7 Signalling procedures between GMM SAPs

3GPP48.018Base Station System (BSS) - Serving GPRS Support Node (SGSN)BSS GPRS protocol (BSSGP)General Packet Radio Service (GPRS)Release 17TS

Tools: ARFCN - Frequency Conversion for 5G NR/LTE/UMTS/GSM

7.1 Paging procedure

When an SGSN initiates the paging procedure for GPRS services as defined in 3GPP TS 24.008, it shall send one or more PAGING-PS PDUs to the BSS.

When instructed by an MSC/VLR to initiate a paging procedure for non-GPRS services as defined in 3GPP TS 24.008, an SGSN shall send one or more PAGING-CS PDUs to the BSS.

These paging PDUs shall contain the information elements necessary for the BSS to initiate paging for an MS within a group of cells.

The SGSN provides an indication of the cells within which the BSS shall page the MS. The levels of resolution within one BSS are: all cells within the BSS, all cells on the BSS within one location area, all cells on the BSS within one routing area, and one BVCI (i.e. cell). A routing area, a location area, or a BSS area is associated with one or more NSEIs. If the cells in which to page the MS are served by several NSEIs then one paging PDU must be sent to each of these NSEIs.

A paging PDU shall be used to generate the corresponding radio interface paging request message(s) to be transmitted at the appropriate time.

It should be noted that each paging PDU relates to only one MS and therefore a BSS may pack pages for different MSs into the relevant 3GPP TS 24.008 or 3GPP TS 44.060 radio interface paging request messages.

In the case of paging for non-GPRS services, the SGSN shall provide the MS’s IMSI and DRX Parameters. The SGSN shall also include the Global CN-Id information element in the paging PDU when this information element is received from the MSC/VLR. The Global CN-Id information element is received from the MSC/VLR if paging using only the IMSI parameter as identifier of the MS is performed via the SGSN when the MSC/VLR applies intra domain connection of RAN nodes to multiple CN nodes as described in 3GPP TS 23.236. The BSS shall then buffer this information element until receiving the paging response from the MS in order to route the paging response to the correct MSC/VLR.

In the case of paging for GPRS services, the SGSN shall provide the MS’s IMSI. If DRX Parameters are available, the SGSN shall also provide the DRX Parameters. If eDRX Parameters are available, the SGSN shall also provide the eDRX Parameters (see sub-clause 7.1a).

NOTE: The IMSI and the DRX Parameters enable the BSS to derive the paging population number. Paging without DRX parameters may require a considerable extension of the paging duration.

An SGSN may provide the BSSGP with MS specific information, enabling a BSS to execute the paging procedure in an MS specific manner. This includes:

– QoS Profile. The Precedence parameter is set by the upper layers (in the SGSN). The SGSN shall set the bit rate parameter to "best effort". The SGSN shall set the transmission mode to unacknowledged. The BSS shall ignore the received bit rate, the BSSGP SDU type, LLC type, and transmission mode parameters;

– PFI or an aggregate BSS QoS profile information which indicates if the page is for signalling, for SMS, for TOM8, for best-effort, or for a specific packet flow. The aggregate BSS QoS profile in this case is used for paging only and is not stored by the BSS. If both of the optional PFI and ABQP IEs are present, the ABQP takes precedence.

– the most recently received downlink Coverage Class and the cell identity for the cell where the Coverage Class was reported by the MS, if previously received from the BSS in the UL-UNITDATA PDU.

– MS Radio Access Capability defining the radio capabilities of the MS. If there is valid MS Radio Access Capability information known by the SGSN for the associated MS, the SGSN shall include it in the PAGING-PS PDU. Otherwise, the MS Radio Access Capability shall not be present.

– Paging Attempt Information consisting of Paging Attempt Count and Intended Number of Paging Attempts information. If Paging Attempt Information is provided then the Paging Attempt Count field shall be increased by one at each new paging attempt performed for delivering a packet data payload available for delivery. If the SGSN cannot determine the number of intended paging attempts the SGSN shall use the code point 0000 corresponding to ´Information not available´.

If an SGSN provides a P-TMSI in a PAGING-PS PDU, then the BSS shall use the P-TMSI to address the MS. If the SGSN does not provide the P-TMSI in the PAGING-PS PDU, then the BSS shall use the IMSI to address the MS.

If the BSS determines that the nominal paging group of the MS occurs too far into the future (e.g. the BSS is unable to buffer the paging request until the next occurrence of the nominal paging group for the indicated MS) it responds to the PAGING-PS PDU by sending a PAGING-PS-REJECT PDU to the SGSN and includes information indicating the time until the next paging occasion therein.

