P-25 (Project 25) is a digital communication format originally developed for government applications. It was designed to enhance interoperability and improve bandwidth efficiency. Over time, P-25 equipment, including repeaters and radios, has entered the amateur radio market via the second-hand market.

Some repeaters are configured in dual-mode, allowing them to operate in both analog and digital modes.

Project 25 standards were developed collaboratively by:

• Association of Public Safety Communications Officials International (APCO)
• National Association of State Telecommunications Directors (NASTD)
• Selected Federal Agencies
• National Communications System (NCS)
• Standardized under the Telecommunications Industry Association (TIA).

P25 is an open architecture system standard that defines digital radio communication architectures for public safety and government organizations. Radios conforming to the P25 standard are interoperable, capable of communicating in analog mode with legacy systems or in digital mode with other P25 radios. As an open standard, any manufacturer can produce P25-compatible equipment.

While P25 was primarily developed for North American public safety, it has been adopted globally for various private applications, including amateur radio.

P25 offers a range of benefits in performance, efficiency, capabilities, and quality. Key advantages relevant to amateur radio include:

P25 digital signals allocate 2800 bits per second of the total 9600 bits per second channel capacity to error correction, resulting in significantly improved voice quality over analog signals, especially in low signal or noisy environments.

• IMBE™ Voice Coder: Converts voice into digital data and applies error correction.
• Error Correction: Corrects small signal errors and eliminates background noise typically present in analog systems.

RepeaterBook tracks P25 repeaters, which typically operate at a bandwidth of 12.5 kHz. Many P25 repeaters are configured for mixed-mode operation, allowing them to pass both analog and digital traffic.

• Network Access Code (NAC): Similar to CTCSS, the NAC controls access to the repeater. The signal is transmitted outside the digital voice frame.
• Talk Groups: P25 talk groups are tracked by RepeaterBook, including default talk groups assigned to specific repeaters.

• RepeaterBook P25 Directory