

an MS when the MS performs GPRS/PS

attach.


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STANDBY t o IDLE(PMM-IDLE t o PMM-DETACHED) (1/3)

z This transitioncan be triggered by MS or SGSN.

z This transition is triggered by the SGSN when tracking

of MS is lost. In this case, SGSN performs an implicitGPRS/PS detach.

z A mobile reachable timer is maintained in the SGSN to

monitor the periodic RA update procedure.

z If the SGSN does not receive RA update request

message from the MS after the timer expires, the MS isconsidered detached.

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STANDBY t o IDLE

(PMM-IDLE t o PMM-DETACHED) (2/3)

z The mobile reachable timer is used only when the MMstate is STANDBY/PMM-IDLE.

z This transition may also be triggered by SGSN when theSGSN receives a Cancel Location message from the

HLR.

z In this case, the MM and the PDP contexts are alreadymoved to the new SGSN that serves the MS, and thecontexts in the old SGSN can be deleted.

z Note that the MS will be associated with the new SGSNin this case.


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STANDBY t o IDLE(PMM-IDLE t o PMM-DETACHED) (3/3)

zThis transition is triggered by the MS when theMS performs implicit detach due to removal of

the SIM card or the battery.

zThis case is defined for UMTS, but not for

GPRS.

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STANDBY t o READY

(PMM-IDLE t o PMM-CONNECTED)

zThis transition is triggered by the MS.

zIn GPRS, this transition occurs when the MSsends an LLC PDU to the SGSN, possibly in

response to a page from the SGSN.zIn UMTS, this transition occurs when the service

request procedure is executed (possibly inresponse of a page from the SGSN) to establishthe PS signaling connection between the MSand the SGSN.


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READY t o STANDBY

(PMM-CONNECTED to PMM-IDLE) (1/2)

z This transition is triggered by either SGSN or MS.

z In GPRS, a READY timer is maintained in the MS andthe SGSN.

z If no LLC PDU is transmitted before the timer expires,then this MM transition occurs.

z The length of the READY timer can only be changed by

the SGSN.

z The MS is informed of the READY timer value changethrough messages such as Attach Accept and RoutingArea Update Accept.

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READY t o STANDBY

(PMM-CONNECTED to PMM-IDLE) (2/2)

zThis MM transition may also occur when theSGSN forces to do so, or when abnormal RLC

condition is detected during radio transmission.

z In UMTS, this MM transition occurs when thePS signaling connection is released or broken(e.g., RRC connection failure), or when the URA

update timer at the RNC expires.


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READY t o IDLE

(PMM-CONNECTED to PMM-DETACHED)

z This transitioncan be triggered by MS or SGSN.

z This transition is triggered by MS or SGSNwhen the MSor the network-initiated GPRS/PS detach is performed.

z This transition is triggered by SGSN when the SGSNreceives a Cancel Location message from the HLR, orwhen the SGSN rejects a

z RA update or an attach request from the MS.

z In UMTS, the PS signaling connection is released afterthis transition.Specifically, both RRC and SCCPconnections are released. In GPRS,

z LLC link is removed after this transition.

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MM and PDP Cont ex t s

zMobility Management (MM) context provides mobilityinformation of an MS.

z Packet Data Protocol (PDP) context providesinformation to support packet delivery between an MS

and the network.

zWhile an MS may be associated with several PDPcontexts, it only has one MM context.

z The MM context is maintained in MS and SGSN.

z The PDP contexts are maintained in MS, SGSN, andGGSN.


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MM Contex t

zThe following fields in the MM context aremaintained in both GPRS and UMTS SGSN:

q IMSI, MM state, P-TMSI, P-TMSI signature,

q International Mobile Equipment Identity (IMEI),

qMobile Station ISDN Number (MSISDN),

q routing area, VLR number, new SGSN address

qMS network access capability,

q authentication triplets, Kc (currently used ciphering key),selected ciphering algorithm, subscribed chargingcharacteristics, and several flags.

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MM SGSN Cont ex t : GPRS vs UMTS

(Loc at ion Inform at ion) (1/2)

zGPRS SGSN maintains cell identity (current cellin READY state, or the last known cell in

STANDBY or IDLE state) and cell identity age

(time elapsed since the last LLC PDU wasreceived from the MS at the SGSN).

zThese two fields are not maintained in UMTS

SGSN because cell tracking is performed by theserving RNC.


