TETRA (TErrestrial Trunked RAdio) is a wireless standard developed by ETSI (the European Telecommunications Standards Institute) in 1995 for mission-critical communications systems. TETRA operates on 25 KHz wideband carriers to create a 4-timeslot TDMA (Time Division Multiple Access) method of communications, comprised of 1 administrative (control) channel and 3 voice or data channels. Additional frequencies at the same location may be combined together with the first control channel, allowing all of the available time slots to be utilized in an efficient manner for voice and data communications. Thus, a 2-carrier site may feature 1 control channel plus 7 voice and data channels. Data operations include short and long messaging, AVL, data transfer, and more.
The Main Control Channel (timeslot 1) has three primary functions: Terminal (user radio) registration, call management, and short data service (SDS) message processing. Terminals may freely roam among base stations on the radio network, similar to cell phone operation. Unlike conventional radio technology, users need not select "where" they are talking, but only to "whom" they would like to speak. If a call is in progress and a user needs to switch base stations, the system automatically processes that switch without the call being lost. SDS messages are essentially text messages intended for humans or machines. For instance, an SDS message might be sent to a device (or multiple devices) to initiate a desired action, such as activating a pump or opening a gate.
TETRA's remaining channels (timeslots 2-4) are used for either voice or data traffic as needed. The system can programmed to prioritize data vs. voice, allocating greater bandwidth and throughput for data operations. The TETRA emergency call system assigns priority levels, allowing an emergency call to override an active call and prioritize emergency response. With one-button activation, an emergency call will be heard and managed as critical traffic.
TETRA is available worldwide, and many public safety users outside the United States utilize the technology. TETRA inherently meets both current and future FCC narrow-banding mandates with its 4-timeslot, 6.25 KHz equivalent TDMA architecture.