If an SGSN provides a TLLI in a PAGING-CS PDU and a radio context identified by the TLLI exists within the BSS, then the paging request message shall be directly sent to the MS. If the SGSN does not provide the TLLI in the PAGING-CS PDU or if no radio context identified by the TLLI exists within the BSS, then the BSS shall use the TMSI, if provided in the PAGING-CS PDU, else the IMSI, to address the MS.

The PAGING-CS PDU consists of the parameters described above for a PAGING-PS PDU (except the P-TMSI, PFI, ABQP and QoS profile parameters) and, optionally, some or all of the following parameters; TMSI, TLLI, Global CN-Id, Channel Needed and eMLPP-Priority. The Channel Needed and eMLPP-Priority information shall be handled transparently by the BSS.

The SGSN may send the BSS a DUMMY-PAGING-PS PDU at any time to determine the time until the next paging occasion for the MS indicated therein. The BSS uses the information provided within the DUMMY-PAGING-PS PDU to calculate the time until the next paging occasion for the indicated MS and includes it within a DUMMY-PAGING-PS-RESPONSE PDU it sends back to the SGSN.

A SGSN that supports Multilateration Timing Advance (MTA) procedure (see 3GPP TS 44.031 and 3GPP TS 43.059 [23]) shall not send a PAGING-PS PDU to a BSS if the MPM timer is running for the corresponding MS. Otherwise, if neither the MPM timer nor Ready timer is running it may send a PAGING-PS PDU indicating ‘positioning event triggered’ in response to receiving a location request (see sub-clause 8b.1). If the SGSN receives a corresponding page response it sends the BSS managing the serving cell a PERFORM-LOCATION-REQUEST PDU and starts the Ready timer. If the UL-UNITDATA PDU containing the page response includes the “MultilaterationTiming Advance”, “MS Sync Accuracy” and “BTS Reception Accuracy Level” information elements the SGSN includes them in the PERFORM-LOCATION-REQUEST PDU it sends to the serving BSS.

In case the network supports restricted use of enhanced coverage (see 3GPP TS 24.008 [46]), and the MS camps on a cell not supporting EC-GSM-IoT, the SGSN may at any time check the status of Enhanced Coverage Restriction (RestrictEC indication in 3GPP TS 24.008 [46]) obtained from HSS/HLR or retrieve the status from the stored MS context and include it in the PAGING-PS PDU (see sub-clauses 10.3.1 and 11.3.141) in order to inform the BSS about restriction on the use of enhanced coverage. The BSS relays the restriction information to the corresponding MS using PS domain paging procedures.

7.1a Paging procedure for Extended Coverage and eDRX

An SGSN may have any combination of uplink and downlink Coverage Class information (previously received from the BSS in an UL-UNITDATA PDU) and eDRX information (negotiated during NAS signalling) available for a given MS when it determines that paging is necessary for that MS. When sending a PAGING-PS PDU to a BSS to trigger paging for a given MS the SGSN shall include the most recently received Coverage Class information and the most recent eDRX value negotiated during NAS signalling for that MS. If the BVC-RESET procedure (see sub-clauses 8.4 and 11.3.84) is performed and indicates a change to the eDRX capability for a BSS, it shall not impact the most recent eDRX value negotiated during NAS signalling for that MS.

In case the network supports restricted use of enhanced coverage (see 3GPP TS 24.008 [46]), and the MS camps on a cell supporting EC-GSM-IoT, the SGSN may at any time check the status of Enhanced Coverage Restriction (RestrictEC indication in 3GPP TS 24.008 [46]) obtained from HSS/HLR or retrieve the status from the stored MS context and include it in the PAGING-PS PDU (see sub-clauses 10.3.1 and 11.3.141) in order to inform the BSS about a restriction on the use of enhanced coverage. The BSS relays the restriction information to the corresponding MS using PS domain paging procedures.

7.1a.1 Coverage Class and eDRX information available

If uplink and downlink Coverage Class information and a negotiated eDRX value are available for the MS the SGSN sends the BSS a PAGING-PS PDU that indicates the available uplink and downlink Coverage Class information and the negotiated eDRX information.

– If a cell in the area targeted for paging supports EC-GSM-IoT and eDRX the MS is paged on the EC-PCH of its EC_CCCH_GROUP using the indicated downlink Coverage Class and eDRX information (see 3GPP TS 45.002 [48]).