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(Loc at ion Inform at ion) (2/2)

z UMTS SGSN maintains the service area code (SAC),and the elapsed time since the last SAC was received atthe SGSN.

z The SAC is used to uniquely identify an area consistingof one or more cells belonging to the same locationarea.

zSAC and the location reporting procedure are used inUMTS for location service (LCS)and other servicessuch as emergency calls .

z These fields are not maintained in GPRS SGSNbecause the concept of SAC does notexist in GPRS.

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(Secur i t y In fo)

z UMTS provides enhanced security functions overGPRS, and thus extra security parameters aremaintained in the UMTS SGSN MM contexts.

z Specifically, UMTS SGSN maintains authentication

vectors, CK (currently used ciphering key), IK (currentlyused integrity key), and KSI (key set identifier).

z GPRS SGSN maintains CKSN (ciphering key sequencenumber of Kc). KSI in UMTS corresponds to CKSN inGSM, and they have the same format.

z The CK parameter in UMTS is equivalent to Kc inGPRS.


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PDP Cont ex t s i n SGSN (1/3)

z PDP route information (PDP context identifier, PDPstate, PDP type, and PDP address)

z Access Point Name (APN) information (APN subscribedand APN in use)

z QoS information (QoS profile subscribed, QoS profilerequested and QoS profile negotiated)

z N-PDU information (GTP-SND and GTP-SNU). TheGTP-SND (GTP-SNU) parameter is the GTP sequencenumber of the N-PDU to be sent from the SGSN to theMS (GGSN).

z Charging information (charging id)

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zOther routing information ( NSAPI, TI, TEID for Gn/Gp,GGSN address in use, and VPLMN address allowed).

z Network layer Service Access Point Identifier (NSAPI) isused by LLC (in GPRS) or RLC (in UMTS) to route the

N-PDUs to appropriate higher layer protocols such assignaling,SMS, or packet data protocols.

z Transaction identifier (TI) is used to represent NSAPI forsome session management signaling messages.

z VPLMN specifies the GPRS/UMTS networks visited bythe MS.


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zSubscribed Charging Characteristics can benormal, prepaid, flat-rate, and /or hot billing.

zIn the early GPRS/UMTS version, chargingcharacteristics for PDP contexts are maintained

in the SGSN.

zIn the latest version, charging characteristics are

included in SGSN MM context.

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PDP SGSN Conte x t : GPRS vs UMT S

(1/2)

zCore Network to Radio Access NetworkConnection

q The UMTS maintains the TEID for the Iu interface and the IP

address of the RNC currently used.

q These two fields are not maintained in the GPRS SGSN

zRadio Resource Information

q The GPRS SGSN maintains radio priority (the RLC/MAC

radio priority level for uplink user data transmission).

q These fields are not kept in UMTS SGSN.


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PDP SGSN Cont ex t : GPRS vs UMT S

(2/2)

zPDU Information

q GPRS SGSN maintains Send N-PDU number (SNDCP

sequence number of the next downlink N-PDU to be sent to

the MS), Receive N-PDU number (SNDCP sequence

number of the next uplink N-PDU to be received from the

MS), packet flow identifier and aggregate BSS QoS profile

negotiated.

q On the other hand, UMTS SGSN maintains PDCP-SND (the

next PDCP sequence number to be sent to the MS) and

PDCP-SNU (the next PDCP sequence number expected

from the MS).

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MM Radio Resourc e In form at ion (1 /2)

z Both GPRS and UMTS, the radio resource informationfor SMS is kept in the MM context, while the radioresource information for user data is maintained in thePDP context.

z The PDP context is defined for data transfer in the userplane. The MM context is defined for mobilitymanagement signaling in the control plane.


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MM Radio Resourc e In form at ion (2 /2)

z SMS is delivered through the control plane by usingcommon channel, which is more efficient than deliverythrough the user plane.

z Through the control plane, the same SMS transferprocedure is used for both CS and PS domains .

z Thus, the radio resource information for SMS is kept inthe MM context.

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MS MM Cont ex t

zIMSI, MM State, P-TMSI, P-TMSI signature

zRouting Area

zMS network access capability

zCKSN/KSI, Ciphering Algorithm

zDRX Parameters


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MS MM Contex t : GPRS vs UMTS

zLocation Information

q GPRS MS maintains cell identity.

q In UMTS, cell tracking is not conducted at the mobility

management layer between the MS and the SGSN.

q Thus, cell identity is not maintained in the MM context of the MS.

