8d Signalling procedures between MBMS 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

8d.1 General

Upon receiving an MBMS-SESSION-START-REQUEST PDU from the SGSN, if the BSS controls cells in any of the MBMS Service Areas in the MBMS service area list the BSS creates an MBMS Service Context, and acknowledges the SGSN using an MBMS-SESSION-START-RESPONSE PDU. More than one MBMS-SESSION-START-RESPONSE PDU can be sent from one BSS to the SGSN for the same MBMS-SESSION-START-REQUEST PDU.

Upon receiving an MBMS-SESSION-UPDATE-REQUEST PDU from the SGSN, the BSS updates the MBMS service area list for the ongoing MBMS broadcast service session and acknowledges the SGSN using an MBMS-SESSION-UPDATE-RESPONSE PDU. More than one MBMS-SESSION-UPDATE-RESPONSE PDU can be sent from one BSS to the SGSN for the same MBMS-SESSION-UPDATE-REQUEST PDU.

At the end of the MBMS Session the BSS receives an MBMS-SESSION-STOP-REQUEST PDU from the SGSN indicating that the MBMS Session can be released. The BSS acknowledges the request to end the MBMS Session by sending the MBMS-SESSION-STOP-RESPONSE PDU to the SGSN. See 3GPP TS 43.246 ([29]).

8d.2 MBMS Session Start

The BSS creates an MBMS Service Context if the BSS controls cells in the MBMS service area list upon reception of an MBMS-SESSION-START-REQUEST PDU from the SGSN.

If the data is received by the BSS and no MBMS bearer is established on the radio interface for that MBMS Session the BSS may buffer the data.

At reception of an MBMS-SESSION-START-REQUEST PDU that leads to an MBMS Service Context creation in the BSS, the BSS shall respond to the SGSN with an MBMS-SESSION-START-RESPONSE PDU with a Cause Value indicating that data transfer shall be initiated on the Point-to-Multipoint BVC from that SGSN.

The SGSN may include the Allocation/Retention Priority information element in the MBMS-SESSION-START-REQUEST PDU. If this information element is received and the BSS supports ARP handling, the BSS shall establish or modify the resources according to the values of the Allocation/Retention Priority IE (priority level, pre-emption indicators) and the resource situation as follows:

– The BSS shall consider the priority level of the requested MBMS bearer, when deciding on the resource allocation.

– The priority levels and the pre-emption indicators may (singularly or in combination) be used to determine whether the MBMS bearer establishment has to be performed unconditionally and immediately. If the requested MBMS bearer is marked as "may trigger pre-emption" and the resource situation requires so, the BSS may trigger the pre-emption procedure which may then cause the forced release of a lower priority bearer which is marked as "pre-emptable". Whilst the process and the extent of the pre-emption procedure is operator-dependent, the pre-emption indicators, if given in the MBMS-SESSION-START-REQUEST PDU, shall be treated as follows:

1. If the Pre-emption Capability IE is set to "may trigger pre-emption", then this allocation request may trigger the pre-emption procedure. The BSS shall only pre‑empt bearers (other MBMS bearers or MS specific bearers) with lower priority, in ascending order of priority.

2. If the Pre-emption Capability IE is set to "shall not trigger pre-emption", then this allocation request shall not trigger the pre-emption procedure.

3. If the Pre-emption Vulnerability IE is set to "pre-emptable", then this connection shall be included in the pre-emption process.

4. If the Pre-emption Vulnerability IE is set to "not pre-emptable", then this connection shall not be included in the pre-emption process.

5. If the Priority Level IE is set to "no priority" the given values for the Pre-emption Capability IE and Pre-emption Vulnerability IE shall not be considered. Instead the values "shall not trigger pre-emption" and "not pre-emptable" shall prevail.

– If the Allocation/Retention Priority IE is not given in the MBMS-SESSION-START-REQUEST PDU, the allocation request shall not trigger the pre-emption process and the connection may be pre-empted and considered to have the value "lowest" as priority level.

– The SGSN shall not include, and the BSS shall ignore, any queuing allowed indication in the Allocation/Retention Priority IE of the MBMS-SESSION-START-REQUEST PDU.

The MBMS Session Repetition Number IE shall be included in the MBMS-SESSION-START-REQUEST PDU in case the MBMS Session Identity IE is included in the same PDU (and vice versa). The MBMS Session Repetition Number IE allows the BSS to recognize retransmissions of a specific session of an MBMS bearer service. The value part of this IE may be used for e.g. deciding whether or not to initiate the counting procedure on a per cell basis (see 3GPP TS 44.018, 3GPP TS 44.060) or, in conjunction with the values of Allocation/Retention Priority IE, whether or not to establish an MBMS radio bearer for the session on a per cell basis.

At reception of an MBMS-SESSION-START-REQUEST PDU with the same TMGI IE and MBMS Session Identity IE as an ongoing MBMS Service Context, the BSS shall respond to the SGSN with an MBMS-SESSION-START-RESPONSE PDU with a Cause Value indicating that data transfer has already been initiated on the Point-to-Multipoint BVC from another SGSN.

