Terrestrial Trunked Radio (TETRA) (formerly known as Trans-European Trunked Radio) is a professional mobile radio

Professional Mobile Radio

Professional mobile radio are field radio communications systems which use portable, mobile, base station, and dispatch console radios...

 and two-way transceiver (colloquially known as a walkie talkie) specification. TETRA was specifically designed for use by government agencies, emergency services, (police

Police

The police is a personification of the state designated to put in practice the enforced law, protect property and reduce civil disorder in civilian matters. Their powers include the legitimized use of force...

 forces, fire departments, ambulance

Ambulance

An ambulance is a vehicle for transportation of sick or injured people to, from or between places of treatment for an illness or injury, and in some instances will also provide out of hospital medical care to the patient...

) for public safety network

Public safety network

A public safety network is a wireless communications network used by emergency services organizations, such as police, fire and emergency medical services, to prevent or respond to incidents that harm or endanger persons or property....

s, rail transportation staff for train radios, transport services and the military

Military

A military is an organization authorized by its greater society to use lethal force, usually including use of weapons, in defending its country by combating actual or perceived threats. The military may have additional functions of use to its greater society, such as advancing a political agenda e.g...

.

TETRA is an European Telecommunications Standards Institute

European Telecommunications Standards Institute

The European Telecommunications Standards Institute is an independent, non-profit, standardization organization in the telecommunications industry in Europe, with worldwide projection...

 (ETSI) standard, first version published 1995; it is mentioned by the European Radiocommunications Committee (ERC).
TETRA uses Time Division Multiple Access

Time division multiple access

Time division multiple access is a channel access method for shared medium networks. It allows several users to share the same frequency channel by dividing the signal into different time slots. The users transmit in rapid succession, one after the other, each using its own time slot. This...

 (TDMA) with four user channels on one radio carrier

Carrier wave

In telecommunications, a carrier wave or carrier is a waveform that is modulated with an input signal for the purpose of conveying information. This carrier wave is usually a much higher frequency than the input signal...

 and 25 kHz spacing between carriers.