Push-to-Talk (PTT), also known as "Press-to-Transmit", is a method of conversing on half-duplex communication lines, including two-way radio, using a momentary button to switch from voice reception mode to transmit mode.
Conventional two-way radios
For commercial, family and amateur two-way radios, PTT is a button that is pressed when needing to transmit with the radio on the tuned frequency or channel. While the PTT button remains unpressed (or "unkeyed"), any radio traffic that is received on the selected channel or frequency is heard through the radio's speaker. Unless the radio support full-duplex operation, received audio is usually muted while the PTT button is pressed. Simultaneous full-duplex transmission and reception on a radio is generally not supported unless either the transmit and receive frequencies have significant separation between the two frequencies, or two different antennas are used with enough distance between them, or a cavity filter is used, due to an effect known as desensing which cancels out received transmissions.
More recently, the PTT concept has been adopted by cellphone carriers as a way to instantaneously send transmissions to other users on the system, emulating walkie-talkie communications on a mobile phone network.
Current use in mobile telephony
Traditional mobile phone networks and devices utilize full-duplex communications, allowing customers to call other persons 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. Such a system does not allow for casual transmissions to be sent to other parties on the network without first dialing them up, like is allowed on two-way radios. Full-duplex operation on mobile phone networks is made possible by using 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 to be already established. Since the system is half-duplex (utilizing a single frequency), only one user can transmit by PTT at a time; the other party is unable to transmit until the transmitting user unkeys their PTT button. Currently, PTT service is supported only between parties on the same mobile carrier service, and users with different carriers are be unable to transmit to each other by PTT. However, the advancement of this service will likely bring interconnectivety of PTT traffic between different networks 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.
Nextel Communications (now merged into Sprint) introduced mobile Push To Talk several years ago using iDEN. The "MOTO Talk" feature by Nextel (affectionately called 'Beep-beep' or 'chirp' by teenagers) includes both on and off iDEN network walkie-talkie service for newer Motorola phone models. The off iDEN-network headset-to-headset 'Direct-connect' feature works for a radius of up to 6 miles.
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.
In Japan, NTT DoCoMo implemented Push-to-Talk in late 2005 with the introduction of new FOMA 902i series handsets. It's billed at 5 yen per push, and has an "unlimited" option for 1000 yen/month.
Currently Cingular Wireless and Alltel offer PTT service using the Kodiak RTX ( Real Time Exchange ) system to deliver PTT speeds "somewhat" comparable to Nextel and SouthernLinc
In Canada the service is provided by several carriers including Telus (Mike), Bell Mobility and Aliant Mobility (10-4). While using the service, customers do not use registered airtime minutes associate with their voice plan. The service is often offered at a discount to those customers who suscribe to a monthly airtime package. Both Bell and Aliant offer the service which allows customers to use the service in the United States without the occurrence of international roaming charges typically associated with cellular use out of country. Customerís using the service in Canada have the ability to contact users across the country without the occurrence of long distance charges. The service is being promoted as a cost effective method for communication which typically runs a high cost. Offered as a solution to businesses and customerís who use a great amount of long distance, PTT (Push-to-Talk)service will greatly change the way many consumers do business. The service allows a caller to simoutaniously communicate with multiple users at different locations. By doing so, this eliminates multiple airtime charges associated with three-way calling. With the cellular number portability coming into effect by March 2007, the PTT (Push-to-Talk) service is expected to increase greatly with the removal of Roamer Access Numbers by August 2006. Roamer Access Numbers had given the freedom for customers to not incur incoming long distance charges by answering calls outside their local calling area if the caller first dials the Roamer Access Number.