Push-to-talk

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Push-to-talk (PTT) is a method of conversing on half-duplex communication lines, including two-way radio, using a button to switch from voice transmission mode to voice reception mode.

How the technology works

The PTT (Push-to-Talk) service uses a combination of half-duplex and full-duplex signals in order for the service to work. The digital two way radio makes use of the half duplex signal. Full-duplex signals allow both parties to talk at the same time by using two separate frequencies. Full-duplex signals are used when a customer places a traditional cellular call. This technology uses one frequency to send information, and another to receive making the voice call possible. As with all cellular service, both full-duplex and half-duplex signals are actually radio frequencies capable of reaching great distances. The PTT (Push-to-Talk) service uses a half-duplex signal, meaning each call using this service only uses one frequency. This technology is commonly used by dispatchers. To use this service the person placing the call must hold on to button while speaking. While they are holding the button, the party on the other end is unable to speak until the button has been released. This avoids any interruption by the other party. Push-to-talk service currently can only communicate with other users of the service under the same wireless carrier.

Current use in mobile telephony

The term has come to be more commonly known as referring to a feature that is available on certain more recent mobile phone models. It allows the mobile phone, when in a special mode, to function as a digital two-way radio in push-to-talk operation (in a fashion similar to the "trunking" feature of newer commercial and public-safety two-way radios). Only one person at a time can talk, by pressing a PTT button, and one or several others can listen instantly. The service connects mobile phone users with each other within seconds. Currently, PTT users have to belong to the same mobile operator's (carrier's) network in order to talk to one another. The evolution of the standards will make such interconnection possible in the near future.

When used with GSM and CDMA networks, the PTT service commonly does not use up the regular airtime minutes that are available for general voice calls.

The Mobile Tornado, Motorola, Nokia, Ericsson, Siemens, Sonim, Huawei, Wireless ZT, etc. versions of PTT are based on 2.5G or 3G packet-switched networks and use SIP and RTP protocols. These particular versions of PTT are called "Push to talk over cellular", which is abbreviated "PoC".

The Open Mobile Alliance is defining PoC as part of the IP Multimedia Subsystem, and a first version of OMA PoC standard was finalized in first half of 2005.

A pre-standard version of PoC is also defined by the industry consortium made up of Motorola, Nokia, Ericsson, Siemens AG, AT&T Wireless, and Cingular Wireless (ATTWS and Cingular merged in September 2004) with the aim of creating a commercial offering enabling inter-operability between vendors.

Several operators are using Pre-Standard Push To Talk Server in GSM / GPRS / EDGE / CDMA / UMTS networks.

Terminal vendors has several variations of software installed on mobile terminals, so there is no 100% compatibility list available.