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Push-to-talk (PTT), also known as Press-to-Transmit, is a method of having conversations or talking on half-duplex communication lines. Including on two-way radio, using a momentary button to switch from voice reception mode to transmit mode.
For example, an air traffic controller usually talks on one radio frequency to all aircraft under his/her supervision. Those under the same frequency can hear other's transmissions while using procedure words such as "over" and "out" to provide order during the conversation. In doing so, they are aware of each other's actions and intentions, and do not hear any background noise from the ones who are not speaking. Similar considerations apply to police radio, the use of business band radios on construction sites, and other scenarios requiring coordination of several parties. Citizens Band is another example of classic push to talk operation.
Push to Talk over Cellular (PoC) is a service option for a cellular phone network which permits subscribers to use their phone as a walkie-talkie with unlimited range. A typical Push-to-Talk connection connects almost instantly. A significant advantage of PoC / PTT is the ability for a single person to reach an active talk group with a single button press; users need not make several telephone calls to coordinate with a group.
Push-to-talk cellular calls similarly provide half-duplex communications — while one person transmits, the other(s) receive. This combines the operational advantages of PTT with the interference resistance and other virtues of mobile phones.
Traditional mobile phone networks and devices utilize full-duplex communications, allowing customers to call other people on a mobile or land-line network and be able to simultaneously talk and hear the other party. Such communications require a connection to be started by dialing a phone number and the other party answering the call, and the connection remains active until either party ends the call or the connection is dropped due to signal loss or a network outage. Therefore, the telephone communication protocol does not allow for casual and immediate transmissions to be sent to other parties on the network. While telephone calls require the lengthy process of dialing, network switching and routing, call setup, and waiting for the other party to answer, a two-way radio has a much quicker protocol because of the immediacy of push to talk communication.
Later 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. 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.
Current use in mobile telephony (PoC)
Full-duplex operation on mobile phone networks is made possible by separate frequencies for transmission and reception. Mobile Push-to-Talk service, offered by some mobile carriers, adds functionality for individual half-duplex transmissions to be sent to another party on the system without needing an existing connection. Since the system is half-duplex, only one user can transmit by PTT at a time; the other party is unable to transmit until the transmitting user unkeys his PTT button. Now, PTT service is not only supported between parties on the same mobile carrier service, users with different carriers will be able to transmit to each other by PTT.
In addition to mobile handsets, the Push-to-Talk service may be complemented with fixed PC applications acting as PTT clients connected to the mobile operator via secured Internet links. A dispatcher is a specialized type of PC Client but designed for heavy load dispatching, i.e. coordinating many issues typically caused when managing large fleets from a dispatch center. In Spain, Telefonica has launched PTT offering with focus on dispatch orientated group communications.
Nextel Communications introduced mobile push-to-talk in August 1993 using iDEN. The first system was turned on in Los Angeles with 134 sites and a capacity for 50,000 dispatch subscribers. The "MOTO Talk" feature by Nextel includes both on- and off-iDEN network walkie-talkie service for newer Motorola phone models. The off-iDEN-network handset-to-handset Direct-Talk feature works such for a radius of up to six miles.
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. There are few full-fledged commercial deployments of OMA PoC. It is very unclear whether OMA PoC will be seriously launched in the European market.
A pre-standard version of PoC is also defined by the industry consortium made up of Motorola, Nokia, Ericsson, Siemens AG and AT&T Mobility with the aim of creating a commercial offering enabling interoperability between vendors.
As of June 2010, no tier-1 operator worldwide has launched OMA. The availability of OMA handsets and fully inter-operable solutions remains to be seen. OMA solutions are missing the most important factor for such a niche market, which is performance. In addition, the standard was mainly adopted by NSN, Nokia, Ericsson and others, none of whom have had major success or background in PTT. OMA PoC also assumed it is a service for consumers, which is not true -- PTT/PoC is an enterprise service. On the bottom line, it is more of a North American phenomenon due to existence of iDEN and other networks. Other parts of the world do not know the service and operators are having a hard time identifying who needs this service.
The latest development in PTT communications is the appearance of apps for this on smartphones like those running Android or Apple iPhones. Some of these applications are cross-platform and some are only available on one platform. TiKL and Zello are near-real-time systems, similar to Nextel, while other apps like Voxer provide real-time streaming voice and recorded messages.
Wireless PTT services includes:
- Push-to-Talk over Cellular Consortium Phase 2 Specifications and Documentation
- Open Mobile Alliance - Push to talk over Cellular (PoC) - Architecture Candidate Version 2.0 – 26 February 2008