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GPRS Signalling Messages and Procedures on the Air interface 1

1MAT, St, Version 1.0

GPRS Signalling messages on GRR, GMM andSM

Dipl.-Ing. Reiner Stuhlfauth, ROHDE&SCHWARZ, Training Center Munich

Summary

GPRS introduces a new kind of signalling to the existing GSM network. There will be anextension of the existing circuit switched protocol plane with a new one, the packet switchedprotocol plane. Therefore some new protocols are included in the GPRS network. Thefollowing figure summarizes the protocol plane between MS and network:

Figure: GPRS protocol plane between MS and network

The SNDCP layer is responsible for mapping the characteristics of the network level onto theunderlying protocols, which are responsible for data transfer over the air interface.Furthermore, the SNDCP multiplexes the data of different applications, the so called PDPcontexts, onto the one transmitting channel created by an established radio link.

The LLC layer forms a logical link for data transfer between network and Mobile Station, MS.What is special about this data transfer link, is that it is completely independent from theunderlying protocols describing or configuring the air interface. In addition, LLC offers someadjacent features like ciphering, acknowledged or unacknowledged data transfer or like in-order-delivery.

Um Network

SNDCP

LLC(Note)

RLC

MAC

Phys. Link

Phys. RF

SNDCP

LLC

RLC

MAC

Phys. Link

Phys. RF

MS

Maps network level characteristics onto

characteristics ofunderlying protocols Logical Link between MS

and SGSN. Independentfrom Radio InterfaceRLC

Reliable link + Radio Link Controlfunctions

MACAccess Control signalling+Mapping of LLC-Framesonto physical channels

Data Coding, Burst Forming,Interleaving, ConvolutionalCoding = Error protection

Modulation, Timingand RF-Generation

RLC = Radio Link ControlMAC = Medium Access Control

SNDCP = SubNetwork Dependent Convergence ProtocolLLC = Logical Link ControlLLC = Logical Link Control



The RLC layer is responsible for establishing a radio link, for maintaining it or for releasing it.This protocol layer specifies a list of signalling messages to be exchanged between MS andthe network to fulfill some tasks concerning the radio link. In this chapter the messages will bedescribed in detail.

The main task of the MAC layer is to organize the access to the radio network. One of itsmain parameters is, for example, the USF value field which identifies the right user of thesucceeding radio block.

The Physical link layer will build the transportation packets, i.e. the radio blocks, and will formthe used GSM Normal Bursts. The training sequence will be included, the interleavingprocedure will be done and the burst will be structured. This layer more or less describes thesame features that are already known from a GSM network, the only difference is the blockdata encoding. While in GSM there was an encoding of speech data onto a Normal Burst, inGPRS there will be an encoding of user data, depending on the used Coding scheme.

The Physical RF layer basically describes the air interface with its modulation scheme, outputpower control, frequency channel and timeslot allocation characteristics. Most of itsparameters are already known in the GSM air interface. The only difference to GSM will be ina future development step the transition to the new modulation scheme 8PSK.



Signalling messages of GPRS Radio Resource management,GRR:

The GRR uses a list of signalling messages to establish, maintain or release a radio linkbetween MS and the network. The used messages of the RLC/MAC layer are the following:

GRR Signalling Messages (Via RLC/MAC)

Downlink Messages

• Packet Access Reject

• Packet Cell Change Order

• Packet Downlink Assignment

• Packet Paging Request

• Packet PDCH Release

• Packet Polling Request

• Packet Power Control/Timing Advance

• Packet PRACH Parameters

• Packet Queueing Notification

• Packet Timeslot Reconfigure

•Packet TBF Release

•Packet Uplink Ack / Nack

•Packet Uplink Assignment

•Packet Downlink Dummy Control Block

•Packet System Information Type 1



•Packet System Information Type 3 bis




GRR Signalling Messages (Via RLC/MAC)

Uplink Messages

• Packet Cell Change Failure

• Packet Control Acknowledgement

• Packet Downlink Ack / Nack

• Packet Uplink Dummy Control Block

• Packet Measurement Report

• Packet Resource Request

• Packet Mobile TBF Status (SMG #30+)

• Packet PSI Status (SMG #30+)



But how do we distinguish between the different messages? In the RLC / MAC block we finda field “message type”. This field indicates which of the following messages is sent.

< Downlink RLC/MAC control message > ::=< MESSAGE_TYPE : bit (6) == 1 00001 > < Packet Access Reject message content > |< MESSAGE_TYPE : bit (6) == 0 00001 > < Packet Cell Change Order message content > |< MESSAGE_TYPE : bit (6) == 0 00010 > < Packet Downlink Assignment message content > |< MESSAGE_TYPE : bit (6) == 0 00011 > < Packet Measurement Order message content > |< MESSAGE_TYPE : bit (6) == 1 00010 > < Packet Paging Request message content > |< MESSAGE_TYPE : bit (6) == 1 00011 > < Packet PDCH Release message content > |< MESSAGE_TYPE : bit (6) == 0 00100 > < Packet Polling Request message content > |< MESSAGE_TYPE : bit (6) == 0 00101 > < Packet Power Control/Timing Advance message content > |< MESSAGE_TYPE : bit (6) == 1 00100 > < Packet PRACH Parameters message content > |< MESSAGE_TYPE : bit (6) == 0 00110 > < Packet Queueing Notification message content > |< MESSAGE_TYPE : bit (6) == 0 00111 > < Packet Timeslot Reconfigure message content > |< MESSAGE_TYPE : bit (6) == 0 01000 > < Packet TBF Release message content > |< MESSAGE_TYPE : bit (6) == 0 01001 > < Packet Uplink Ack/Nack message content > |< MESSAGE_TYPE : bit (6) == 0 01010 > < Packet Uplink Assignment message content > |< MESSAGE_TYPE : bit (6) == 1 00101 > < Packet Downlink Dummy Control Block message content > |< MESSAGE_TYPE : bit (6) == 1 10001 > < PSI1 message content > |< MESSAGE_TYPE : bit (6) == 1 10010 > < PSI2 message content > |< MESSAGE_TYPE : bit (6) == 1 10011 > < PSI3 message content > |< MESSAGE_TYPE : bit (6) == 1 10100 > < PSI3 bis message content > |< MESSAGE_TYPE : bit (6) == 1 10101 > < PSI4 message content > |< MESSAGE_TYPE : bit (6) == 1 10110 > < PSI5 message content > |< MESSAGE_TYPE : bit (6) == 1 10000 > < PSI6 message content > |< MESSAGE_TYPE : bit (6) == 1 11000 > < PSI7 message content > |< MESSAGE_TYPE : bit (6) == 1 11001 > < PSI8 message content > |< MESSAGE_TYPE : bit (6) == 1 10111 > < PSI13 message content > ! < Unknown message type : bit (6) = < no string > < Default downlink message content > > ;

