Common Air Interface

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.

Starting around 2012, products became available with the newer phase 2 modulation protocol, the older protocol known as P25 became P25 phase 1. P25 phase 2 products require the more advanced AMBE vocoder, which allows audio to pass through a more compressed bitstream and provides two TDMA voice channels in the same channel spacing (12.5 kHz) that phase 1 provides one voice channel. The two protocols are not directly compatible. However, phase 2 radios are capable of using both phase 1 modulation and analog FM modulation, per the standard.

P25 fills a similar role as TETRA or DMR protocols. However, TETRA and DMR both use TDMA technology, similar to P25 phase 2, and are generally only used for trunked radio systems (using a control channel), whereas P25 phase 1 is not TDMA and can more easily directly replace a conventional (non-trunked) radio system.

Public safety radios have been upgraded from analog to digital since the 1990s because of an increased use of data on radio systems for such features as GPS location, trunking, text messaging, metering, and encryption.

Various user protocols and different public safety radio spectrum made it difficult for Public Safety agencies to achieve interoperability and widespread acceptance. However, lessons learned during disasters the United States faced in the past decades have forced agencies to assess their requirements during a disaster when basic infrastructure has failed. To meet the growing demands of public safety digital radio communication, the United States Federal Communications Commission (FCC) at the direction of the United States Congress initiated a 1988 inquiry for recommendations from users and manufacturers to improve existing communication systems. Based on the recommendations, to find solutions that best serve the needs of public safety management, in October 1989 APCO Project 25 came into existence in a coalition with:

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