Also known as: P25, Project 25, APCO-25, APCO P25
Project 25 (P25, also APCO-25) is a suite of open standards for digital land-mobile radio developed for public-safety and government users in North America. It defines how radios, repeaters, and trunked systems carry digital voice and data, with the explicit goal of interoperability between equipment from different manufacturers.1
Overview
P25 was created so that police, fire, and emergency-medical agencies could replace incompatible analog systems with a common digital standard. It specifies the air interface (how bits travel over the radio link), the vocoder used for voice, the trunking signalling, and inter-system interfaces. P25 comes in two air-interface generations: Phase 1, which uses FDMA, and Phase 2, which uses TDMA to double channel capacity.
Technical characteristics
| Property | Phase 1 | Phase 2 |
|---|---|---|
| Access | FDMA | TDMA (2 slots) |
| Channel | 12.5 kHz | 12.5 kHz / 2 = 6.25 kHz equiv. |
| Modulation | C4FM (and CQPSK) | H-CPM / H-DQPSK |
| Vocoder | IMBE | AMBE+2 |
| Symbol rate | 4800 baud (9600 bps) | — |
Both phases can be deployed conventionally or as a trunked system coordinated by a control channel.
History
P25 standardisation began in the late 1980s under APCO, with Phase 1 documents published by the TIA from the mid-1990s.1 Phase 2 followed to address spectrum-efficiency mandates, introducing TDMA so two voice conversations could share one 12.5 kHz channel.
Deployment
P25 is the dominant digital standard for U.S. state and federal public safety, and is used by many large metropolitan systems. Systems are catalogued in databases such as RadioReference, which list their control-channel frequencies and talkgroups.
Decoding it with GopherTrunk
GopherTrunk decodes both P25 Phase 1 and Phase 2: it locks the control channel, follows channel grants to voice channels, and runs the matching vocoder to produce audio. See the protocol landscape lesson for how P25 compares with other systems, and the Status page for current coverage.
Sources
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Project 25 — Wikipedia, for P25 history, the Phase 1/Phase 2 air interfaces, and the TIA/APCO standardisation. ↩ ↩2