< Uplink RLC/MAC control message > ::=< MESSAGE_TYPE : bit (6) == 000000 > < Packet Cell Change Failure message content > |< MESSAGE_TYPE : bit (6) == 000001 > < Packet Control Acknowledgement message content > |< MESSAGE_TYPE : bit (6) == 000010 > < Packet Downlink Ack/Nack message content > |< MESSAGE_TYPE : bit (6) == 000011 > < Packet Uplink Dummy Control Block message content > |< MESSAGE_TYPE : bit (6) == 000100 > < Packet Measurement Report message content > |< MESSAGE_TYPE : bit (6) == 001010 > < Packet Enhanced Measurement Report message content > |< MESSAGE_TYPE : bit (6) == 000101 > < Packet Resource Request message content > |< MESSAGE_TYPE : bit (6) == 000110 > < Packet Mobile TBF Status message content > |< MESSAGE_TYPE : bit (6) == 000111 > < Packet PSI Status message content > |< MESSAGE_TYPE : bit (6) == 001000 > < EGPRS Packet Downlink Ack/Nack message content > |

< MESSAGE_TYPE : bit (6) == 001001 > < Packet Pause message content > |< MESSAGE_TYPE : bit (6) == 001010 > < Additional MS Radio Access Capabilities message content>;



The description of the single messages is given as follows, in chronological order,corresponding to the message type field:

Packet Access RejectThis message is sent on the PCCCH or PACCH by the network to the mobile station toindicate that the network has rejected the MSs access request. This message may containfields addressing more than one mobile station. The direction of this message is network toMS.

Packet Control AcknowledgementThis message is sent on the PACCH from the mobile station to the network. The message isformatted either as an RLC/MAC control block or as 4 identical access bursts. If sent as aresponse to a Packet Polling Request message, this latter message shall specify the formatof the Packet Control Acknowledgement message. Otherwise, the System Informationparameter CONTROL_ACK_TYPE indicates which format the mobile station shall use.The access burst format is 11 bits or 8 bits long. If the System Information parameterACCESS_BURST_TYPE indicates 11-bit access, the mobile station shall transmit the 11-bitformat. If the System Information parameter ACCESS_BURST_TYPE indicates 8-bit access,the mobile station shall transmit the 8-bit format. The mobile station shall transmit the accessburst four times, once in each TDMA frame of the uplink radio block.This message is sent in the direction of mobile station to network.

Packet Cell Change FailureThis message is sent on the PACCH from the mobile station to the network to indicate that acommanded cell change order has failed.

Packet Cell Change OrderThis message is sent on the PCCCH or PACCH by the network to the mobile station to tellthe mobile station to leave the current cell and change to a new cell. This message is theGPRS replacement for the Handover command (HND_CMD) message in GSM.

Packet Channel RequestThis message is sent in random mode on the PRACH. It does not follow the basic format of aGSM Normal Burst. For this message, the MS will use the short Access Burst, just as knownin GSM. The possible formats of this burst are 8 or 11-bit format. The total length of this burstis still the same, only the synchronisation sequence of this burst will be shorter, while themessage length will be enhanced to 11 bit instead of 8 bit length which is already known.The message is 11 bits or 8 bits long. If the System Information parameterACCESS_BURST_TYPE indicates 11-bit access, the mobile station shall transmit the 11-bitformat. If the System Information parameter ACCESS_BURST_TYPE indicates 8-bit access,the mobile station shall transmit the 8-bit format.

EGPRS Packet Channel RequestThis message is sent by EGPRS capable MSs in cells supporting EGPRS and using 11 bitACCESS BURST TYPE. This message is sent to perform EGPRS one-phase accessrequest, EGPRS short access request or EGPRS two-phase access request. For all otherpurposes (page response, cell update etc.) the standard PACKET CHANNEL REQUESTmessage shall be used.



Packet Downlink Ack/NackThis message is sent on the PACCH from the mobile station to the network to indicate thestatus of downlink RLC data blocks received and to report the channel quality of the downlink.The mobile station may optionally initiate an uplink TBF.

EGPRS Packet Downlink Ack/NackThis message is sent on the PACCH from the mobile station to the network to indicate thestatus of downlink RLC data blocks received and to report the channel quality of the downlink.The mobile station may optionally initiate an uplink TBF or request a temporary suspension ofthe downlink TBF.

Packet Downlink AssignmentThis message is sent on the PCCCH or PACCH by the network to the mobile station toassign downlink resources to the mobile station.For a mobile station assigned to operate in the fixed allocation MAC mode, the network mayassign regularly repeating intervals during which the mobile station shall measure neighbourcell power levels. A mobile allocation or reference frequency list received as part of thisassignment message shall be valid until a new assignment is received or each TBF of the MSis terminated.

Packet Downlink Dummy Control BlockThis message is sent on the PCCCH or PACCH by the network to the mobile station as a fillmessage with either of the optional parameters PAGE_MODE and PERSISTENCE_LEVEL,or with no content.

Packet Uplink Dummy Control BlockThis message is sent on the PACCH from the mobile station to the network when the mobilestation has no other block to transmit.

Packet Measurement ReportThis message is sent on the PACCH from the mobile station to the network to reportmeasurement results. The message may contain measurement results from the NetworkControl measurements or from the Extended measurements, but not both simultaneously.More than one message may be required depending on the number of measurements toreport.