Instead, it is maintained between the MS and the UTRAN.

z Security Informationq UMTS MS maintains extra security parameter CK next.

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MS MM Cont ext : GPRS vs UMTS

(Radio Resourc e Info )

zGPRS MS maintains radio priority SMS.

z In UMTS, the SMS as well as signaling are deliveredthrough dedicated control channels.

zThus the radio priority is not maintained in the UMTSMS.

z The GPRS MS maintains the MS radio accesscapability,(e.g., multislot capability and power class)

z The UMTS MS maintains UE capability (e.g., powercontrol, code resource, UE mode, and PDCP capability).


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Relat ionship betw een MM Stat es &

Contex t s (3/3)

zREADY (PMM-CONNECTED)

q In the READY/PMM-CONNECTED state, valid MM contexts

are maintained in the MS and the SGSN.

q As in the STANDBY/PMM-IDLE state, the MS may initiate

PDP context activation and deactivation.

q In this state, the signaling connection is established in

UMTS.

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At t ac h: St ep 1 (1 /3)

zThe MS initiates the attach procedure bysending the Attach Request message to the

SGSN.

zIn GPRS, besides the MS network accesscapability, the message includes parameterssuch as MS radio access capability.

zThese radio related parameters are not includedin UMTS Attach Request message.


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zThe UMTS message includes the ``follow onrequest'' field to indicate if there is pending

uplink traffic that needs Iu connection after theattach procedure is completed.

zThis field is not needed in GPRS because the Iu

interface does not exist.

zFurthermore, the security parameters for UMTSand for GPRS are different.

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zWhen the receives the attach request at the endof Step 1, there are several possibilities:q If the MS has changed SGSN since last detach, then Step 2 is

executed so that the new SGSN can obtain the MS identity (i.e.,IMSI) form the old SGSN.

q If the MS has not changed SGSN, then the received P-TMSI is usedby the SGSN to identify the MM context of the MS.

q If the MM context has not been deleted since last detach (i.e., theMS is known by the new SGSN), then Steps 2-6 are skipped, andStep 7 is executed.

q Otherwise (the MS is not known by the old and the new SGSNs),Step 2 is skipped, and Step 3 is executed.


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At t ach : St ep 2

z The MS is known by the old SGSN.

z The new SGSN sends the Identification Requestmessage to the old SGSN.

z The P-TMSI is used to obtain the IMSI andauthentication information from the old SGSN.

z If the old SGSN cannot find the MM context for the MS,

then Step 3 is executed.

zOtherwise the IMSI is returned to the new SGSN, andStep 4 is executed.

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At t ac h: St eps 3-5

z Step 3. The MS is unknown in both the old and the new SGSNs.

The new SGSN asks the MS to supply IMSI through the Identity

Request and Response messages exchange.

z Step 4. Authentication is mandatory if the MM context of the MS

has been deleted since last detach. The equipment (IMEI) may be

optionally checked.

z Step 5. If the MS has moved from the old SGSN to the new SGSN

since last detach or if the MS is performing the first attach, then the

RA update procedure is executed so that the new SGSN can obtain

the current MM context of the MS.


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At t ach : St ep 6

z If the Gs interface does not exist, then this step isskipped.

zOtherwise (Gs exists), the attach type in Step 1 ischecked.

z If attach type indicates (1) combined PS/CS attach or (2)PS attach and the MS is already CS attach, then LA

update is performed.z The LA update is required so that the SGSN-VLR

association is established and the VLR can maintaincurrent LA information of the MS.

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At t ach : St ep 7

z For GPRS, if attach is successful, then the SGSNselects radio priority SMS and sends the Attach Acceptmessage to the MS.

z P-TMSI is included in the message if the SGSN

allocates a new P-TMSI.

z In UMTS, radio priority SMS is not maintained in mobilitymanagement.

z However, this parameter is still reserved in the UMTSAttach Accept message in order to support handoffbetween UMTS and GSM networks.


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At t ac h: St eps 8 and 9

z Step 8. If P-TMSI or TMSI have been changed, the MSsends the Attach Complete message to the SGSN toacknowledge receipt of the TMSIs.

z Step 9. If TMSI has been changed, the SGSN sends theTMSI Reallocation Complete message to the VLR.

z After PS attach, the MS is in the READY (for GPRS) orthe PMM-CONNECTED (for UMTS) state and MMcontexts are established in the MS and the SGSN.