At reception of an MBMS-SESSION-START-RESPONSE PDU, the SGSN shall either start data transfer or not depending on the received Cause Value.

After transmission of the MBMS-SESSION-START-RESPONSE PDU, the BSS shall wait at least the time specified in the value part of the Time to MBMS Data Transfer IE included in the MBMS-SESSION-START-REQUEST PDU and at most a time exceeding by 5 seconds the value part of the Time to MBMS Data Transfer IE for the first reception of the associated data before the BSS validates whether or not there is another SGSN that previously has sent an MBMS-SESSION-START-REQUEST PDU.

If after the start of the data flow associated to an MBMS Service Context, the BSS does not receive data for at least 30 seconds and the BSS has not received the MBMS-SESSION-STOP-REQUEST PDU, the BSS validates whether or not there is another SGSN that previously has sent an MBMS-SESSION-START-REQUEST PDU.

If, in any of the two cases mentioned above, another SGSN has sent an MBMS-SESSION-START-REQUEST PDU, the BSS shall send an MBMS-SESSION-START-RESPONSE PDU to such an SGSN with a Cause Value indicating that data transfer shall be initiated on the Point-to-Multipoint BVC from that SGSN. Otherwise, the BSS shall end the MBMS Service Context.

In any case, the BSS will send an MBMS-SESSION-START-RESPONSE PDU with a Cause Value indicating that the MBMS Service Context has been released due to interrupted data flow to the SGSN that previously has been ordered to perform data transfer.

If the BSS does not support any MBMS Service Area in the MBMS Service Area Identity List the BSS will send an MBMS-SESSION-START-RESPONSE PDU to the SGSN with Cause Value indicating that none of the listed MBMS Service Areas are supported by the BSS.

Figure 8d.2: MBMS Session Start procedure

8d.2.1 Abnormal Conditions

In any failure case in BSS the BSS may send an MBMS-SESSION-START-RESPONSE PDU including a Cause Value indicating the reason for the failure.

If an MBMS-SESSION-START-RESPONSE PDU is not received in response to an MBMS-SESSION-START-REQUEST PDU within T11 seconds, then the MBMS-SESSION-START-REQUEST PDU shall be repeated a maximum of MBMS-SESSION-START-REQUEST-RETRIES attempts. After MBMS-SESSION-START-REQUEST-RETRIES + 1 attempts the procedure is stopped and the O&M is informed.

8d.3 MBMS Session Stop

The SGSN may terminate an MBMS Session in the BSS by sending the MBMS-SESSION-STOP-REQUEST PDU to the BSS. The SGSN shall include the MBMS Stop Cause IE in the MBMS-SESSION-STOP-REQUEST PDU to indicate to the BSS if the MBMS Session termination has been ordered by an upstream node or if the SGSN itself has decided to terminate the MBMS Session (due to e.g. that the last MS that has an active MBMS UE Context for the MBMS Session within the SGSN has left the routing area(s) handled by the BSS).

The BSS ends an MBMS Service Context upon reception of an MBMS-SESSION-STOP-REQUEST PDU, including the MBMS Stop Cause IE indicating that an upstream node is terminating the MBMS Session, from the SGSN and acknowledges with an MBMS-SESSION-STOP-RESPONSE PDU.

At reception of an MBMS-SESSION-STOP-REQUEST PDU including the MBMS Stop Cause IE indicating that the SGSN is terminating the MBMS Session, the BSS shall validate whether or not there is another SGSN that previously has sent an MBMS-SESSION-START-REQUEST PDU or, in case of an MBMS broadcast service session, an MBMS-SESSION-UPDATE-REQUEST PDU.

If another SGSN has sent an MBMS-SESSION-START-REQUEST PDU or an MBMS-SESSION-UPDATE-REQUEST PDU, the BSS shall send an MBMS-SESSION-START-RESPONSE PDU or an MBMS-SESSION-UPDATE-RESPONSE PDU, respectively, to such an SGSN with a Cause Value indicating that data transfer shall be initiated on the Point-to-Multipoint BVC from that SGSN. Otherwise, the BSS shall end the MBMS Service Context. The BSS shall then acknowledge the MBMS-SESSION-STOP-REQUEST PDU by sending an MBMS-SESSION-STOP-RESPONSE PDU to the SGSN.

Figure 8d.3: MBMS Session Stop procedure

8d.3.1 Abnormal Conditions

If an MBMS-SESSION-STOP-RESPONSE PDU is not received in response to an MBMS-SESSION-STOP-REQUEST PDU within T11 seconds, then the MBMS-SESSION-STOP-REQUEST PDU shall be repeated a maximum of MBMS-SESSION-STOP-REQUEST-RETRIES attempts. After MBMS-SESSION-STOP-REQUEST-RETRIES + 1 attempts the procedure is stopped and the O&M is informed.

8d.4 MBMS Session Update

Upon reception of an MBMS-SESSION-UPDATE-REQUEST PDU from the SGSN for an ongoing MBMS broadcast service session, the BSS creates an MBMS Service Context if the BSS controls cells in the MBMS service area list and there is no ongoing MBMS Service Context identified with the same TMGI IE and, if available, MBMS Session Identity IE in the BSS.