Packet Measurement OrderThis message is sent on the PCCCH or PACCH by the network to a mobile station givinginformation for NC and EXT measurement reporting and network controlled cell reselection. Ifnot all information fits into one message, the remaining information will be sent in otherinstances of the Packet Measurement Order message.

Packet Mobile TBF StatusThis message is sent from the mobile station to the network on the uplink PACCH to indicateerroneous messages have been received relating to either a downlink or an uplink TBF.



Packet Enhanced Measurement ReportThis message is sent either on the PACCH, if in packet transfer mode, or on an assignedblock on a PDTCH, from the mobile station to the network to report enhanced measurementresults. The message contains measurement results from the Network Controlmeasurements. More than one message may be required depending on the number ofmeasurements to report.

Packet Paging RequestThis message is sent on the PCCCH by the network to trigger channel access by up to fourmobile stations, for either TBF or RR connection establishment. It may also be sent onPACCH to a mobile station in packet transfer mode to indicate page request for RRconnection establishment. The mobile stations are identified by either IMSI, TMSI or P-TMSI.Depending on the method used to identify the mobile station, 1 - 4 mobile stations can beaddressed in the message. Special requirements for the transmission of this message onPACCH applies, see specifications GSM 05.02.

Packet PDCH ReleaseThis message is sent on PACCH by the network to notify all mobile stations listening to thatPDCH that one or more PDCHs will be immediately released and become unavailable forpacket data traffic.

Packet Polling RequestThis message is sent on the PCCCH or PACCH by the network to the mobile station to solicita PACKET CONTROL ACKNOWLEDGEMENT message from the mobile station.

Packet Power Control/Timing AdvanceThis message is sent on PACCH by the network to the mobile station in order to update themobile station timing advance or power control parameters.

Packet PRACH ParametersThis message is sent on the PCCCH by the network to all mobile stations within the cell toupdate the PRACH parameters in between Packet System Information messages containingPRACH parameters.

Packet Queueing NotificationThis message is sent on the PCCCH by the network to the mobile station to notify the mobilestation that it is being placed in a queue. The message allocates a Temporary QueueingIdentity to the mobile station.

Packet Resource RequestThis message is sent on the PACCH by the mobile station to the network to request a changein the uplink resources assigned.

Packet PSI StatusThis message is sent on the PACCH from the mobile station to the network to indicate whichPSI messages the mobile station has received.



Packet System Information Type 1This message is sent by the network on the PBCCH or PACCH giving information for Cellselection, for control of the PRACH, for description of the control channel(s) and optionalglobal power control parameters.

Packet System Information Type 2This message is sent by the network on PBCCH and PACCH giving information aboutreference frequency lists, cell allocation, GPRS mobile allocations and PCCCH descriptionsbeing used in the cell.PSI2 also contains Non-GPRS cell options applicable for non-packet access.

Packet System Information Type 3This message is sent by the network on the PBCCH or PACCH giving information about theBCCH allocation (BA_GPRS) in the neighbour cells and cell selection parameters for servingcell and non-serving cells.

Packet System Information Type 3 bisThis message is sent by the network on the PBCCH and PACCH giving information about theBCCH allocation in the neighbour cells and cell selection parameters for non-serving cells.

Packet System Information Type 4This message is optionally sent by the network on the PBCCH and PACCH giving informationdirecting the mobile station to make interference measurements.

Packet System Information Type 5This optional message is sent by the network on the PBCCH giving information formeasurement reporting and network controlled cell reselection.

Packet System Information Type 6This optional message is sent by the network on the PBCCH or PACCH to provide broadcastinformation required by non-GSM networks.

Packet System Information Type 7This optional message is sent by the network on the PBCCH or PACCH to provide broadcastinformation required by non-GSM networks.

Packet System Information Type 8This message is optionally sent by the network on the PBCCH and PACCH giving informationabout Cell Broadcast Channel configuration.

Packet System Information 13This message may be broadcast by the network on the PACCH or on the PCCCH. Themessage provides the mobile station with GPRS cell specific access-related information.

Packet TBF ReleaseThis message is sent on the PACCH by the network to the mobile station to initiate release ofan uplink or downlink TBF.



Packet Uplink Ack/NackThis message is sent on the PACCH by the network to the mobile station indicating the statusof the received RLC data blocks. This message may also update the timing advance andpower control parameters. A fixed allocation mobile station may also be assigned uplinkresources.

Packet Uplink AssignmentThis message is sent on the PCCCH or PACCH by the network to the mobile station toassign uplink resources. The mobile station may be addressed by TFI (=Temporary FlowIdentifier), TQI (=Temporary Queing Identifier), or Packet Request Reference, dependingupon the procedure used. A mobile allocation or reference frequency list received as part ofthis assignment message shall be valid until a new assignment is received or each TBF of theMS are terminated.

Packet Timeslot ReconfigureThis message is sent on the PACCH by the network to the mobile station to assign uplinkand/or downlink resources. A mobile allocation or reference frequency list received as part ofthis assignment message shall be valid until a new assignment is received or each TBF of theMS are terminated.

Additional MS Radio Access CapabilitiesThis message is sent on the PACCH by the mobile station to the network to inform aboutradio access capabilities of the mobile station.



Signalling messages for GPRS Mobility Management

Ready Timer procedureThe value of the READY timer may be negotiated between the MS and the network using theGPRS attach or GPRS routing area updating procedure.The READY timer, T3314 is used in the MS and in the network per each assigned P-TMSI tocontrol the cell updating procedure. When the READY timer is running or has beendeactivated, the MS shall perform cell update each time a new cell is selected. When the READY timer has expired the MS shall:

- perform the routing area updating procedure when a routing area border is crossed;

- not perform a cell update when a new cell is selected.

All other GMM procedures are not affected by the READY timer.

Periodic routing area updatingPeriodic routing area updating is used to periodically notify the availability of the MS to thenetwork. The procedure is controlled in the MS by the periodic RA update timer, T3312. Thevalue of timer T3312 is sent by the network to the MS in the messages ATTACH ACCEPTand ROUTING AREA UPDATE ACCEPT. The value of the timer T3312 shall be unique withinan RA.The timer T3312 is reset and started with its the initial value when the READY timer isstopped or expires. The timer T3312 is stopped and shall be set to its initial value for the nextstart when the READY timer is started. If after a READY timer negotiation the READY timervalue is set to zero, the timer T3312 is reset and started with its initial value .If the initialREADY timer value is zero, the timer T3312 is reset and started with its initial value when theROUTING AREA UPDATE REQUEST message is transmitted.When timer T3312 expires, the periodic routing area updating procedure shall be started andthe timer shall be set to its initial value for the next start.