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Detach

z The PS detach procedures are basically the same for both GPRSand UMTS.

z The network or the MS may request detach explicitly.

z When PS detach is executed, the MS will not receive the SGSN-

based service.z Implicit PS detach is executed by the network (without notifying the

MS) if the mobile reachable timer expires or when radio path isdisconnected due to errors.

z After implicit PS detach is performed, the MS's MM context isdeleted after an implementation dependent timeout period.

z The PS detach procedure also inactivates the PDP contexts.


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Loca t ion Updat e

z In location management, the MS informs the network ofits location through RA and LA update procedures.

z The update procedures are executed in two situations.

z Normal location update is performed when the MSdetects that the location has been changed.

z Periodic location update is exercised even if the MSdoes not move. That is, the MS periodically reports its

``presence'' to the network.

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Per iod ic Loc at ion Updat e (1 /9)

z Periodic RA update allows the network to detect if anMS is still attached to the network.

z A periodic RA update timer is maintained in both the MSand the SGSN.

z Every time this timer expires, the MS performs periodicRA update.

z The periodic RA update timer value is set by the SGSN,and is sent to the MS through the RA Update Accept orthe Attach Accept messages when the MS visits an RA.

z This value cannot be changed before the MS leaves theRA.


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Periodic Loc at ion Up dat e (4/9) : PS/CSat t ac hed MS (CASE I)-1/2

zFor a PS/CS attached MS in Network Mode I,periodic RA update is performed, and LA updatemust not be performed.

zIn this case, the VLR relies on SGSN to receiveperiodic RA updates.

zIf the SGSN detects that the MS is lost, theSGSN detaches the MS, and notifies the VLR ofthis detach by the IMSI Detach Indicationmessage.

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Periodic Loc at ion U pdat e (5/9) : PS/CS

at t ac hed MS (CASE I)-2/2

zFor normal location update, combined RA/LAupdate is performed when the MS changes

locations.

zIn Network Mode II, the RA update (to theSGSN) and LA update (to the VLR) areperformed separately.

zIn this case, the LA update is always performedbefore RA update.


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at t ac hed MS (CASE II)-3/5

zClass A MS (GPRS) or PS/CS MS (UMTS)

q For Network Mode I, if there are inter-SGSN or inter-LA

crossings during CS connection, then at the end of the CS

connection, combined RA/LA update is executed to modify

the SGSN-VLR association.

q For Network Mode II, if there are inter-LA crossings during

CS connection, then at the end of the CS connection, LA

update is performed.

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zClass B MS (GPRS)

q During CS connection, the MS does not execute any RA/LA

updates.

q For Network Mode I, at the end of the CS connection, RA

update is performed if inter-RA crossings occur in CS

connection, and combined RA/LA update is performed if

inter-SGSN or inter-LA crossings occur in CS connection.


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zClass B MS (GPRS)

q For Network Mode II, at the end of the CS connection, RA

update is performed if inter-RA or inter-SGSN crossings

occur in CS connection.

q LA update is performed if inter-LA crossings occur in CS

connection.

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Combi ned RA/LA Up dat e: St ep 1 (1/2)

zThe MS sends the Routing Area UpdateRequest message to the new SGSN.

zThis message is not ciphered so that the new

SGSN can process the message.

zFor both GPRS and UMTS, the update type canbe RA update, periodic RA update, combined

RA/LA update, or combined RA/LA update withIMSI attach.


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z The follow on request' parameter is used in UMTS to indicate if the

Iu connection should be kept for pending uplink traffic. This

parameter does not exist in GPRS.

z In GPRS, before the BSS passes the message to the SGSN, it adds

the cell global identity information (including cell, RA and LA

identities).

z In UMTS, the RNC adds the routing area identity information

(including RA and LA identities).

z For inter-SGSN update, Steps 2-9 are executed. Otherwise (intra-

SGSN update), these steps are skipped.

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z To obtain the MM and PDP contexts of the MS, the newSGSN sends the SGSN Context Request message tothe old SGSN.

z Basically, the old SGSN validates the old P-TMSI

signature, and returns the MM and the PDP contexts ofthe MS using the SGSN Context Response message.

z The old SGSN starts a timer. The MM context in the oldSGSN is deleted when both of the following conditionsare satisfied: the timer expires, and the old SGSNreceives the Cancel Location message from the HLR.


