The OMA's PoC efforts will enable the use of SIP as the signaling protocol over cellular data networks. The OMA believes that with the increasing bandwidth offered by next-generation cellular, all PTT communication can take place on the data side of the cellular network, without impacting any voice bandwidth. That approach clearly distinguishes PoC from iDEN or Kodiak Networks' Real-Time Exchange, another circuit-switched PTT technology. The OMA explicitly mentions supporting cellular networks developed by the 3GPP and 3GPP2 industry groups, which provide the 3G specs GSM and CDMA, respectively.
Major mobile telecommunications players, including Motorola, Nokia, Siemens and Sony Ericsson, along with a handful of domestic and international cell carriers, such as AT&T Wireless, are backing OMA.
The PoC standard extends the SIP framework to facilitate PTT communication. SIP messages are used end-to-end for session dialog between all devices and servers, with some new procedures added to the SIP stack that are relevant to the handling of wireless SIP UAs (User Agents). Unlike a standard SIP UA, in which other UAs or proxies communicate directly with the end device, PoC introduces a server that caches the status and settings of the wireless device. When a wireless UA joins the network or changes its status, it updates the PoC server using "publish" and "notify" messages. When another device wants to contact this particular UA, it checks with the PoC server to determine if the wireless device is ready to receive PTT requests, then proceeds accordingly. This process effectively prevents unnecessary wireless bandwidth use.
Status, supported natively in SIP as "presence," is important to PTT technology. Through presence information, the phones and network know if a user is available.
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