The IMS: IP Multimedia Concepts And Services, Second Edition

6.3. User plane

The user plane of PoC includes three components, as illustrated in Figure 6.9: the media flow (consisting of talk bursts), talk burst control and quality feedback.

The participating PoC server relays talk bursts, talk burst control messages and quality feedback measurements back and forth between the controlling PoC server and the PoC client, except in the following scenarios:

• the PoC client has a pre-established session with the PoC server;

• the PoC client and PoC server support simultaneous sessions;

• the PoC server needs media transport logging to support charging funtionality;

• the PoC server is used for transcoding services or other media adaptations;

• the PoC server needs to support lawful interception;

• the PoC server acts as a gateway between two different talk burst control protocols.

Figure 6.9. User-plane protocol entities.

The PoC server – in performing both the participating and controlling function – acts as an RTP translator. A translator, as opposed to an RTP mixer, only passes media packets from one IP flow to another, without re-packetizing or alteration to the media format.

6.3.1. Talk bursts

A talk burst is simply a single burst of media flowing from a participant to the controlling PoC server. The controlling PoC server distributes the talk burst to all of the participants of the session. The participants receive the media flow, and render it via their ...