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z This timer mechanism ensures that if the MS initiates another inter-

SGSN routing area update before the current update procedure is

completed, the old SGSN still keeps the MM context.

z In GPRS, the old SGSN stops assigning SNDCP N-PDU numbers

to downlink N-PDUs received. The old SGSN will forward buffered

packets to the new SGSN at Step 4a.

z In UMTS, packet forwarding is not performed between the SGSNs.

Also, the temporary logical link identity (TLLI) included in the GPRSSGSN Context Request message is not found in the UMTS

message.

78

Com bined RA/LA Upda t e : St ep 3

z If the old P-TMSI signature checking at Step 2 fails,security function involving MS, BSS/UTRAN, newSGSN, and the HLR is performed.

z If this security procedure also fails, then the old SGSNcontinues as if the SGSN Context Request message isnever received and this procedure exits.

zOtherwise (security check successes), Step 4 isexecuted.


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Com bined RA/LA Updat e: St eps 8 an d 9

z The HLR inserts the subscriber data to the new SGSN.

z For each PDP context, the new SGSN checks if the context is new,

active, or inactive.

z If the PDP context is active, then extra tasks are performed by the

SGSN.

z For example, the SGSN compares if the received QoS subscribed

value is the same as the value of the QoS negotiated parameter.

z If not, the SGSN should initiate the PDP context modification

procedure to adjust the QoS parameters of the context.

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Combine d RA/LA Upd at e: St eps 10 -12

z Steps 10-12 are executed if the new SGSN detects that the LA hasbeen changed or the update type in Step 1 indicates combinedRA/LA update with IMSI (CS) attach.

z Step 10 (LA Update). Through a table lookup technique, the SGSNtranslated RA identity (RAI) into the VLR number and sends the

Location Update Request message to the VLR (after Step 8 isexecuted). The VLR creates or updates the SGSN-VLR associationby storing the SGSN number.

z Step 11. The standard GSM location update procedure isperformed.

z Step 12. The new VLR allocates a new TMSI and responds withLocation Update Accept to the SGSN. Allocation of TMSI is optionalif the VLR is not changed.


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Combine d RA/LA Upd at e: St eps 13 -15

z Step 13. The new SGSN sends the Routing Area Update Acceptmessage to the MS.

q In GPRS, the new SGSN also confirms all mobile-originated N-PDUssuccessfully transferred before the start of the update procedure.

z Step 14. The MS sends the Routing Area Update Completemessage to the new SGSN to confirm the reallocation of the TMSI.

q In GPRS, the MS also confirms all received mobile-terminated N-PDUsbefore the RA update procedure started.

q This information is used by the new SGSN to check if the packetsforwarded from the old SGSN have been received by the MS. If so, theseredundant packets are discarded.

z Step 15. If a new TMSI has been received by the MS, then theTMSI Reallocation Complete message is sent to the VLR.

84

Comb ined RA/LA Updat e (GPRS vs UMTS)

z In terms of RA update, the major differences betweenUMTS and GPRS are the following:

q In GPRS, packet forwarding is performed between old and newSGSN during RA update.

q In UMTS, packet forwarding is handled at the RNC level, and theSGSN is not involved.

q In the RA update, the UMTS MS may determine if the Iu connectionshould be maintained, which is not needed in GPRS.

z Note that for a pure intra-SGSN RA update, Steps 2-12,14 and 15 are not executed. For a pure inter-SGSN RAupdate, Steps 10-12, and 15 are not executed.


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SRNC and DRNC (2/6)

z Packets are routed between the MS and the GGSN. TheMS communicates with two Node Bs (B1 and B2).

z In WCDMA , an MS is allowed to transmit signal throughmultiple radio paths connected to different Node Bs, andthe signals are merged in a network node (RNC1).

z In a packet routing path between the core network andthe MS, the RNC that directly connects to the SGSN iscalled the serving RNC (SRNC).

86

SRNC and DRNC (3/6)

zWhen the MS moves toward Node B3, the radio linkbetween the MS and B1 is removed due to radio pathloss, and the radio link between B3 and the MS isadded.

z In this case, B3 is connected to RNC2, and an Iur linkbetween RNC1 and RNC2 is established so that thesignal received by B3 can be forwarded to RNC1through RNC2.

z RNC1 then combines the signals from B2 and B3, andforwards it to SGSN1. In this case, RNC1 is the SRNC,and RNC2 is called the drift RNC (DRNC).


