THE IMS: IP Multimedia Concepts and Services, 3rd Edition

6

Push to Talk Over Cellular

Push to talk over Cellular (PoC) provides a direct one-to-one and one-to-many voice communication service. The idea is simple. Users select the individuals or groups they wish to talk to, and then press the push to talk key to start talking. The session is connected in real time. Push to talk sessions are one-way communication: while one person speaks, the other(s) only listens. The turns to speak are requested by pressing the push to talk key and granted on a first-come-first-served basis. Push to talk speech is usually connected without the recipients answering and heard through the phone's built-in loudspeaker. Alternatively, a user can choose to receive push to talk sessions only after accepting an invitation. If more privacy is needed, they can also listen to sessions through an earphone or headset.

The push to talk service is based on multiunicasting. Each sending client sends packet data traffic to a dedicated push to talk application server and, in the case of a group session, the server then duplicates the traffic to all the recipients (see Figure 6.1). No multicasting is performed either in the access or core network and mobility management is carried out by the radio network. This is why the push to talk solution works transparently over cellular and fixed networks. PoC session control and other signalling is based on Session Initiation Protocol (SIP) and voice traffic is carried through a Real-time Transport Protocol/Real-time Transport ...