GPRS attach procedureThe GPRS attach procedure is used for two purposes:

- normal GPRS attach, performed by the MS to IMSI attach for GPRS services only. Thenormal GPRS attach procedure shall be used by GPRS MSs in MS operation mode C,independent of the network operation mode. It shall also be used by GPRS MSs in MSoperation modes A or B if the network operates in network operation mode II or III .

- combined GPRS attach procedure, used by GPRS MSs in MS operation modes A or Bto attach the IMSI for GPRS and non-GPRS services provided that the network operatesin network operation mode I.

With a successful GPRS attach procedure a GMM context is established.



GPRS detach procedureThe GPRS detach procedure is used:- to detach the IMSI for GPRS services only. Independent of the network operationmode, this procedure is used by all kind of GPRS MSs;- as a combined GPRS detach procedure used by GPRS MSs operating in MSoperation mode A or B to detach the IMSI for GPRS and non-GPRS services, or for non-GPRS services only, if the network operates in network operation mode I; or- in the case of a network failure condition to indicate to the MS that a re-attach withsuccessive activation of previously active PDP contexts shall be performed.After completion of a GPRS detach procedure or combined GPRS detach procedure forGPRS and non-GPRS services, the GMM context is released.The GPRS detach procedure shall be invoked by the MS if the MS is switched off, the SIMcard is removed from the MS or if the GPRS or non-GPRS capability of the MS is disabled.The procedure may be invoked by the network to detach the IMSI for GPRS services. TheGPRS detach procedure causes the MS to be marked as inactive in the network for GPRSservices, non-GPRS services or both services.If the GPRS detach procedure is performed, the PDP contexts are deactivated locally withoutpeer to peer signalling between the SM and LLC entities in the MS and the network.

MS initiated GPRS detach procedure initiationThe GPRS detach procedure is initiated by the MS by sending a DETACH REQUESTmessage. The detach type information element may indicate “GPRS detach with switchingoff”, “GPRS detach without switching off”, “IMSI detach”, “GPRS/IMSI detach with switchingoff” or “GPRS/IMSI detach without switching off”. If the MS is not switched off, timer T3321shall be started after the DETACH REQUEST message has been sent. If the detach typeinformation element value indicates “IMSI Detach” the MS shall enter GMM-REGISTERED.IMSI-DETACH_INITIATED, otherwise the MS shall enter the state GMM-DEREGISTERED-INITIATED. If the detach type information element value indicates “IMSIDetach” or “GPRS/IMSI Detach”, state MM IMSI DETACH PENDING is entered.

Network initiated GPRS detach procedure initiationThe network initiates the GPRS detach procedure by sending a DETACH REQUESTmessage to the MS. The DETACH REQUEST message shall include a detach type IE. Inaddition, the network may include a cause IE to specify the reason for the detach request. Thenetwork shall start timer T3322. If the detach type IE indicates “re-attach not required” or “re-attach required", the network shall deactivate the PDP contexts and deactivate the logicallink(s), if any, and shall change to state GMM-DEREGISTERED-INITIATED.

Routing area updating procedureThis procedure is used for:

- normal routing area updating to update the registration of the actual routing area of anMS in the network. This procedure is used by GPRS MSs in MS operation mode C andby GPRS MSs in MS operation modes A or B that are IMSI attached for GPRS and non-GPRS services if the network operates in network operation mode II or III;

- combined routing area updating to update the registration of the actual routing andlocation area of an MS in the network. This procedure is used by GPRS MSs in MSoperation modes A or B that are IMSI attached for GPRS and non-GPRS servicesprovided that the network operates in network operation mode I; or



- periodic routing area updating. This procedure is used by GPRS MSs in MS operationmode C and by GPRS MSs in MS operation modes A or B that are IMSI attached forGPRS or for GPRS and non-GPRS services independent of the network operationmode;

- IMSI attachment for non-GPRS services when the MS is IMSI attached for GPRSservices. This procedure is used by GPRS MSs in MS operation modes A or B, if thenetwork operates in network operation mode I.

P-TMSI reallocation procedureA temporary mobile station identity for GPRS services, the Packet-TMSI (P-TMSI), is used foridentification within the radio interface signalling procedures. The structure of the P-TMSI isspecified in GSM 03.03 [10]. The P-TMSI has significance only within a routing area. Outsidethe routing area it has to be combined with the routing area identification (RAI) to provide foran unambiguous identity.The purpose of the P-TMSI reallocation procedure is to provide identity confidentiality, i.e. toprotect a user against being identified and located by an intruder.Usually, P-TMSI reallocation is performed at at least each change of a routing area. (Suchchoices are left to the network operator).The reallocation of a P-TMSI is performed by the unique procedure defined in this section.This procedure can only be initiated by the network in state GMM-REGISTERED.

Authentication and ciphering procedureThe purpose of the authentication and ciphering procedure is threefold:- to permit the network to check whether the identity provided by the MS is acceptable or

not.- to provide parameters enabling the MS to calculate a new GPRS ciphering key; and- to let the network set the ciphering mode (ciphering/no ciphering) and algorithm.

The authentication and ciphering procedure can be used for either:- authentication only;- setting of the ciphering mode and the ciphering algorithm only; or- authentication and the setting of the ciphering mode and the ciphering algorithm.The authentication and ciphering procedure is always initiated and controlled by the network.

Identification procedureThe identification procedure is used by the network to request an MS to provide specificidentification parameters to the network e.g. International Mobile Subscriber Identity,International Mobile Equipment Identity



Paging for GPRS services

Paging is used by the network to identify the cell the MS has currently selected, or to promptthe mobile to re-attach, if necessary, as a result of network failure. If the MS is not GPRSattached when it receives a paging for GPRS services, the MS shall ignore the paging.