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SRNC and DRNC (4/6)

zDRNC transparently routes the data through theIub and the Iur interfaces.

zIt only performs Layer 1 and partial Layer 2functionality (e.g., MAC for common and shared

channels).

zThus, the RLC connections are defined between

the SRNC and the MS, and the DRNC isbypassed.

88

SRNC and DRNC (6/6)

z Suppose that the MS moves away from B2, and theradio link between the MS and B2 is disconnected.

z In this case, the MS does not communicate with anyNode Bs connected to RNC1.

z The routing path is now MS--B3--RNC2--RNC1--

SGSN1--GGSN.

z In this case, it does not make sense to route packets

between the MS and the core network through RNC1.


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PS SRNC Relo c at ion

zSRNC relocation may be performed to removeRNC1 from the routing path.

zSuppose that RNC2 connects to SGSN2. Thenafter RNC relocation, the packets are routed tothe GGSN through RNC2 and SGSN2. At thispoint, RNC2 becomes the serving RNC.

zSRNC relocation may also be executed whenhard handoff occurs (and the MS is in the PMM-CONNECTED state).

90

PS SRNC Reloc at ion w i t h Hard

Han dof f (2/2)

zBefore the relocation, the communication path isGGSN--SGSN1--RNC1--B2--MS, and the MS is

not connected to any Node Bs of RNC2.

zDuring hard handoff and SRNC relocation, theradio link connected to the MS is switched fromB2 to B3.

zAfter the relocation, the communication path isGGSN--SGSN2--RNC2--B3--MS.


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CS SRNC Reloc at ion (2/2)

zThe SRNC relocation procedures for PS andCS services are different.

zBefore CS relocation, the call path is MSC1RNC1--RNC2--MS.

zAfter the relocation, the call path is MSC1--MSC2--RNC2--MS, and MSC1 becomes theanchor MSC.

92

PS SRNC Relo c at ion Proc edu re (1/2)

zThis procedure is only performed for an MS inPMM-CONNECTED state

q Before CS relocation, the call path is MSC1RNC1

RNC2MS.

q After the relocation, the call path is MSC1MSC2RNC2

MS, and MSC1 becomes the anchor MSC.


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PS SRNC Reloc at ion Proc edure : Steps 1

and 2

z Step 1. RNC1 determines that RNC2 is the target forrelocation, and informs SGSN1 of this decision throughthe Relocation Required message.

z Step 2. If both RNC2 and RNC1 are connected to SGSN1 (intraSGSN relocation), Steps 2-4 are skipped, and the relocation

procedure proceeds to Step 5. Otherwise, it is an inter-SGSN

SRNC relocation. In this case, suppose that RNC2 connects to

SGSN2. Then SGSN1 sends the MM and the PDP contexts of the

MS to SGSN2 by using the Forward Relocation Request message.

94

PS SRNC Reloc at ion Proc edure : Steps 3

and 4

z Step 3. SGSN2 and RNC2 exchange the RelocationRequest and Response message pair to establish the Iuuser plane transport bearers between SGSN2 andRNC2, and exchange routing information required for

packet delivery.

z Step 4. SGSN2 sends the Forward RelocationResponse message to SGSN1. The message indicatesthat SGSN2 and RNC2 are ready to receive thedownstream packets buffered in RNC1 (i.e., the packetsthat have not been acknowledged by the MS).


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PS SRNC Relo c at ion : St ep 5

z Step 5. SGSN1 sends the Relocation Commandmessage to RNC1. This message instructs RNC1 to

forward the buffered downstream packets to RNC2.

z Note that Steps 1-5 reserve the core network resources

for the new path. Before Step 6 is executed, the packetsare routed through the old path.

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z RNC1 receives the Relocation Command message, it starts the

data-forwarding timer.

z Expiration of this timer will be checked at Step 12.

zRNC1 sends the Relocation Commit message to RNC2, whichprovides information about the buffered packets (e.g.,

sequence numbers) to be tunneled to RNC2.

z RNC1 stops exchanging packets with the MS, and forwards the

buffered packets (which are sent from GGSN) to RNC2.

z RNC2 switches all bearers from the RNC1 to the SGSN.


