Project 25

Project 25 (P25 or APCO-25) is a suite of standards for digital radio communications for use by federal, state/province and local Public safety organizations in North America to enable them to communicate with other agencies and mutual aid response teams in emergencies. In this regard, P25 fills the same role as the European Terrestrial Trunked Radio (TETRA) protocol, but the two are not interoperable. The major difference between the two is P-25 is expected to work jointly with existing analog systems. Therefore, it uses "simulcast" method for control. "Simulcast" refers to the use of the same set of control channels throughout a given area, which are "simultaneously broadcast, or simulcast" in the region. In contrast, TETRA uses "Multicast," which means the control channel is embedded, therefore there is no need to use a separate channel to broadcast control signals. TETRA provides 4 slots per channel, which means four voice calls can be handled on one channel. This is similar to the GSM cellular system, which allows eight slots or eight users per channel.