TETRA, or TEsterial TRunked RAdio, is an ETSI radio standard, develped in 1995, and remains a very popular, professional, mission-critical communications system. TETRA uses a 25 kHz signal, above 300 MHz, to create a 4 time slot TDMA method of communications -- 1 administrative channel, and 3 data or voice channels. Subsequent frequencies at the same location may tie into the first administrative channel, allowing all of the available time slots to be either voice or data communications. Thus, a 2 carrier site may feature 1 control channel, and 7 voice and data channel operation. Data operations include IP and circuit switching. A transmitter at a tower site is called a base station.
TETRA's Main Control Channel (timeslot 1) has three main functions: terminal (radio) registration, call management, and short data service (SDS) data. Terminals may freely roam among base stations on the radio network, just like a cell phone. Unlike analog technology, users do not need to select "where" they are talking, but "who" they would like to talk to. If a call is in progress, and a user needs to switch base stations, the system processes the switch without the call being lost, just like a cell phone. SDS messages are text messages, intended for humans or machines, as you can send a text message to another device (or multiple devices), and have them do something, such as turn on a pump, or open a gate.
TETRA's remaining channels (timeslots 2-4) are used for either voice or data traffic. IP traffic may be used, and some systems will prioritize data vs. voice, allowing the data to have more bandwidth when the voice channels are not in use. TETRA features a robust emergency call system. The radio system has a level of priority, with the emergency call having the ability to cancel an active call to make room for the emergency. With one-button activation, an emergency call will be heard and managed as programmed by the customer.
TETRA is available worldwide, and many public safety users outside of the United States enjoy daily use of the technology. TETRA may be comparable with America's pricey P25, however TETRA has features beyond P25 that users may be interested in. TETRA costs a bit more, but offers more, than Digital Mobile Radio's Tier III technology, so it is an ideal fit for organizations that require mission critical communications, but would also like advanced radio functionality such as a full-duplex telephone connection via the radio.
TETRA is considered narrowbanded by the FCC, actually, with its 6.25 kHz time slots, it is narrower than the current mandate.