– If a cell in the area targeted for paging supports EC-GSM-IoT but not eDRX the MS is paged on the EC-PCH of its EC_CCCH_GROUP using the lowest eDRX cycle (see 3GPP TS 45.002 [48]) only if the PAGING-PS PDU indicates the lowest eDRX cycle. Otherwise the MS is not paged.

– If a cell in the area targeted for paging does not support EC-GSM-IoT the MS is paged on the PCH using the indicated eDRX information if the MS Radio Access Capability IE (see sub-clause 11.3.22) indicates that the MS supports GPRS/EGPRS. The MS is not paged if the MS Radio Access Capability IE indicates that the MS does not support GPRS/EGPRS.

7.1a.2 Coverage Class information not available, eDRX information available

If uplink and downlink Coverage Class information is not available but a negotiated eDRX value is available for the MS the SGSN sends the BSS a PAGING-PS PDU that indicates the negotiated eDRX information. The MS is paged on the PCH, using the indicated eDRX information, in the cell(s) in the area targeted for the paging.

7.1a.3 Coverage Class information available, eDRX information not available

If uplink and downlink Coverage Class information is available but a negotiated eDRX value is not available for the MS the SGSN sends the BSS a PAGING-PS PDU that indicates the available Coverage Class and DRX information (if available).

– If a cell in the area targeted for paging supports EC-GSM-IoT the MS is paged on the EC-PCH of its EC_CCCH_GROUP (see 3GPP TS 45.002 [32]) using the indicated downlink Coverage Class information and the lowest eDRX cycle.

– If a cell in the area targeted for paging does not support EC-GSM-IoT the MS is paged on the PCH using DRX (the SGSN may include DRX information in the PAGING-PS PDU) if the MS Radio Access Capability IE indicates that the MS supports GPRS/EGPRS. The MS is not paged if the MS Radio Access Capability IE indicates that the MS does not support GPRS/EGPRS.

7.1b Paging Procedure for Extended Coverage with Paging Indication

If the downlink Coverage Class information included in the PAGING-PS PDU indicates CC3 or CC4 and the MSRAC capability indicates that the MS supports monitoring of the EC-PICH, the BSS triggers for cells in the area targeted for paging and broadcasting the support of EC-PICH, the sending of a paging indication in the EC-PICH block, corresponding to the paging block for the paging group of the MS, before scheduling the paging message in the paging block. If the MSRAC capability does not indicate support for EC-PICH monitoring, the BSS does not trigger the sending of a paging indication before scheduling the paging message for the MS.

7.2 Radio Access Capability Update procedure

The BSS may request an MS’s current Radio Access capability and/or its IMSI by sending to an SGSN a RA‑CAPABILITY-UPDATE PDU which includes the TLLI of the MS and a Tag. The allocation of the Tag is implementation specific. The BSS then starts timer T5.

The SGSN shall respond by sending a RA-CAPABILITY-UPDATE-ACK PDU which includes the TLLI of the MS, the Tag received in the corresponding RA-CAPABILITY-UPDATE PDU, and an RA-Cap-UPD-Cause field; the IMSI of the MS is also included when known. The BSS shall stop timer T5.

If the RA-Cap-UPD-Cause is set to "OK", then an MS Radio Access Capability field and the IMSI shall be present. The received MS’s radio access capability, if valid, shall then replace any radio access capability previously associated with the MS. If the RA-Cap-UPD-Cause is not set to "OK", then neither the MS Radio Access Capability nor the IMSI shall be present in the RA-CAPABILITY-UPDATE-ACK PDU.

7.2.1 Abnormal conditions

If an SGSN receives a RA-CAPABILITY-UPDATE PDU which includes an unknown TLLI, it shall answer with a RA‑CAPABILITY-UPDATE-ACK PDU which includes the RA-CAP-UPD-Cause set to the value "TLLI unknown".

If an SGSN receives a RA-CAPABILITY-UPDATE PDU which includes a known TLLI, but there are no Radio Access parameters or IMSI known to the SGSN for the associated MS, the SGSN shall reply to the request with a RA‑CAPABILITY‑UPDATE-ACK PDU in which the RA-CAP-UPD-Cause is set to: "no RA capability or IMSI available".

If a BSS receives a RA-CAPABILITY-UPDATE-ACK PDU containing a Tag which is different from the last transmitted Tag by the BSS, it shall ignore the reception of this PDU.

