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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 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.
In autumn of 1993, innovation in Push-to-Talk technology was pushed further by Roger Wood with the release of the iDEN software platform and handset as an overhaul of the MIRS system previously developed for Nextel Communications. The first system was turned on in Los Angeles with 134 sites and a capacity for 50,000 dispatch subscribers. Wood and a small band of engineers led the introduction of iDEN "i" series handsets optimized for the software application of the same name in 1996 – one of the first purely mobile software applications capable of Internet connectivity – and software / device combination led to an explosion in non-voice mobile applications from simple messaging to audio books. Each handset effectivley had its own IP address, opening up new possibilities for instant communications other than voice packets. 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 defined 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 a few full-fledged commercial deployments of OMA PoC. The most-prominent US deployment is AT&T Mobility's Enhanced Push-to-Talk service powered by Kodiak Networks. Kodiak-powered PTT is a carrier-grade PTT solution that's compliant with the second version of the OMA PoC standard. The Enhanced PTT service launched in November 2012. Another commercial deployment of an OMA PoC PTT service is Bell Mobility's Next Generation Push-to-Talk in Canada. It launched in April 2012 and is powered by the Kodiak Network PTT platform.
A pre-standard version of PoC was 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.
Currently PTT/PoC is an enterprise service. It has been more of a North American phenomenon due to existence of iDEN and other PTT networks. However recently there has been interest in enhancing PoC to meet the needs of US and European Public Safety for next generation Mission Critical PTT over LTE (Long Term Evolution).
OMA Push-to-Talk over Cellular (PoC) Commercial Platforms
The latest development in PTT communications is the appearance of apps for this on smartphones like those running Android, BlackBerry devices or Apple iPhones. Some of these applications are cross-platform and some are only available on one platform. Wireless carrier-grade PTT systems have adapted to and adopted the smartphone platform by providing downloadable apps that support their PTT systems across many mobile platforms. Carriers like AT&T, Bell Mobility, C Spire, Sasktel, Sprint, and Verizon support downloadable PTT client applications for their real-time PTT systems. There are many over-the-top (OTT) applications that offer PTT capabilities, but do not have carrier-level support at the network level. TiKL and Zello are near real-time systems, similar to Nextel, other apps like Voxer provide both streaming voice and recorded messages. The newest app on the market is Prip. Unlike other PTT applications, Prip offers real time Push-to-Talk. The app was built by Motorola and works with any U.S. mobile carrier. 
Wireless Carrier PTT Services
- AT&T Mobility AT&T Enhanced PTT
- Bell Mobility Next Generation PTT
- C Spire Wireless Push-to-Talk
- Sasktel 4G Push-to-Talk
- Sprint Direct Connect
- Verizon Wireless Push to Talk
- Rush, Susan, Wireless Leaders Band For PTT Interoperability, retrieved 2009-03-03
- Vern Seward (September 28, 2012). "Voxer: What iMessage Should Be". The Mac Observer. Retrieved October 23, 2012.
- 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