Push to talk

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, the PTT or pressel is a switch 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 supports 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 (PTT PoC)

PTT PoC or Push to Talk over Cellular is a feature similar to walkie-talkie that is provided over a cellular phone network. A typical Push to Talk connection connects almost instantly. One significant advantage of PoC is allowing a single person to reach an active talk group at a button press, thus users no longer need to make several calls to coordinate with a group.

Push-to-talk calls are half duplex communications — while one person speaks, the other(s) listen. 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, as is provided by 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 will be unable to transmit to each other by PTT. However, the advancement of this service will likely bring interconnectivity of PTT traffic between different networks in the near future.

In addition to mobile handsets, the Push-to-Talk service might be complemented with fixed PC applications acting as PTT clients connected to the mobile operator via secured Internet links. A specialized type of PC Client is a dispatcher, similar to a PC client but designed for heavy load dispatching. That is, 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, as has Push To Talk UK Ltd on it is YouPoC Service.

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 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 for a radius of up to six miles.

Sprint plans on implementing Qualcomm's QChat on their EV-DO Revision A network. QChat has connection times of less than a second, which brings it in line with Nextel's MOTO Talk connection speeds. This will in time replace their ReadyLink Push-to-Talk technology.

The Mobile Tornado, inTechnology PLC UK, Motorola, Nokia, Ericsson, Siemens, Sonim, Wireless Technologies Finland, Wireless Zeta Telecomunicaciones, Clarity, 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. 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.

See also

* IP Multimedia Subsystem
* IMPS

References