If a BSS sends a RA-CAPABILITY-UPDATE PDU to an SGSN and the RA-CAPABILITY-UPDATE-ACK is not returned within a period T5 with the same Tag value as provided in the request, the RA-CAPABILITY-UPDATE procedure shall be repeated a maximum of RA-CAPABILITY-UPDATE-RETRIES attempts. The Tag value shall be changed by the BSS at each new retry.

7.3 Radio Status procedure

A BSS and an MS radio interface communication status may change due to the following:

1) the MS goes out of coverage and is lost;

This condition is signalled by setting the Radio Cause value to "Radio contact lost with MS".

2) the link quality is too bad to continue the communication;

This condition is signalled by setting the Radio Cause value to "Radio link quality insufficient to continue communication".

3) the BSS has ordered the MS to perform a cell reselection.

This condition is signalled by setting the Radio Cause value to "Cell reselection ordered".

4) the BSS is preparing to order the MS to perform a cell-reselection to a new cell and internal re-routing of packets to the new cell is not possible.

This condition is signalled by setting the Radio Cause value to "Cell reselection preparation".

5) the BSS has detected that the packet cell change order has failed.

This condition is signalled by setting the Radio Cause value to "Cell reselection failure".

Conditions 1) and 2) indicate that attempts to communicate between an MS and an SGSN via this cell should be suspended or abandoned. An SGSN shall stop sending LLC-PDUs to the cell for the MS. The criteria for deciding whether condition 1) or 2) has occurred is not in the scope of the present document.

The conditions for resuming a suspended or abandoned communication between an MS and SGSN are defined in 3GPP TS 24.008.

Condition 3) indicates that the SGSN should wait for a cell update before resuming the transmission of LLC-PDUs to the BSS.

Condition 4) indicates that the SGSN shall suspend downlink transmission of LLC-PDUs. This condition shall only be signalled if the Enhanced Radio Status feature has been negotiated. For this condition the SGSN shall wait for either:

a) a cell update from the MS in a new Cell . In this case the SGSN should resume downlink transmission in the new Cell.

b) or a new RADIO-STATUS PDU from the BSS with a different Radio Cause value. In this case the SGSN should follow the procedures specified for that Radio Cause value.

Condition 5) indicates that the SGSN shall resume the transmission of LLC-PDUs to the BSS in case the downlink transmission has been suspended. This condition shall only be signalled if the Enhanced Radio Status feature has been negotiated.A BSS shall signal these exception conditions to an SGSN by sending a RADIO-STATUS PDU. It shall contain a reference to the MS, either TLLI or TMSI or IMSI, and an indication of the exception condition, i.e. the Radio Cause value.

After receipt of a RADIO-STATUS PDU with cause value indicating Condition 1-4, the SGSN may try to locate the mobile station in case any downlink LLC PDU needs to be sent to the mobile station, as it can not expect to receive systematically an uplink LLC PDU from the mobile station or a RADIO-STATUS PDU with cause value indicating Condition 5 from the BSS to resume the downlink transfer. To this avail, the SGSN should send a PAGING-PS PDU towards the mobile station.

7.4 SUSPEND procedure

If the MS signals to the BSS that it wishes its GPRS service to be suspended, the BSS shall send a SUSPEND PDU to the SGSN and start timer T3. Actions within the SGSN while an MS is suspended are not specified, but paging is typically stopped. The SUSPEND PDU contains:

– the TLLI of the MS; and

– the Routeing Area of the MS as received in the Layer 3 Um interface message GPRS Suspension Request (see 3GPP TS 44.018).

For each SUSPEND PDU received by an SGSN, a SUSPEND-ACK PDU shall be returned to the BSS. Upon reception of the SUSPEND-ACK PDU, the BSS shall stop T3. The SUSPEND-ACK PDU contains:

– the TLLI of the MS as received in the SUSPEND PDU;

– the Routeing Area of the MS as received in the SUSPEND PDU; and

– the Suspend Reference Number.

The SGSN generates the Suspend Reference Number in a manner that it enables it to differentiate between different SUSPEND PDUs relating to the same MS.

7.4.1 Abnormal conditions

If a SUSPEND-ACK PDU is not received for a SUSPEND PDU within T3 seconds, then the SUSPEND PDU procedure shall be repeated a maximum of SUSPEND-RETRIES attempts. After SUSPEND-RETRIES attempts the procedure is stopped and the O&M system is informed.

If a SUSPEND-ACK PDU is received for an MS that is already marked as suspended, then the SUSPEND-ACK PDU is ignored.

