8b Signalling Procedures between LCS SAPs

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

8b.1 Location Procedure

When the SGSN receives a location request, and the BSS supports LCS, the SGSN starts the location procedure by sending a PERFORM-LOCATION-REQUEST PDU. An exception case is when the SGSN supports the MTA procedure and receives a location request (i.e. a “MAP Provide Subscriber Location” message is received from the GMLC) for a MS that supports the MTA procedure (determined by the MS Radio Access Capability of the target MS – see sub-clause 11.3.22) in which case it shall proceed as follows:

– If the MPM timer is running for the target MS (see sub-clause 8b.2.1) it shall send a system failure indication to the corresponding GMLC.

– If the MPM timer is not running but the Ready timer is running for the target MS the SGSN sends a PERFORM-LOCATION-REQUEST PDU to the BSS managing the serving cell.

– If neither the MPM nor Ready timer is running, the SGSN sends a PAGING-PS PDU indicating ‘positioning event triggered’ (see sub-clause 10.3.1) to each BSS in the set of BSSs managing cells in the paging area corresponding to the target MS and waits for a page response as described in sub-clause 7.1.

The SGSN shall provide the BVCI and the NSEI indicating the PTP functional entity (i.e. the cell) upon which the last LLC-PDU was received from the MS as well as the Cell ID received together with that LLC-PDU. The SGSN shall also provide the IMSI. If the SGSN has valid DRX Parameters for a TLLI, then the SGSN shall include them in the PDU. If the SGSN has valid eDRX Parameters for a TLLI, then the SGSN shall include them in the PDU. The SGSN shall also provide the downlink Coverage Class associated with the MS, if previously received from the BSS in the UL-UNITDATA PDU.

If LLC ciphering is not activated at SGSN, SGSN may include “MTA Access Security Required” in the PERFORM LOCATION REQUEST PDU, if integrity protection for the radio access part of the MTA procedure is required. BSS upon receipt of this IE shall request SMLC to perform the location request. The “MTA Access Security Required” may indicate MTA Access Security method or the BSS Duplication Detection method is to be performed. For the case where the MTA Access Security method is performed, on receipt of “MTA Signature” and “MTA Sequence” included in the response sent by SMLC to BSS, the BSS forwards them to SGSN in PERFORM-LOCATION-RESPONSE-PDU as “MTA Signature” IE and “MTA Sequence” IE. SGSN upon receiving PERFORM-LOCATION-RESPONSE-PDU checks the presence of “MTA Signature” and “MTA Sequence “, and if present, generates a local MTA Signature using “MTA Sequence”. The generated local MTA Signature is then compared against received “MTA Signature”.

If MS Radio Access Capability information is available in the SGSN for the associated MS, the SGSN shall include it in the PERFORM-LOCATION-REQUEST PDU.

The Location Type indicates which type of location information the SGSN is requesting. The LCS capability IE reports the PS LCS capabilities of the MS and is included by the SGSN if it has been received from the MS. LCS Priority and LCS QoS are provided if available in the SGSN. The SGSN may provide the IMEI of the Mobile Station.

On receipt of the PERFORM-LOCATION-REQUEST PDU for positioning of the target MS, the BSS transfers the positioning request to the SMLC according to the procedures defined in 3GPP TS 43.059 and 3GPP TS 49.031 and awaits the result. An exception case is when the BSS receives a PERFORM-LOCATION-REQUEST PDU for which the “MultilaterationTiming Advance”, “MS Sync Accuracy” and “BTS Reception Accuracy Level” information elements are included in which case the BSS proceeds as follows:

– If a packet data transfer is ongoing for the target MS (see 3GPP TS 44.018 [25]) the BSS waits for up to 5 seconds for the packet data transfer to complete (i.e. all TBF resources are released) before sending a positioning request to the SMLC (see 3GPP TS 43.059 and 3GPP TS 49.031).

– Upon completing the ongoing packet data transfer (within 5 seconds) or if no packet resources are assigned upon receiving the PERFORM-LOCATION-REQUEST PDU the BSS sends a positioning request to the SMLC, includes the “MultilaterationTiming Advance”, “MS Sync Accuracy” and “BTS Reception Accuracy Level” information elements therein and awaits the positioning result.

– If packet resources remain assigned for more than 5 seconds after receiving the PERFORM-LOCATION-REQUEST PDU the BSS returns a PERFORM-LOCATION-RESPONSE PDU with a LCS cause value indicating ‘Target MS Unreachable’ (see sub-clause 11.3.58) and aborts the Location procedure.

The BSS then returns the result of positioning to the SGSN in the PERFORM-LOCATION-RESPONSE PDU and stops the MPM Timer (if running). This PDU contains the PTP BVCI indicating the PTP functional entity (i.e. the cell) upon which the last LLC-PDU was received from the MS, a location estimate and optionally positioning data. Upon reception of the PERFORM-LOCATION-RESPONSE PDU the SGSN stops the MPM timer (if running).

If assistance data was instead requested by the SGSN for an MS, the BSS transfers the request to the SMLC according to the procedures defined in 3GPP TS 43.059 and 3GPP TS 49.031 and awaits the result. If the Requested GPS or GANSS Assistance Data IE was received from the MS, it is forwarded to the BSS. If the SMLC indicates to the BSS that it was able successfully to transfer this to the MS, the BSS shall return a PERFORM-LOCATION-RESPONSE PDU to the SGSN. This PDU shall contain the PTP BVCI indicating the PTP functional entity (i.e. the cell) upon which the last LLC-PDU was received from the MS but no other optional or conditional information elements. The absence of an LCS Cause parameter in this case implies that the transfer was successful.

