TETRA

Terrestrial Enhanced(changed from European) Trunked Radio (TETRA) is a specialist Professional Mobile Radio and walkie talkie standard used by police, fire departments, ambulance and military. Its main advantage over technologies such as GSM are:
the much lower frequency used, which permits very high levels of geographic coverage with a smaller number of transmitters, cutting infrastructure cost.
fast call set-up - a one to many group call is generally set-up within 0.5 seconds compared with the many seconds that are required for a GSM network.