Upon reception of an MBMS-SESSION-UPDATE-REQUEST PDU with the same TMGI IE and, if available, MBMS Session Identity IE as an ongoing MBMS Service Context but with (a) new MBMS Service Area(s) added to the MBMS Service Area Identity List IE, the BSS may send assignments for the ongoing MBMS broadcast service session to the mobile stations in the new MBMS Service Area(s) and repeat notifications to the mobile stations in the old MBMS Service Area(s).

Upon reception of an MBMS-SESSION-UPDATE-REQUEST PDU with the same TMGI IE and, if available, MBMS Session Identity IE as an ongoing MBMS Service Context but without (an) old MBMS Service Area(s) included any longer in the MBMS Service Area Identity List IE, the BSS shall release MBMS radio bearers relevant to the ongoing MBMS broadcast service session in the old MBMS Service Area(s).

The MBMS Session Information IE shall denote a Broadcast MBMS Session.

The Allocation/Retention Priority IE, if available, and the MBMS Session Repetition Number IE, if available, shall be handled by the BSS as described in the MBMS Session Start procedure (see sub-clause 8d.2).

If the Allocation/Retention Priority IE is not present in the MBMS-SESSION-UPDATE-REQUEST PDU, the allocation request shall not trigger the pre-emption process and the connection may be pre-empted and considered to have the value "lowest" as priority level.

If the data is received by the BSS and no MBMS bearer is established on the radio interface for that MBMS Session, the BSS may buffer the data.

At reception of an MBMS-SESSION-UPDATE-REQUEST PDU that leads to an MBMS Service Context creation in the BSS, the BSS shall respond to the SGSN with an MBMS-SESSION-UPDATE-RESPONSE PDU with a Cause Value indicating that data transfer shall be initiated on the Point-to-Multipoint BVC from that SGSN.

At reception of an MBMS-SESSION-UPDATE-REQUEST PDU with the same TMGI IE and, if available, MBMS Session Identity IE as an ongoing MBMS Service Context and including new and/or removing old MBMS Service Area(s), the BSS shall respond to the SGSN with an MBMS-SESSION-UPDATE-RESPONSE PDU with a Cause Value indicating either that data transfer shall continue on the Point-to-Multipoint BVC from that SGSN (see note) or that data transfer has already been initiated on the Point-to-Multipoint BVC from another SGSN.

NOTE: The Cause Value indicating that data transfer shall continue on the Point-to-Multipoint BVC from that SGSN is set to the same one denoting that data transfer shall be initiated on the Point-to-Multipoint BVC from that SGSN, i.e. ‘0001’ (see sub-clause 11.3.74).

If the BSS does not support any MBMS Service Area in the MBMS Service Area Identity List IE, the BSS shall send an MBMS-SESSION-UPDATE-RESPONSE PDU to the SGSN with a Cause Value indicating that none of the listed MBMS Service Areas are supported by the BSS.

At reception of an MBMS-SESSION-UPDATE-RESPONSE PDU, the SGSN shall either start/continue data transfer or not depending on the received Cause Value.

After transmission of the MBMS-SESSION-UPDATE-RESPONSE PDU, the BSS shall wait at least the time specified in the value part of the Time to MBMS Data Transfer IE included in the MBMS-SESSION-UPDATE-REQUEST PDU and at most a time exceeding by 5 seconds the value part of the Time to MBMS Data Transfer IE for the first reception of the associated data before the BSS validates whether or not there is another SGSN that previously has sent an MBMS-SESSION-UPDATE-REQUEST PDU or an MBMS-SESSION-START-REQUEST PDU with the same content of the received MBMS-SESSION-UPDATE-REQUEST PDU.

If, in any of the two cases mentioned above, another SGSN has sent an MBMS-SESSION-UPDATE-REQUEST PDU or an MBMS-SESSION-START-REQUEST PDU, the BSS shall send an MBMS-SESSION-UPDATE-RESPONSE PDU or an MBMS-SESSION-START-RESPONSE PDU, respectively, to such an SGSN with a Cause Value indicating that data transfer shall be initiated on the Point-to-Multipoint BVC from that SGSN. Otherwise, the BSS shall end the MBMS Service Context.

In any case, the BSS shall send an MBMS-SESSION-UPDATE-RESPONSE PDU with a Cause Value indicating that the MBMS Service Context has been released due to interrupted data flow to the SGSN that previously has been ordered to perform data transfer.

Figure 8d.4: MBMS Session Update procedure

8d.4.1 Abnormal Conditions

In any failure case in BSS the BSS may send an MBMS-SESSION-UPDATE-RESPONSE PDU including a Cause Value indicating the reason for the failure.

If an MBMS-SESSION-UPDATE-RESPONSE PDU is not received in response to an MBMS-SESSION-UPDATE-REQUEST PDU within T11 seconds, then the MBMS-SESSION-UPDATE-REQUEST PDU shall be repeated a maximum of MBMS-SESSION-UPDATE-REQUEST-RETRIES attempts. After MBMS-SESSION-UPDATE-REQUEST-RETRIES + 1 attempts the procedure is stopped and the O&M is informed.