Paging for GPRS services using P-TMSI:The network shall initiate the paging procedure for GPRS services using P-TMSI when GMMsignalling messages or user data is pending to be sent to the MS while the Mobile Reachabletimer is running. The network may page only GPRS MSs which are GMM-REGISTERED andidentified by a local P-TMSI.

Paging for GPRS services using IMSI:Paging for GPRS services using IMSI is an abnormal procedure used for error recovery in thenetwork.

The network may initiate paging using IMSI if the P-TMSI is not available due to a networkfailure.

GPRS Session management

The main function of the session management (SM) is to support PDP context handling of theuser terminal. The SM comprises procedures for- identified PDP context activation, deactivation and modification; and- anonymous PDP context activation and deactivation.SM procedures for identified access can only be performed if a GMM context has beenestablished between the MS and the network. If no GMM context has been established, theMM sublayer has to initiate the establishment of a GMM context by use of the GMMprocedures as described in chapter 4. After GMM context establishment, SM uses servicesoffered by GMM. Ongoing SM procedures are suspended during GMM procedure execution.For anonymous access, no GMM context is established.

Session management states

In this section, the SM states are described for one SM entity. Each SM entity is associatedwith one PDP context. The following picture describes the SM states in the MS and on thenetwork side.



Session management states in the MS

In this section, the possible states of an SM entity in the mobile station are described. Asillustrated in the figure, there are four SM states in the MS.

PDP-INACTIVE

This state indicates that no PDP context exists.

PDP-ACTIVE-PENDING

This state exists when PDP context activation is requested by the MS.

PDP-INACTIVE-PENDING

This state exists when deactivation of the PDP contexts is requested by the MS.

PDP-ACTIVE

This state indicates that the PDP context is active.

PDP-ACTIVE-PENDING

PDP-ACTIVE

PDP-INACTIVE


DR (ACTIV. PDP CONTX. REQ)

DI (ACTIV. PDP CONTX. REJ) DI (DEACTIV. PDP CONTX. REQ)

DR: GMMSM-DATA-REQUEST (Message), i.e. message sent by an MSDI: GMMSM-DATA-INDICATION (Message), i.e. message received by an MS

DI (DEACTIV. PDP CONTX. REQ)

DR (DEACTIV. PDPCONTX. ACC)

DI (DEACTIV. PDP CONTX. ACC)

DR (DEACTIV. PDPCONTX. ACC)

DI (REQ PDP CONTX. ACTIV)

DR (MOD PDP CONTXT ACC)

DI (ACTIV. PDP CONTX. ACC)

DR (ACTIV. PDP CONTX. REQ)

DR (DEACTIV. PDP CONTX. REQ)

DI (MOD PDP CONTXT REQ)

Figure: Session management states in the MS (overview)



Session management states on the network side

In this section, the possible states of an SM entity on the network side are described. Asillustrated in figure 6.2/GSM 04.08, there are five SM states on the network side.

PDP-INACTIVE

This state indicates that the PDP context is not active.

PDP-ACTIVE-PENDING

This state exists when the PDP context activation is initiated by the network.


This state exists when deactivation of the PDP context is requested by the network.

PDP-ACTIVE

This state indicates that the PDP context is active.

PDP-MODIFY-PENDING

This state exists when modification of the PDP context is requested by the network.

DR: GMMSM-DATA-REQUEST (Message), i.e. message sent by networkDI: GMMSM-DATA-INDICATION (Message), i.e. message received by the network

PDP-INACTIVE-PEND

DR (DEACTIV. PDP CONTX. REQ) DI (DEACTIV. PDP CONTX. ACC)

DI (DEACTIV. PDP CONTX. REQ)

DR (DEACTIV. PDP CONTX. ACC)

DI (ACTIV. PDP CONTX. REQ)

DR (ACTIV. PDP CONTX. ACC)

PDP ACTIVE

PDP-MODIFY-PEND

DR (MODIFY. PDP CONTX. REQ)

DI (MODIFY. PDP CONTX. ACC)

PDP-ACTIVE-PEND

DI (REQ PDP CONTX. ACTIV. REJ)

DR (REQ PDP CONTX. ACTIV)

PDP-INACTIVE

DI (ACTIV. PDP CONTX. REQ)

DR (ACTIV. PDP CONTX. ACC)

DR (ACTIV. PDP CONTX. REJ)

Figure Session management states on the network side (overview)



Session Management procedures

PDP context activation

The purpose of this procedure is to establish a PDP context between the MS and the networkfor a specific QoS on a specific NSAPI. The PDP context activation may be initiated by theMS or the initiation may be requested by the network.

Successful PDP context activation initiated by the mobile station

In order to request a PDP context activation, the MS sends an ACTIVATE PDP CONTEXTREQUEST message to the network, enters the state PDP-ACTIVE-PENDING and starts thetimer T3380. The message contains the selected NSAPI, PDP type and, if the MS requests astatic address, the PDP address. The MS shall ensure that the selected NSAPI is notcurrently being used by another Session Management entity in the MS.Upon receipt of an ACTIVATE PDP CONTEXT REQUEST message, the network selects aradio priority level based on the QoS negotiated and may reply with an ACTIVATE PDPCONTEXT ACCEPT message. Upon receipt of the message ACTIVATE PDP CONTEXTACCEPT the MS shall stop timer T3380 and enter the state PDP-ACTIVE. The MS shallinitiate establishment of the logical link for the LLC SAPI indicated by the network with theoffered QoS and selected radio priority level if no logical link has been already established forthat SAPI. If the offered QoS parameters received from the network differ from the QoSrequested by the MS, the MS shall either accept the negotiated QoS or initiate the PDPcontext deactivation procedure. If the LLC SAPI indicated by the network can not besupported by the MS, the MS shall initiate the PDP context deactivation procedure.

