8.36 MBMS Session Start

25.4133GPPRelease 17TSUTRAN Iu interface Radio Access Network Application Part (RANAP) signalling

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

8.36.1 General

The purpose of the MBMS Session Start procedure is to request the UTRAN to notify UEs about an upcoming MBMS Session of a given MBMS Bearer Service and to establish a MBMS RAB and MBMS Iu signalling connection for this MBMS Session. The MBMS Session Start procedure is triggered by the CN (PS domain).

The procedure uses connection oriented signalling.

8.36.2 Successful Operation

Figure 46: MBMS Session Start procedure. Successful operation.

The CN initiates the procedure by sending a MBMS SESSION START message.

The MBMS SESSION START message shall contain:

– TMGI;

– MBMS Bearer Service Type;

– MBMS Session Identity, if available;

– Iu Signalling Connection Identifier IE;

– RAB parameters (including e.g. Allocation/Retention Priority);

– PDP Type Information, if available;

– PDP Type Information extension, if available;

– MBMS Session Duration;

– MBMS Service Area;

– Frequency Layer Convergence Flag, if available;

– RA List of Idle Mode UEs, if available;

– Global CN-ID IE, only when the MBMS SESSION START message is sent from a CN node towards an RNC for which the sending CN node is not the default CN node;

– MBMS Session Repetition Number, if available;

– Time to MBMS Data Transfer;

– MBMS Counting Information, if available.

Upon reception of the MBMS SESSION START message, the RNC shall store the Iu Signalling Connection Identifier IE for the duration of the MBMS Iu signalling connection. The Iu Signalling Connection Identifier IE contains an Iu signalling connection identifier which is allocated by the CN. The value for the Iu Signalling Connection Identifier IE shall be allocated so as to uniquely identify an Iu signalling connection for the involved CN node.

The Global CN-ID IE contains the identity of the CN node that sent the MBMS SESSION START message, and it shall, if included, be stored together with the Iu signalling connection identifier. If the Global CN-ID IE is not included, the MBMS SESSION START message shall be considered as coming from the default CN node.

Upon reception of the MBMS SESSION START message, the RNC shall store, if not already, and remember the TMGI IE, the RAB parameters IE and the other attributes of the session as part of the MBMS Service Context. The TMGI IE contains the TMGI identifier which uniquely identifies the MBMS Bearer Service.

If the RNC receives the MBMS SESSION START message including the Session Re-establishment Indicator IE for a service which is already ongoing, it shall, if supported, accept this message and replace the MBMS context for that service (see TS 23.007 [53]).

If the MBMS Bearer Service Type IE is set to "Multicast", upon reception of the MBMS SESSION START message, the RNC shall initiate allocation of requested resources for the MBMS RAB at a proper point in time if at least one of the following two conditions is fulfilled:

– the RNC controls at least one cell contained in the indicated MBMS Service Area and, if the RA List of Idle Mode UEs IE is included in MBMS SESSION START message, at least one RNC´s RA is contained in this list, or if Empty/Full RA List of Idle Mode UEs IE included in MBMS SESSION START message is set to "fulllist",

– the RNC serves UEs consuming radio resources from cells contained in the indicated MBMS Service Area.

If no mapping is configured for a certain MBMS Service Area Identity in the MBMS Service Area IE in the RNC it shall simply ignore it

The RNC may optimise the point in time when the resource allocation is initiated based on the Time to MBMS Data Transfer IE.

In case the RA List of Idle Mode UEs IE is included in MBMS SESSION START message but none of above conditions is fulfilled, the RNC may decide to wait for either an update of the RA List of Idle Mode UEs or a UE linking to finally establish the MBMS RAB. If the RNC decides so, it shall report it immediately to the CN in the MBMS SESSION START RESPONSE message with the cause value "Successful MBMS Session Start – No Data Bearer Necessary" and maintain an Iu signalling connection.

If the Empty/Full RA List of Idle Mode UEs IE included in MBMS SESSION START message is set to "fulllist", the RNC shall initiate the MBMS Notification over the Uu interface in all the cells under its control which belong to the indicated MBMS service area.

If the MBMS Bearer Service Type IE is set to "Broadcast", the MBMS Counting Information IE shall be included in the MBMS SESSION START message. If the MBMS Counting Information IE is set to "counting", the RNC may apply MBMS counting.

If the MBMS Bearer Service Type IE is set to "Broadcast" upon reception of the MBMS SESSION START message, the RNC shall initiate allocation of requested resources for the MBMS RAB at a proper point in time if it controls at least one cell contained in the indicated MBMS Service Area.

If the MBMS Bearer Service Type IE is set to "Multicast" the MBMS Counting Information IE shall be ignored.

The allocation of requested resources shall be made according to the values of the Allocation/Retention Priority IE (priority level, pre-emption indicators) and the resource situation as follows:

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

– The Queuing Allowed IE shall be ignored for MBMS RAB.

– The priority levels and the pre-emption indicators may (singularly or in combination) be used to determine whether the MBMS RAB establishment has to be performed unconditionally and immediately. If the requested MBMS RAB is marked as "may trigger pre-emption" and the resource situation requires so, the RNC may trigger the pre-emption procedure which may then cause the forced release of a lower priority RAB 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 message, 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. UTRAN shall only pre‑empt RABs (other MBMS RABs or UE specific RABs) 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 message, 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. Moreover, queuing shall not be allowed.

The UTRAN shall use the PDP Type Information IE or PDP Type Information extension IE to configure any compression algorithms.

The MBMS SESSION START message may contain the MBMS Synchronisation Information IE, consisting of the

– MBMS HC Indicator IE;

– IP Multicast Address IE;

– GTP DL TEID IE;

– IP Source Address IE, if available.

