Push-to-talk over Cellular
Push-to-talk over Cellular (PoC) was the first IMS-based service deployed by several mobile operators because it does not require the deployment of new radio technologies. PoC can run on top of low-bandwidth and high-delay links, which are inappropriate for running other types of services, such as voice calls.
PoC is a walkie-talkie type of service. Users press (and hold) a button when they want to say something, but they do not start speaking until their terminal tells them to do so (usually by beeping). At this point, users say whatever they want to say and signal the end of their speech by releasing the button.
Unlike regular voice calls, which are full-duplex, PoC is a half-duplex service; that is, only one user can speak at a time.
PoC sessions can have more than two participants. At a given time, one user speaks and the rest listen (as in the two-party case). A simple way of understanding a multiparty PoC is a group of friends going to the movies. One at a time, they take turns to tell the rest which movie they want to watch, at which point they can make the final choice (usually after some extra rounds of discussions).
Even though Push-to-talk was originally designed to be a voice-only service, it currently supports different media types such as streamed video and instant messages.
25.1 PoC Standardization
When the definition of the PoC service started there were several incompatible PoC specifications. Many were not based on the IMS, ...