Successful PDP context activation requested by the network

In order to request a PDP context activation, the network sends a REQUEST PDP CONTEXTACTIVATION message to the MS and starts timer T3385. If available, the APN shall beincluded in the REQUEST PDP CONTEXT ACTIVATION message.Upon receipt of a REQUEST PDP CONTEXT ACTIVATION message, the MS shall thaneither initiate the PDP context activation procedure as described in the previous section orshall reject the activation request by sending a REQUEST PDP CONTEXT ACTIVATIONREJECT message. The value of the reject cause IE of the REQUEST PDP CONTEXTACTIVATION REJECT message shall indicate the reason for rejection, e.g. “insufficientresources to activate another context”.The ACTIVATE PDP CONTEXT REQUEST message sent by the MS in order to initiate thePDP context activation procedure shall contain the PDP address, PDP Type and APNrequested by the network in the REQUEST PDP CONTEXT ACTIVATION message.Upon receipt of the ACTIVATE PDP CONTEXT REQUEST message, the network shall stoptimer T3385.The same procedures then apply as described for MS initiated PDP context activation.



MS NetworkACTIVATE PDP CONTEXT REQUEST

Start T3380

ACTIVATE PDP CONTEXT ACCEPT

ACTIVATE PDP CONTEXT REJECTStop T3380

Stop T3380

or

Figure: MS initiated PDP context activation procedure

MS Network

REQUEST PDP CONTEXT Start T3385

Start T3380

ACTIVATE PDP CONTEXT ACCEPT Stop T3380

ACTIVATE PDP CONTEXT REQUEST

Stop T3380 ACTIVATE PDP CONTEXT REJECT

Stop T3385

or

or

REQUEST PDP CONTEXT ACTIVATION REJECT Stop T3385

Figure: Network initiated PDP context activation procedure

PDP context modification procedure

The PDP context modification procedure is invoked by the network in order to change theQoS negotiated during the PDP context activation procedure or at previously performed PDPcontext modification procedures. The procedure can be initiated by the network at any timewhen a PDP context is active.In order to initiate the procedure, the network sends the MODIFY PDP CONTEXT REQUESTmessage to the MS and starts the timer T3386. The message shall contain the new QoS andthe radio priority level that shall be used by the MS at the lower layers for the transmission ofdata related to the PDP context.Upon receipt of this message the MS shall reply with the MODIFY PDP CONTEXT ACCEPTmessage if the MS accepts the new QoS and the indicated LLC SAPI.If the MS does not accept the new QoS or the indicated LLC SAPI, the MS shall initiate thePDP context deactivation procedure for the PDP context - the reject cause IE value of theDEACTIVATE PDP CONTEXT REQUEST message shall indicate “QoS not accepted”.



PDP context deactivation procedure

The purpose of this procedure is to deactivate an existing PDP context between the MS andthe network. The PDP context deactivation may be initiated by the MS or by the network.

PDP context deactivation initiated by the MS

In order to deactivate a PDP context, the MS sends a DEACTIVATE PDP CONTEXTREQUEST message to the network, enters the state PDP-INACTIVE-PENDING and startstimer T3390. The message contains the transaction identifier in use for the PDP context to bedeactivated and a cause code that typically indicates one of the following causes:

# 25: LLC or SNDCP failure;# 26: insufficient resources;# 36: regular PDP context deactivation; or# 37: QoS not accepted.

The network shall reply with the DEACTIVATE PDP CONTEXT ACCEPT message. Uponreceipt of the DEACTIVATE PDP CONTEXT ACCEPT message, the MS shall stop timerT3390. Both the MS and the network shall initiate local release of the logical link if it is notused by another PDP context.

PDP context deactivation initiated by the network

In order to deactivate a PDP context, the network sends a DEACTIVATE PDP CONTEXTREQUEST message to the MS and starts timer T3395. The message contains thetransaction identifier in use for the PDP context to be deactivated and a cause code thattypically indicates one of the following causes:

# 25: LLC or SNDCP failure;# 36: regular PDP context deactivation;# 38: network failure; or# 39: reactivation requested.

The MS shall, upon receipt of this message, reply with a DEACTIVATE PDP CONTEXTACCEPT message. Upon receipt of the DEACTIVATE PDP CONTEXT ACCEPT message,the network shall stop the timer T3395. Both the MS and the network shall initiate localrelease of the logical link if it is not used by another PDP context.



MS Network

DEACTIVATE PDP CONTEXT REQUESTStart T3390

DEACTIVATE PDP CONTEXT ACCEPTStop T3390

Figure: MS initiated PDP context deactivation procedure

MS Network

Start T3395

DEACTIVATE PDP CONTEXT ACCEPTStop T3395

DEACTIVATE PDP CONTEXT REQUEST

Figure: Network initiated PDP context deactivation procedure

Messages for GMM and SM

P-TMSI reallocation commandThis message is sent by the network to the MS to reallocate a P-TMSI.

P-TMSI reallocation completeThis message is sent by the MS to the network to indicate that reallocation of a P-TMSI hastaken place.

Authentication and ciphering requestThis message is sent by the network to the MS to initiate authentication of the MS identity. Inaddition, the ciphering mode is set, indicating whether ciphering will be performed or not.

Authentication and ciphering responseThis message is sent by the MS to the network in response to an Authentication andciphering request message.

Authentication and ciphering rejectThis message is sent by the network to the MS to indicate that authentication has failed (andthat the receiving MS shall abort all activities).

Identity requestThis message is sent by the network to the MS to request submission of the MS identityaccording to the specified identity type. E.g. responses are IMSI, TMSI, P-TMSI or IMEI.



Identity responseThis message is sent by the MS to the network in response to an identity request messageproviding the requested identity.

Routing area update requestThis message is sent by the MS to the network either to request an update of its location fileor to request an IMSI attach for non-GPRS services.

Routing area update acceptThis message is sent by the network to the MS to provide the MS with GPRS mobilitymanagement related data in response to a routing area update request message.

Routing area update completeThis message shall be sent by the MS to the network in response to a routing area updateaccept message if a P-TMSI and/or a TMSI has been assigned and/or if there areestablished LLC connections.

Routing area update rejectThis message is sent by the network to the MS in order to reject the routing area updateprocedure.

