P25

P25, commonly known as APCO Project 25, is a suite of digital radio standards for public-safety communications. It was developed by the Association of Public-Safety Communications Officials (APCO) to enable interoperable voice and data radios across agencies and vendors. The standards are published by the Telecommunications Industry Association (TIA) as the ANSI/TIA-102 series. P25 is primarily used in North America, with some international adoption.

The system aims to provide open interoperability, backward compatibility with analog systems where feasible, robust encryption, and support for data communications in addition to voice. It defines the Common Air Interface (CAI) for digital voice and control signaling, plus multiple traffic modes, including Phase I FDMA and Phase II TDMA. Phase II doubles channel efficiency by placing two logical voice channels into each 12.5 kHz channel. Voice is encoded with the IMBE vocoder, and digital error protection and signaling enable trunking and roaming.

Encryption and data services are optional: users may deploy AES- or DES-based encryption, as well as packet data, short data, and telemetry services over P25 networks. Interoperability is a central goal, with certification programs ensuring equipment from different manufacturers can communicate on the same system.

P25 has become a de facto standard for public-safety radio in many jurisdictions, enabling cross-agency cooperation and multi-vendor procurement, though the pathway to full modernization varies by region and agency.