Content for  TS 23.280  Word version:  19.1.0

The MC service server may use an MBMS bearer for application level control signalling, according to this subclause. An MBMS bearer for application level control signalling is typically used for the purposes beyond the benefit for using MBMS for resource efficiency, e.g. for improved MC service performance (KPIs), handling of high load scenarios. The MBMS bearer for application level control signalling may be used to transmit the following messages:

• Transmission control (e.g. call setup and floor control)
• MBMS bearer announcement for media bearers
• Group application paging
• Group dynamic data (e.g. status of the group)
• Group state (e.g. emergency alerts)

An MBMS bearer for application level control signalling is activated in a service area that is larger than the estimated service for media bearers. The service area for the media bearers mainly based on counting of group members in each defined service area. The MBMS bearer for application level control signalling is also activated with a QoS that is better than MBMS media bearers since the packet loss requirements are much stricter. The MC service client shall not send responses to group-addressed application level control signalling unless instructed or configured to respond.

The procedure in Figure 10.7.3.4.2-1 shows only one of the receiving MC service clients using an MBMS bearer.

The MBMS announcement may be done on either a unicast bearer or a MBMS bearer. Using a unicast bearer for MBMS bearer announcement provides an interactive way of doing announcement. The MC service server will send the MBMS bearer announcement message to the MC service client regardless if there is an MBMS bearer active or the MC service client can receive the data on the MBMS bearer with sufficient quality. The benefit of the existing procedure is that it gives a secure way to inform the MC service client about the MBMS bearer and how to retrieve the data on the MBMS bearer. When there is more than one MBMS bearer active in the same service area for MC service, there are not the same reasons to use unicast bearer for additional MBMS bearer announcement. Instead a MBMS bearer for application level control signalling can be used to announce additional MBMS bearers. The MBMS bearer announcement messages are sent on an MBMS bearer used for application control messages. This bearer will have a different QoS setting compared to an MBMS bearer used for media, since application signalling messages are more sensitive to packet loss.

The procedure defined below enables the MC service to announcement a new MBMS bearer. Pre-conditions:

• An MBMS bearer used for MC service application control messages must have been pre-established and announced to the MC service client.
• Additional MBMS bearer information may have already been announced to the client.

The same procedure can also be used to modify existing MBMS bearer announcement information. Example of such modification could be addition of UDP ports or modification of codec in the SDP.

The MC service client and MC service server use this procedure to report and take action on the MBMS bearer quality. An MC service client monitors an MBMS bearer to receive MC service media. Based on the received quality (e.g. radio level quality, transport level quality), the MC service client needs to inform the MC service server that the MC service client is able to receive the MC service media on the MBMS bearer with sufficient quality or not able to receive the MC service media on the MBMS bearer with sufficient quality. Furthermore, based on the received quality, the MC service client may notify the MC service server at which MBMS reception quality level it has received the MC service media on the MBMS bearer. The issue can be more complex since the MC service client needs to estimate the quality of the bearer even in the scenario when there are no data currently transmitted on the MBMS bearer (e.g. between MCPTT group call). The reason for this is that an MC service client that has entered an area with significantly degraded MBMS quality, might not even notice that an MC service communication is ongoing, meanwhile the MC server still assumes that the MC service client can receive the media being broadcasted. To estimate the MBMS bearer quality, for example as an equivalent BLER (Block Error Rate), when no data is sent is implementation specific. This estimation can be dependent on for example the modulation and coding scheme (MCS) and measurements from the reference signals from the eNB(s). Other metrics (e.g. RTP packet loss) may be used to estimate the MBMS bearer quality.

The MC service client shall indicate the ability of the MC service client to receive the MBMS bearer. Pre-conditions:

• There is an MBMS bearer activated and the MBMS bearer information is announced to the MC service client
• The MC service client is located in the MBMS broadcasting area
• The MC service UE monitors SIB-13 (or SIB-20) and (SC-)MCCH to receive the modulation and coding scheme
• The MC service UE monitors the cell specific reference signal and when MBSFN transmission is used, the MBSFN specific reference signals