GMM StatusThis message is sent by the MS or by the network at any time to report certain errorconditions listed in the information element GMM cause. The purpose of the GMM causeinformation element is to indicate the reason why a GMM request from the mobile station isrejected by the network. Some possible field values are:

• IMSI unknown in HLR• Illegal MS• Illegal ME• GPRS services not allowed• GPRS services and non-GPRS services not allowed• MS identity cannot be derived by the network• Implicitly detached• PLMN not allowed• Location Area not allowed• Roaming not allowed in this location area• MSC temporarily not reachable• Network failure• Congestion• Etc.

GMM InformationThis message is sent by the network at any time to send certain information to the MS. Thisinformation could be: Full name for network, Short name for network, Network time zone,Universal time and time zone, LSA Identity, ...



Activate PDP context requestThis message is sent by the MS to the network to request activation of a PDP context.

Activate PDP context acceptThis message is sent by the network to the MS to acknowledge activation of a PDP context.

Activate PDP context rejectThis message is sent by the network to the MS to reject activation of a PDP context.

Request PDP context activationThis message is sent by the network to the MS to initiate activation of a PDP context.

Request PDP context activation rejectThis message is sent by the MS to the network to reject initiation of a PDP context activation.

Modify PDP context requestThis message is sent by the network to the MS to request modification of an active PDPcontext.

Modify PDP context acceptThis message is sent by the MS to the network to acknowledge the modification of an activePDP context.

Deactivate PDP context requestThis message is sent to request deactivation of an active PDP context.

Deactivate PDP context acceptThis message is sent to acknowledge deactivation of the PDP context requested in thecorresponding Deactivate PDP context request message.

Activate AA PDP context requestThis message is sent by the MS to the network to initiate activation of an AA PDP context. AA= Anonymous Access PDP context request.

Activate AA PDP context acceptThis message is sent by the network to the MS to acknowledge the activation of an AA PDPcontext.

Activate AA PDP context rejectThis message is sent by the network to the MS to reject the activation of an AA PDP context.

Deactivate AA PDP context requestThis message is sent to request deactivation of an active AA PDP context.

Deactivate AA PDP context acceptThis message is sent to acknowledge deactivation of an AA PDP context requested by thecorresponding Deactivate AA PDP context request message.



SM StatusThis message is sent by the network or the MS to pass information on the status of theindicated context and report certain error conditions.

How can you identify the current GPRS MM or SM message? This is given by the informationelement type in the Layer 3 message:

Message types for GPRS mobility management

Bits8 7 6 5 4 3 2 1

0 0 - - - - - - Mobility management messages

0 0 0 0 0 0 0 1 Attach request0 0 0 0 0 0 1 0 Attach accept0 0 0 0 0 0 1 1 Attach complete0 0 0 0 0 1 0 0 Attach reject0 0 0 0 0 1 0 1 Detach request0 0 0 0 0 1 1 0 Detach accept

0 0 0 0 1 0 0 0 Routing area update request0 0 0 0 1 0 0 1 Routing area update accept0 0 0 0 1 0 1 0 Routing area update complete0 0 0 0 1 0 1 1 Routing area update reject

0 0 0 1 0 0 0 0 P-TMSI reallocation command0 0 0 1 0 0 0 1 P-TMSI reallocation complete0 0 0 1 0 0 1 0 Authentication and ciphering req0 0 0 1 0 0 1 1 Authentication and ciphering resp0 0 0 1 0 1 0 0 Authentication and ciphering rej0 0 0 1 0 1 0 1 Identity request0 0 0 1 0 1 1 0 Identity response0 0 1 0 0 0 0 0 GMM status0 0 1 0 0 0 0 1 GMM information

Message types for GPRS session management

Bits8 7 6 5 4 3 2 1

0 1 - - - - - - Session management messages

0 1 0 0 0 0 0 1 Activate PDP context request0 1 0 0 0 0 1 0 Activate PDP context accept0 1 0 0 0 0 1 1 Activate PDP context reject

0 1 0 0 0 1 0 0 Request PDP context activation0 1 0 0 0 1 0 1 Request PDP context activation rej.

0 1 0 0 0 1 1 0 Deactivate PDP context request0 1 0 0 0 1 1 1 Deactivate PDP context accept

0 1 0 0 1 0 0 0 Modify PDP context request0 1 0 0 1 0 0 1 Modify PDP context accept

0 1 0 1 0 0 0 0 Activate AA PDP context request0 1 0 1 0 0 0 1 Activate AA PDP context accept0 1 0 1 0 0 1 0 Activate AA PDP context reject

0 1 0 1 0 0 1 1 Deactivate AA PDP context request0 1 0 1 0 1 0 0 Deactivate AA PDP context accept

0 1 0 1 0 1 0 1 SM Status



GPRS Radio Interface: Encapsulation:

The user data will be encapsulated into the different protocol planes before transmitting overthe GPRS air interface. The following figure indicates this encapsulation procedure in theexample of a Mobile Station.

Figure: Encapsulation of user data in different protocol layer

Please remark that there is no fixed relation between the protocol headers and the user data.More or less all protocol headers will have a dynamic length, depending on the context. Thislength will also vary through a communication link, result will be always an variating data rateof the output data.The Frame Check Sequence, FCS in the LLC frame structure is necessary, because inGPRS it is possible, that the air interface, let’s say the RLC / MAC layer is not using any kindof forward error correction. This will be in case of Coding Scheme CS-4. For this reason, theLLC layer must have an error detection mechanism. Together with the error detectionmechanism in the RLC/MAC layer, the Block Check Sequence, BCS this will be a doubleerror detection mechanism. But the length of the BCS is changing between 16 or 40 bit, i.e.the protection or detection quality is very good concerning the block header, the RLC andMAC header but it does not have the best quality of protection over all the data field.