If a SUSPEND PDU refers to an MS which is unknown in the SGSN, then a SUSPEND-NACK PDU is returned containing a cause value (Cause value: Unknown MS). The BSS shall stop the SUSPEND procedure.

If the Suspend procedure is supported on the Gn interface, in case of an inter-SGSN suspend procedure the MS shall not be treated as unknown in the SGSN when the RA indicated in the SUSPEND PDU is not served by the SGSN.

7.5 RESUME procedure

When the reason why a GPRS-attached MS was suspended disappears, i.e.:

– it leaves dedicated mode, disconnecting the MS from the MSC; or

– it is handed over to a cell that supports DTM;

the BSS shall either a) instruct the MS to initiate the Routeing Area Update procedure, or b) signal to the SGSN that an MS’s GPRS service shall be resumed.

If the BSS executes a), then no further action is required.

If the BSS executes b), then the BSS shall send a RESUME PDU containing the same Suspend Reference Number received in the SUSPEND-ACK PDU to the SGSN and start timer T4. The RESUME PDU contains:

– the TLLI of the MS;

– the Routeing Area of the MS; and

– the Suspend Reference Number.

For each RESUME PDU received by an SGSN, a RESUME-ACK PDU shall be returned to the BSS. Upon reception of the RESUME-ACK PDU, the BSS shall stop T4. The RESUME-ACK PDU contains:

– the TLLI of the MS; and

– the Routeing Area of the MS.

7.5.1 Abnormal conditions

If a RESUME-ACK PDU is not received for a RESUME PDU within T4 seconds, then the RESUME PDU procedure shall be repeated a maximum of RESUME-RETRIES attempts. After RESUME-RETRIES attempts the procedure is stopped, the O&M system is informed and the MS shall be instructed to initiate the Routeing Area Update procedure.

If a RESUME-ACK PDU is received for an MS that is not suspended, then the RESUME-ACK PDU is ignored.

If a RESUME PDU refers to an MS which is unknown in the SGSN, then a RESUME-NACK PDU is returned containing a cause value (Cause value: Unknown MS). The BSS shall stop the RESUME procedure and the MS shall be instructed to initiate the Routeing Area Update procedure.

7.6 MS Registration Enquiry

7.6.1 General

This procedure shall be supported by a BSS if it supports CS/PS coordination enhancements in a MOCN configuration (see sub-clause 6.6) or in a GWCN configuration (see sub-clause 6.7).

The purpose of the MS Registration Enquiry procedure is to aquire registration information for a given MS from the core network. More precisely, triggered e.g. by the Rerouting procedure in the CS domain (see 3GPP TS 48.008 [14]), the BSS sends an enquiry to the SGSN(s) whether a given MS is already registered with any of the core network operators in the shared network.

7.6.2 Registration enquiry

The MS Registration Enquiry procedure is triggered by the BSS sending the MS REGISTRATION ENQUIRY message to the SGSN containing the IMSI for the MS for which the registration enquiry concerns.

To avoid a possible change of serving operator when the MS performs inter-RAT mobility from E-UTRAN to GERAN (see 3GPP TS 23.251 [43]), the MS registration enquiry may also need to be sent from the SGSN to the MME of the same CN operator. For this reason the BSS includes the MME Query indicator in the MS REGISTRATION ENQUIRY message.

The MS REGISTRATION ENQUIRY message is sent to each SGSN connected to the BSS. However, in order to minimise the signalling in the core network, the MME Query indicator shall only be included in one of the MS REGISTRATION ENQUIRY messages addressing the same CN operator.

The SGSN responds with the MS REGISTRATION ENQUIRY RESPONSE message indicating whether the MS is served by (one of) the CN operator(s) sharing the network.

7.6.3 Registration response

If the MS is found to be registered with (one of) the CN operator(s) sharing the network, the SGSN shall return the MS REGISTRATION ENQUIRY RESPONSE message to the BSS with the IMSI and the PS Registered Operator IE (containing the serving CN operator) included in the message.

If the MS is not registered with (any of) the sharing CN operator(s), the SGSN excludes the PS Registered Operator IE from the MS REGISTRATION ENQUIRY RESPONSE message.

Upon reception of the MS REGISTRATION ENQUIRY RESPONSE message with the PS Registered Operator IE included in the message, the BSS may terminate the MS Registration Enquiry procedure regardless if one or more SGSNs have not yet responded to the MS REGISTRATION ENQUIRY message.