P-25 is a digital format originally developed for government applications. It was designed for interoperability and improved bandwidth efficiency.
The equipment, repeaters, and radios have been sold to amateur radio operators on the second-hand market.
Some repeaters can be set to dual-mode to allow for analog and digital modes.
Project 25 (P25) is a set of standards produced through the joint efforts of the Association of Public Safety Communications Officials International (APCO), the National Association of State Telecommunications Directors (NASTD), selected Federal Agencies and the National Communications System (NCS), and standardized under the Telecommunications Industry Association (TIA).
P25 is an open architecture, user driven suite of system standards that define digital radio communications system architectures capable of serving the needs of Public Safety and Government organizations. A P25 radio is any radio that conforms to the P25 standard in the way it functions or operates. P25 compliant radios can communicate in analog mode with legacy radios and in either digital or analog mode with other P25 radios. The P25 standard exists in the public domain, allowing any manufacturer to produce a P25 compatible radio product.
Although developed primarily for North American public safety services, P25 technology and products are not limited to public safety alone and have also been selected and deployed in other private system applications, worldwide.
P25 has many various benefits in performance, efficiency, capabilities and quality. Key P25 technology benefits pertinent to amateur radio include:
With 2800 bits per second of the total 9600 bits per second channel capacity allocated to error correction, P25 digital signals have improved voice quality over standard analog signals, especially at low or noisy RF carrier levels. The IMBE™ voice coder converts voice information into digital data and then the data is protected using error correction codes. The error correction is able to correct for small errors in the received signal. Since the audio is digitally encoded, the background noise typically present in analog systems is also removed.
Repeaterbook.com understands that P25 is a digital format that operates at a bandwidth of 12.5 kHz, but many repeaters can be set up for a mixed-mode to pass analog traffic. P25 can control access to the repeater with a NAC (Network Access Code). It is much like CTCSS, but the signal is sent outside of the digital voice frame.
P25 incorporates talk groups and the available talk groups are tracked by Repeaterbook.com. P25 repeaters may also a default talkgroup, which is also tracked by Repeaterbook.com.