Content for TS 26.346 Word version: 19.1.0
8A Group Communication delivery method
8A.1 Overview
8A.2 Transport method
8A.3 Signalling of GCS application service content
8A.4 ROHC for Group Communication delivery method
8A.5 SDP exchange for FEC support with the Group Communication delivery method
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8A Group Communication delivery method |R13| p. 105
8A.1 Overview p. 105
The MBMS Group Communication delivery method delivers a UDP/IP packet flow to the UE. The BM-SC provides Group Communication delivery by receiving UDP/IP packets and forwarding them over the MBMS path provided by the MBMS Bearer Service. Both IPv4 and IPv6 may be used by the Group Communication delivery method.
Figure 8A.1 depicts a reference model of the GCSE architecture with a MCPTT Server as the GCS AS, and shows how GCS application data is ingested by the BM-SC via the MB2 interface.
Figure 8A.1: Reference Model of GCS Architecture for MCPTT service delivery over eMBMS via BM-SC group communication delivery method
(⇒ copy of original 3GPP image)
(⇒ copy of original 3GPP image)
8A.2 Transport method p. 106
The application transport protocol on top of the MB2 UDP/IP is transparent to the BM-SC and is not defined in this specification.
Upon reception of GCS AS UDP/IP packets, the BM-SC removes the UDP/IP header and performs UDP/ IP encapsulation of the user plane IP data that was received over the MB2-U interface.
If requested by the GCS-AS over the MB2 interface, the BM-SC shall perform ROHC header compression over the broadcast link (between BM-SC and UE). The header compression shall be applied on the encapsulated UDP/IP packets according to the limitations in clause 8A.4.
If FEC protection is requested by the GCS AS, the BM-SC shall use the MBMS FEC scheme as specified in clause 8.2.2.
8A.3 Signalling of GCS application service content p. 106
8A.3.1 General |R17| p. 106
The GCS AS signals the GCS application service content to the UE over the GC1 interface.
The GCS application client in the UE will use the signalled information provided by the GCS AS to enable reception of GCS application data over the appropriate MBMS Bearer(s). This signalling shall include TMGI associated to this MBMS bearer, multicast IP address, UDP port and source IP address. The MCPTT server signals group calls over MBMS by combining the announcement of the MBMS bearer (see TS 23.280), which provides the TMGI and session description parameters, and signalling messages, which indicates the delivery start over MBMS of a group call and provides additional media parameters (see TS 23.379).
8A.3.2 Void
8A.4 ROHC for Group Communication delivery method |R15| p. 106
8A.4.1 General p. 106
If ROHC is enabled for the GC delivery session at reference point MB2, then the following conditions shall apply:
- Only the U-mode shall be used throughout the lifetime of the session. Other modes shall not be used.
- A single ROHC channel per GC delivery session shall be used.
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Exactly one of the following alternatives as specified in RFC 3095 shall be used:
- the RTP/UDP/IP profile with identifier 0x0001,
- the UDP/IP profile with identifier 0x0002, or
- the uncompressed IP profile with identifier 0x0000.
- UDP flow encoded with profile 0x0001 or profile 0x0002 shall use different context IDs.
- A common context ID may be used for all flows encoded with profile 0x0000.
- The LARGE_CIDS flag may be set to true. LARGE_CIDS value can be derived from the value from the MAX_CID flag.
- ROHC segmentation shall not be used and the MRRU value shall be set to 0
8A.5 SDP exchange for FEC support with the Group Communication delivery method |R15| p. 107
The GCS AS can request the BM-SC to apply FEC to protect the flow delivered with the group communication delivery method, as specified in TS 23.468 and TS 29.468, by including the FEC-Request AVP in the MBMS-Bearer-Request AVP. The FEC-Request AVP contains an SDP describing the FEC framework configuration information (see Section 5.5 of RFC 6363), which includes identification of the set of source flows, repair flows, and other FEC parameters.
To describe the FEC framework configuration information,
- the exchanged SDP shall include the sender IP address, using the source-filter attribute ("a=source-filter:") as specified in clause 8.3.1.1,
- the exchanged SDP shall include the destination IP address using the "connection data" field ("c="), as specified in clause 8.3.1.2,
- the exchanged SDP shall include, at session level, a FEC declaration attribute ("a=FEC-declaration") as specified in clause 7.3.2.8, a "a=FEC-OTI-extension" attribute specified in clause 8.3.1.8 and a "a=mbms-repair" attribute, as specified in clause 8.3.1.8,
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for each RTP/UDP source flow to be protected by FEC, the exchanged SDP shall include a media block
- where the media ('m-line') provides the destination port, as specified in clause 8.3.1.2, and uses 'UDP/MBMS-FEC/RTP/AVP' or 'UDP/MBMS-FEC/RTP/SAVP' as protocol identifier (see clause 8.2.2.13a),
- with the "a=FEC" attribute, specified in clause 7.3.2.8.
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for each repair flow, the exchanged SDP shall include a media block
- where the media ('m-line') provides the destination port, as specified in clause 8.3.1.2, and uses 'UDP/MBMS-REPAIR' as protocol identifier (see clause 8.2.2.13a),
- with the "a=mbms-flowid" attribute, as specified in clause 8.3.1.9.