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z Immediately after RNC2 is successful switched atStep 6, RNC2 sends Relocation Detect message to

SGSN2.

z The purpose of this message is to inform the SGSN2

that RNC2 is starting the SRNC operation, and theCN should switch the packet routing path from

RNC1 to RNC2.

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PS SRNC Reloc at ion : St ep 8 (1/2)

z RNC2 restarts the RLC connections.

z RNC2 and the MS exchange information to identify the last

upstream packets received by RNC2 and the last downstream

packets received by the MS. This is achieved by exchanging

the RNTI Reallocation and Complete message pair.

z In the RNTI Reallocation message, RNC2 provides RA, LA, and

possibly RRC information.

z Since the RA has been changed, the MS also triggers the RA

update procedure shown at Step 13.


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PS SRNC Reloc at ion : St ep 8 (2/2)

z After the MS has reconfigured itself, it sends the RNTI ReallocationComplete message to the RNC2.

z The packet exchange with the MS can start.

z Note that the message RNTI Reallocation is not found in 3GPPspecifications (specifically, 3GPP 25.331 ). Instead, we found thattwo messages, UTRAN Mobility Information and UTRAN MobilityInformation Confirm, can be used by UTRAN to allocate a new

RNTI and to convey other UTRAN mobility related information to anMS.

z Note that in Steps 6-8, the UTRAN connection point is moved fromRNC1 to RNC2.

q In this period, packet exchange between the MS and network is stoppedfor lossless relocation.

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z After Step 7, the SGSN2 switches the user plane fromRNC1 to RNC2.

z For inter SGSN SRNS relocation, SGSN2 and everycorresponding GGSN exchange the Update PDPContext Request and Response message pair to modifythe GGSN address, SGSN TEID,and QoS profilenegotiated stored in the GGSN PDP context.

z This operation switches the GGSN connection fromSGSN1 to SGSN2.


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Combined Hard Handof f w i th SRNC

Relocat ion

zThe message flow is similar to the PS SRNCRelocation with the following differences.

q The SRNC relocation procedure (without hard handoff) is initiated

by RNC1 without involving the MS.

q For combined hard handoff with SRNC relocation, at the

beginning, RNC1 decides that the MS is involved, and the MSreconfigures physical channel immediately after Step 5. Thus,

RNTI relocation at Step 8 is not needed.

q Also, in the combined procedure, the SRNC context (of RNC1)must be forwarded through the path SGSN1--SGSN2--RNC2.

104

UMTS-GPRS Int ersy st em Chang e

z When a GPRS/UMTS dual mode MS moves from a cell supportingGSM/GPRS radio technology to a cell supporting WCDMA radiotechnology (or vice versa), a UMTS-GPRS intersystem change maytake place.

z To provide this feature, mechanisms should exist to derive the area

identities (for LA, RA, and cell) and the routing-related informationfrom one system to another.

z We describe UMTS-GPRS intersystem change using simpleexamples where the GSM/GPRS cells and the UMTS cells areconnected to the same SGSN.

z In this case, the SGSN supports both the Gb and IuPS interfaces.


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UMTSGPRS

zFor SGSN change from UMTS to GPRS, if theMS is in the PMM-IDLE state, then the normalGPRS RA update procedure is executed.

zIf the MS makes the intersystem change

decision when it is in the PMM-CONNECTED

state, then it stops the transmission to thenetwork, and the following steps are executed

for intra SGSN change.

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UMTSGPRS: St ep 1

[Step 1.] An LLC link is established between the MS and

the SGSN.

z The MS sends the Routing Area Update Request

message to the SGSN through the new BSS.

z This step is exactly the same as Step 1 of normal RA

update procedure initiated by a GPRS MS.


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UMTSGPRS: St ep 2

[Step 2.] The SGSN exchanges the SRNS Context Request andResponse messages with the old SRNS to obtain the followinginformation:

z GTP-SND and GTP-SNU are used to resume transmission to theGGSN.

z PDCP-SNU is used to resume transmission to the MS for loss-lessrelocation.

z The SGSN converts the PDCP sequence number into the SNDCPsequence number and saved it in the GPRS PDP context.

z The SRNS stops sending packets to the MS, and starts bufferingthe packets received from the GGSN.