Protocol layer inside MS:

Data

TCP/IP

SNDCP

Information fieldLLC FCS

Data fieldRLC/MAC BCS

456 bit

Max length = 1600 octets

184 - 440 bit, depending on CS-1 - CS-4 16 or 40 bit, depending on CS

Application

IP

SNDCP

LLC

RLC/MAC = new data

= previouslayerRF

Convolutional encoding



GPRS: Identifiers on the data transport over the air interface

In GPRS there are some questions to be answered before handing out the received data tothe right application. For this reason, there are some different identifiers that have to be usedin certain protocol layers. The following picture demonstrates these most importantIdentifiers, to be used for reaching the final destination if we transmit a Packet Data Unit,PDU. The 3 identifiers, TFI, SAP and NSAPI are cascaded in the following way:

Figure: Cascading of the identifiers in GPRS

The first question to be answered, if a BTS transmits a PDU, is who is the owner of this datapacket, i.e. which Mobile Station should receive this downlink data. For this reason there is anidentifier, TLLI, Temporary Logical Link Identifier, that is derived or calculated out of thePTMSI, the subscriber identity number. Because of the length of this TLLI we also use a shortform, the so called Temporary Flow Identifier, TFI. The TFI is only 5 bits in length, so there aremaybe some mismatches after assignment, that’s the reason, why in the first downlink datablocks, there will always be the inclusion of the TLLI, that is much longer and therefore theprobability of misunderstanding is much lower.This identifier is directly concerned to the air interface, it will be a question of the receiver, ifhe shall hand out this data unit to higher layers of if he shall discard it, because it was sent tosomebody else. For this reason, the TLLI or TFI is included in the RLC/MAC header.

If the right Mobile Station, MS has received the data unit, there are now some morepossibilities. The next question to be answered is, which higher layer protocol should receivethe data unit, for example, is this received data unit a signalling message of the GPRSmobility management or is it some user data, packed into an SNDCP header? To answer thisquestion, we use the Service Access Point Identifier, SAPI included in the transport layerframe, the LLC frame. This identifier indicate the higher layer (in OSI Modell = Layer 3)protocol that will be the owner of the received packet data unit. In GPRS we can find here the

TLLI orTFI

SAPI NSAPIRF

RLC/MAC

LLC

SNDCPTOM SMS SM/GMM

SAPI

PDP PDP PDP

NSAPI

IdentifiesMS

Identifies Layer 3 Protocol

Identifies PDP - context



distinction between, GPRS Mobility Management, GPRS Session Management, SMSTransfer, User Data Unit via SNDCP or GSM foreign signalling messages, tunnelled with theTunneling of Messeges protocol, TOM.

In our next step, we assume that this received packet data unit is to be forwarded to the userapplication, i.e. by the received data it is about a SNDCP data unit. But as we have seen, inGPRS it is possible, that we can find more than one activated application, or the so calledPDP context that is activated. Now it is the task of this receiving layer to forward the incomingdata to the right application. Every PDP context is identified with this parameter, NetworkService Access Point Identifier, NSAPI.Each NSAPI points to the corresponding application.



GPRS: Assignment of Radio Resource

In GPRS there are in principle two different ways how to assign radio ressource. First wehave to assure, that in GPRS uplink and downlink are completely different from each otherand will be assigned in 2 steps. Packet Uplink Assignment and Packet Downlink Assignment.

Packet Downlink Assignment:This assignment consists of the main parameters:TFI: Temporary Flow Identifier. Included in every RLC/MAC Block, identifies the owner of thisradio block.Timeslot Allocation: Bitmap of 8 Bits, that indicates, which timeslot has been assigned fordownlink data transfer.TBF Starting time: To indicate, when the data transfer will begin. Starting time refering to theGSM clock timing, or multiframe number.Frequency parameters: Which frequency channel shall be used .

Consequence will be for the Mobile Station, to receive every downlink radio block on theassigned timeslots and decide after receiving by checking the TFI field in the RLC header ifthe radio must to be forwarded to higher layer protocols or will be discarded.

Packet Uplink Assignment:This uplink assignment contains as important Information Elements, IE the followingparameters:

TFI and TLLI: Assignment of the identifier of the TBF, to be included in every radio block.Channel Coding Command: Which Coding Scheme should be used.

Packet Timing Advance: Field that contains the Timing Advance value to be used, the TimingAdvance Index for the update procedure on PTCCH and the timeslot, where the PTCCH isactive.

Frequency parameters: Which frequency channel shall be used .

If dynamic allocation:

USF value as 3 bit field, for every assigned timeslot

Power control parameter: the necessary parameters to calculate the transmittingpower of the MS. E. g. alpha, gamma and p0 parameter. See formula on other chapterfor details.

TBF Starting time: when will the TBF start.



Figure: usage of the USF field in the uplink TBF establishment

If fixed allocation:

Downlink control timeslot: This indicates, which downlink timeslot should be monitoredfor PACCH blocks.

Timeslot allocation: to indicate which timeslots should be used by the MS on the uplink

Bitmap allocation: Refering to the 52 Multiframe: which radio blocks will be assigned tothe MS for uplink data transfer.

Packet Timing Advance: Field that contains the Timing Advance value to be used, theTiming Advance Index for the update procedure on PTCCH and the timeslot, where thePTCCH is active.

Power control parameter: the necessary parameters to calculate the transmittingpower of the MS. E. g. alpha, gamma and p0 parameter. See formula on other chapterfor details.

downlink

USFRadio Block B(n)

USF = R1

USF = R2

USF = R3

USF = Free

USF = Free

Monitoring Paging ChannelSend Access Burst

uplink

Radio Block B(n+1)

Monitoring USF in Block B(n)Sends in Block B(n+1) or in Blocks B(n+1) - B(n+4)

Fixed Allocation = Fixed number of octets to betransmitted in N bundled Timeslots

Dynamic Allocation = USF points to next Radio Blockon uplink RF-Channel for use by MS

Extended Dynamic Allocation = USF points to next 4 Radio Blocks on uplink RF-Channel for use by MS



Figure: Example: Assignment of radio ressource in fixed allocation mode

B0 - B11 = Radio Blocks T = Frame used for PTCCH X = Idle frame

B9 B10 B11B6 B7 B8B3 B4 B5B0 B1 B2 T X XT

52 TDMA Frames

Timeslot Allocation 0 1 2 3 4 5 6 700001011Bitmap example

Allocation Bitmap

010111000110111000111000101001100111



TN4

TN6

TN7

PDCH

= not assigned Radio Block = assigned Radio Block

example

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Signalling messages gprs e - pudn.comread.pudn.com/downloads161/ebook/733566/GPRS/GPRS_chap13.pdf · The training sequence will be included, ... GPRS Signalling Messages and Procedures