In case of successful MBMS RAB establishment, if the MBMS Synchronisation Information IE was not received within the MBMS SESSION START message, the RNC shall include the Transport Layer Address IE and the Iu Transport Association IE in the MBMS SESSION START RESPONSE message. The RNC may answer successfully even though the MBMS resources have not been established in all relevant cells.

If NNSF is active, the RNC may receive from several CN nodes for a certain MBMS Bearer Service the MBMS SESSION START message. In this case, if the RNC decides to establish the requested MBMS RAB, it shall only establish one MBMS Iu bearer and shall inform the selected CN node accordingly i.e. with MBMS SESSION START RESPONSE message including the Transport Layer Address IE and the Iu Transport Association IE.

If the MBMS Synchronisation Information IE was received within the MBMS SESSION START message, even if the RNC received related information more than once due to NNSF, if supported, it shall not include the Transport Layer Address IE and the Iu Transport Association IE in the MBMS SESSION START RESPONSE message and shall join the indicated IP Multicast group only once for the indicated MBMS Service Id. In case of successful joining the indicated IP Multicast group, the RNC shall inform all the CN nodes from which it has received a MBMS SESSION START message for that MBMS service with the Cause IE set to "Successful MBMS Session Start – IP Multicast Bearer established". If the IP Source Address IE is contained in the MBMS Synchronisation Information IE, the RNC shall use this information for joining the IP Multicast group.

If the RNC receives from several CN nodes for a certain MBMS Bearer Service the MBMS SESSION START message and all the MBMS SESSION START messages include the RA List of Idle Mode UEs IE and does not include the Session Re-establishment Indicator IE, the RNC shall, if supported, maintain an MBMS Iu signalling connection towards all the CN nodes and inform them accordingly i.e. with MBMS SESSION START RESPONSE message and cause value "Successful MBMS Session Start – No Data Bearer Necessary" to all the CN nodes except the one, if any, towards which the RNC confirmed the successful MBMS RAB establishment.

The MBMS Session Repetition Number IE may be included in the MBMS SESSION START message in case the MBMS Session Identity IE is included in the same message. The MBMS Session Repetition Number IE may be used by RNC to recognise retransmissions of a particular session of a MBMS Bearer Service with identical contents. This IE may be used for counting purpose.

When the Frequency Layer Convergence Flag IE is set to "no-FLC-flag", the RNC is being requested to not apply any frequency layer convergence mechanism. The service shall then be delivered to all cells of all the MBMS Service Area Identities indicated in the MBMS Service Area IE.

Transmission and reception of a MBMS SESSION START RESPONSE message terminate the procedure in the UTRAN and in the CN respectively.

8.36.3 Unsuccessful Operation

Figure 47: MBMS Session Start procedure. Unsuccessful operation.

If the RNC is not capable of correctly processing the request (e.g. the MBMS resources could not be established at all in any cell), the CN shall be informed by the MBMS SESSION START FAILURE message.

If the MBMS Bearer Service Type IE is set to "Broadcast" upon reception of the MBMS SESSION START message and the RNC doesn’t have any cell contained in the indicated MBMS Service Area, it shall report it immediately to the CN in the MBMS SESSION START FAILURE message with the cause value "MBMS – No cell in MBMS Service Area".

If NNSF is active and the RNC received from several CN nodes for a certain MBMS Bearer Service the MBMS SESSION START message, but not all of the MBMS SESSION START messages include the RA List of Idle Mode UEs IE, the RNC shall inform the respective CN nodes accordingly i.e. with MBMS SESSION START FAILURE message and cause value "MBMS – Superseded Due To NNSF" to all the CN nodes except the one towards which the RNC confirmed the successful MBMS RAB establishment with MBMS SESSION START RESPONSE message.

When UTRAN reports failure of the MBMS Session Start procedure, the cause value should be precise enough to enable the core network to know the reason for the failure. Typical cause values are: "MBMS – Superseded Due To NNSF", "Requested Traffic Class not Available", "Invalid RAB Parameters Value", "Requested Maximum Bit Rate not Available", "Requested Guaranteed Bit Rate not Available", "Requested Transfer Delay not Achievable", "Invalid RAB Parameters Combination", "Condition Violation for Guaranteed Bit Rate", "Iu Transport Connection Failed to Establish", "No Resource Available".

Transmission and reception of a MBMS SESSION START FAILURE message terminate the procedure in the UTRAN and in the CN respectively.

8.36.4 Abnormal Conditions

If, for a MBMS RAB requested to be set up, the PDP Type Information IE and/or PDP Type Information extension IE is not present, the RNC shall continue with the procedure.

If an MBMS SESSION START message from a given CN Node provides a TMGI IE that is used for an already established and running MBMS Session provided by the same CN Node, and the indicated MBMS Service Area IE refers to an MBMS Service Area that is partially or completely overlapping with the MBMS Service Area of the already established and running MBMS Session, and the Session Re-establishment Indicator IE is not included in the MBMS SESSION START message, then the RNC shall return an MBMS SESSION START FAILURE message with the cause value "TMGI in Use and overlapping MBMS Service Area ".

If an MBMS SESSION START message from a given CN Node provides a TMGI IE that is used for an already established and running MBMS Session provided by another CN Node, and the indicated MBMS Service Area IE refers to a different MBMS Service Area that is partially overlapping with the MBMS Service Area of the already established and running MBMS Session, and the Session Re-establishment Indicator IE is not included in the MBMS SESSION START message, then the RNC shall return an MBMS SESSION START FAILURE message with the cause value "TMGI in Use and overlapping MBMS Service Area ".