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UMTSGPRS: St eps 3 a nd 4

[Step 3.] Security functions may be executed as in Step 3 of RAUpdate procedure.

z If the MS is not allowed to attach in the RA, or if subscriptionchecking fails, then the SGSN rejects this RA update.

[Step 4.] At this point, the SGSN is ready to receive packets.

z The SGSN sends the SRNS Data Forward Command message tothe old SRNS, which instructs the SRNS to forward the bufferedpackets to the SGSN.

z The SRNS starts a data-forwarding timer. Before this timer expires,the Iu connection between the SRNS and the SGSN will bemaintained (see Step 6).


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[Step 5.] For the packets received by the old SRNS from the SGSN,

but have not been sent to the MS, the packets are tunneled back

from the SRNS to the SGSN.

[Step 6.] When the SGSN timer set at Step 4 expires, the Iu Release

Command and Complete messages are exchanged to release the

Iu connection.

z If the type parameter in the Routing Area Update Request message

at Step 1 is combined RA/LA update (for Network Mode I), or if the

LA has been changed, then the SGSN triggers LA update that

involves the SGSN, new VLR, old VLR and the HLR.

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[Step 7.] The SGSN updates the MM and PDP contexts. New P-TMSI

and new TMSI may be allocated.

z The SGSN sends the Routing Area Update Accept message to the

MS.

[Step 8.] The MS returns the Routing Area Update Complete message

to the SGSN if a new P-TMSI is allocated or if the MS needs to

acknowledge the packets received from the network.

z If a new TMSI is allocated to the MS, then the SGSN sends a TMSI

Reallocation Complete message to the new VLR (this message is

not shown).


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GPRSUMTS

z For SGSN change from GPRS to UMTS, if the MS is inthe STANDBY state, then the normal UMTS RA update

procedure is executed.

z If the MS makes the intersystem change decision when

it is in the READY state, then it stops the transmission tothe network by disconnecting the LLC link.

112

GPRSUMTS: St eps 1 an d 2

z [Step 1.] The MS establishes an RRC connection to thenew SRNS, and sends the Routing Area UpdateRequest to the SGSN through the SRNS.

z [Step 2.] The SGSN stops the transmission to the oldBSS. The security functions may be executed amongSGSN, SRNS, and MS.

q If the type parameter in the Routing Area Update Request message at Step1 is combined RA/LA update (for Network Mode I), or if the LA has beenchanged, then the SGSN triggers LA update that involves the SGSN, newVLR, old VLR and the HLR.


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GPRSUMTS: St ep 3

z[Step 3.] The SGSN updates the MM and PDPcontexts for the MS. A new P-TMSI may be

allocated.

q The SGSN sends the Routing Area Update Accept message

to the MS.

q Reception of the new P-TMSI is acknowledged by the MS

through the Routing Area Update Complete message.

q If a new TMSI is allocated to the MS, then the SGSN sends a

TMSI Reallocation Complete message to the new VLR (this

message is not shown).

114

GPRSUMT S: St eps 4-6

z [Step 4.] If the MS has pending uplink data or signaling,it sends the Service Request message to the SGSN.

z [Step 5.] The SGSN requests the SRNS to set up theradio bearer between the SRNS and the MS. The N-

PDU sequence numbers in GPRS PDP context of theSGSN is used to derive PDCP sequence numbers forthe next packets to be delivered in the UTRAN radiobearer.

z [Step 6.] Packet transmission is resumed betweenSGSN, SRNS, and MS


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GPRSU MT S v s U MT SGPRS

zA major difference between the two messageflows is that packet forwarding is not required in

intra SGSN change from GPRS to UMTS. Thereason is that in GPRS, the packets are

buffered in SGSN.

zNote that for inter SGSN change from GPRS toUMTS, packet forwarding will occur from the oldSGSN to the new SGSN.

Summary

z This talk describes mobility management evolution from GPRS toUMTS.

z In the UMTS architecture, the radio access network UTRAN isintroduced. Most radio management functions handled in GPRScore network have been moved to UTRAN in UMTS.

z The GPRS mobility management functionality has been significantlymodified to accommodate UMTS. We pronounced the differencesbetween the GPRS and the UM- TS procedures.

z For further reading for UMTS core network architecture, the readeris referred to 3GPP 23.121. The UMTS protocol stacks areintroduced in 3GPP 23.060. The complete 3GPP specifications canbe found in www.3gpp.org.

Documents

09 UMTS System