Otherwise, if the deciphering keys were requested for LCS broadcast assistance data, the BSS transfers the request to the SMLC according to the procedures defined in 3GPP TS 43.059 and 3GPP TS 49.031 and awaits the result. If the BSS receives the deciphering keys, the BSS shall send them to the SGSN in a PERFORM-LOCATION-RESPONSE PDU containing also the PTP BVCI indicating the PTP functional entity (i.e. the cell) upon which the last LLC-PDU was received from the MS.

8b.1.1 Unsuccessful Operation

If the BSS fails to respond to the PERFORM-LOCATION-REQUEST PDU it returns a PERFORM-LOCATION-RESPONSE PDU with a LCS cause value indicating the failure cause.

If the BSS receives a failure indication from the SMLC it shall send a PERFORM-LOCATION-RESPONSE PDU to the SGSN with the LCS cause value that it received from the SMLC and stop the MPM timer (if running). Upon reception of the PERFORM-LOCATION-RESPONSE PDU the SGSN shall stop the MPM timer (if running).

8b.1.2 Abnormal Conditions

The following condition may occur:

If the SGSN needs to abort previously initiated location request, it shall send the PERFORM-LOCATION-ABORT PDU to the BSS. This PDU shall include the PTP BVCI indicating the PTP functional entity (i.e. the cell) upon which the last LLC-PDU was received from the MS. As a result of reception of this PDU the BSS shall abort activities related to positioning of the target MS or assistance data delivery. The BSS shall return a PERFORM-LOCATION-RESPONSE PDU with a cause value indicating the abortion of location request. The SGSN may reattempt the positioning request after the PERFORM-LOCATION-RESPONSE PDU is received from the BSS, but not before the PDU is received.

If the P-TMSI is reallocated for a target MS during the location procedure, the SGSN shall abort the location procedure.

If a SUSPEND PDU is received for a target MS during the location procedure, the SGSN shall abort the location procedure.

If a Routing Area Update request is received from a target MS during the location procedure, the SGSN shall abort the location procedure.

If an Inter NSE Cell Change, within the same routing area, occurs for a target MS during the location procedure, the SGSN shall provide the new NSEI and new BVCI in the FLUSH-LL PDU sent to the BSS, in order for the BSS to maintain the on-going location procedure, if possible. In case the BSS is unable to maintain the on-going location procedure, then a location abort shall be triggered by the BSS towards the SMLC.

8b.1.3 Overload

For location requests initiated by the SGSN, the BSC may employ the same procedures defined for an SMLC in 3GPP TS 49.031 to alleviate an overload condition in the BSS.

8b.2 Position Command Procedure

The position command procedure is used to convey an embedded RRLP message between the BSS and the MS.

8b.2.1 Position Command

The BSS initiates the position command procedure by sending the POSTION-COMMAND PDU to the SGSN. The procedure is only valid while a location procedure for the target MS is ongoing.

The POSITION-COMMAND PDU shall include the RRLP Flags and the RRLP APDU information elements and the PTP BVCI indicating the PTP functional entity (i.e. the cell) upon which the last LLC-PDU was received from the MS. The RRLP APDU information element carries the RRLP message and the RRLP Flags information element carries control information for RRLP.

The SGSN shall extract the RRLP message from the RRLP APDU information element and forward it, together with the RRLP Flags, to the MS in a TOM message carried in an LLC-PDU, see 3GPP TS 44.064.

An exception case is when the POSITION-COMMAND PDU contains the Multilateration Timer IE (i.e. the MTA procedure has been selected by the SMLC, see sub-clause 11.3.136) and the SGSN determines that it has neither coverage class nor eDRX information available for the target MS. In this case the SGSN sends a POSITION-RESPONSE PDU to the BSS with a ‘LCS cause’ value indicating ‘Target MS Unreachable’ (see sub-clause 11.3.58) and aborts the Location procedure. Otherwise, the SGSN proceeds as follows:

– A MPM timer is started for the target MS and the SGSN shall not attempt to deliver downlink data to that MS or page that MS for as long as the MPM timer continues to run.

– The DL-UNITDATA PDU used to relay the RRLP message to the BSS includes a Timing Advance Request IE (see sub-clause 10.2.1) if the POSITION-COMMAND PDU includes a Timing Advance Request IE (see sub-clause 10.5.4) indicating that timing advance information is needed.

If an UL-UNITDATA PDU is sent from the BSS to the SGSN for the target MS when a corresponding MPM timer is running then both the BSS and SGSN shall stop their respective MPM timers and abort the location procedure for that MS.

8b.2.2 Position Response

The SGSN initiates the position response procedure when it receives a TOM message in an LLC-PDU carrying an RRLP message for a target MS. The procedure is only valid while a location procedure for the target MS is ongoing.

When the SGSN receives a TOM message in an LLC-PDU carrying an RRLP message for a target MS, the SGSN shall extract the RRLP message and forward it to the BSS in a POSITION-RESPONSE PDU. The RRLP message shall be included in the RRLP APDU information element. The RRLP Flags information shall be extracted from the TOM header and be included in the RRLP Flags information element The POSITION-RESPONSE PDU shall also include the PTP BVCI indicating the PTP functional entity (i.e. the cell) upon which the last LLC-PDU was received from the MS.

8b.2.3 Unsuccessful Operation

If the SGSN fails to process the POSITION-COMMAND PDU it returns a POSITION-RESPONSE PDU with a LCS cause value indicating the failure cause.

If a POSITION-COMMAND PDU is received by the SGSN while a location procedure for the target MS is not ongoing a POSITION-RESPONSE PDU with a LCS cause value indicating this failure cause is returned.

If a POSITION-RESPONSE PDU is received by the BSS while a location procedure for the target MS is not ongoing the BSS shall ignore the PDU.