What is Project 25?
To put it simply, Project 25 (P25) is a Land Mobile Radio (two-way radio) wireless communications standard that originated in North America in the mid-1990’s. The standard was the brain child of the Association of Public Safety Communications Officials (APCO for short). The primary goal of P25 was to standardize the multitude of proprietary features being used in the LMR market for public safety application in the name of interoperability with a protocol which would meet the needs for mission critical communications use while being spectrally efficient in preparation for narrowbanding from 25 kHz channels to 12.5 kHz channels (a hard date set for January 1, 2013) by using pioneering (for the 1990’s) digital modulation technology.
Project 25 was originally written for two types of systems, conventional and trunked. In a conventional system, the infrastructure operates like standard two-way radio infrastructure, the radios can operate in either a direct peer-to-peer setup or in a repeated setup where the radios send transmissions to a fixed repeater which then retransmits the received signal to extend the usable range. In a trunked configuration, a group of repeaters are clustered together and access to the system is dynamically assigned based on channel availability (very similar to how a phone trunk line works). The key take away from a trunking system is that it can handle much higher traffic loads compared to conventional repeaters with the same number of channel resources (repeaters).
How Does P25 Work?
P25 is essentially a serial data protocol which operates at 4800 baud. The primary modulation scheme for P25 is Continuous 4-level Frequency Modulation (C4FM, not to be confused with the C4FM modulation used by NXDN and Yaesu’s System Fusion digital protocols) and is used by infrastructure in non-simulcast operation and by the uplink via the subscriber radios. This is done by varying the deviation frequency of the signal to represent two bits per symbol for a total throughput of 9600 bps. The levels are as follows:
| Binary Bits | Symbol | Deviation Level |
| 01 | +3 | +1800 Hz |
| 00 | +1 | +600 Hz |
| 10 | -1 | -600 Hz |
| 11 | -3 | -1800 Hz |
This makes the maximum deviation around ±1.8 kHz and the occupied bandwidth around 8.1 kHz which is narrower than an analog 11.25 kHz narrowband (± 2.5 kHz max. deviation). Both fit into a 12.5 kHz channel. There are several other types of modulation used in different applications which will be touched on later.
Using an AMBE 2+ vocoder (original spec was for IMBE but all modern radios use AMBE 2+ as it has otherwise succeeded the IMBE chip) 20 ms of audio is encoded/decoded to/from 88 bits of information. Essentially in every second of voice transmission, 4400 bits are used to represent voice data, 2400 bits are used for signaling (radio ID, talkgroup ID, encryption ID, etc) and 2800 bits are used for error correction. There’s a lot more to the structure of the frames, and for that I highly recommend checking out Codan’s P25 Training Guide which is at version 4.0 at the time of this writing.
P25’s Phases
P25 is made up of developmental phases. At the time of this writing, there are three and each has different purposes and developmental paths.
Phase 1
Phase 1 is sometimes simply referred to as P25. It was the first real push to create the standard. Phase 1 includes conventional and trunked operation in a Frequency Division Multiple Access (FDMA) format. This really just means that like regular analog FM, there is only a single talk path available on a channel at any given time. Phase 1 has an alternate downlink modulation scheme of Continuous Quadrature Phase Shift Keying (CQPSK) which is sometimes referred to as Linear Simulcast Modulation (LSM) and is used for simulcast applications (mostly in 7/800 MHz).
Phase 2
Phase 2 was originally a update to create a P25 version that could be narrowed into a 6.25 kHz channel (much like how NXDN accomplishes 6.25 kHz). What Phase 2 ended up being was a TDMA solution in the existing 12.5 kHz channel (much like DMR). Unlike other TDMA solutions, Phase 2 is only functional in a trunked environment. Phase two uses two different modulation schemes, Harmonized – Differential Quadrature Phase Shift Keying (H-DQPSK) on the downlink at 12,000 bps (from the repeaters) and Harmonized – Continuous Phase Modulation (H-CPM) at 6,000 bps on the uplink. The key note the control channel still operates at 9600 bps C4FM or CQPSK (which is what allows Phase 2 to be backwards compatible with Phase 1 SM’s).
Phase 3
Phase 3 was originally a mobile data solution but has been abandoned due to the technological explosion of mobile data solutions such as 3G/LTE and 5G has rendered Phase 3 as originally intended as obsolete.
Similarities Between P25 and Analog
With P25, access control is handled very similarly to how it is handled with analog. In an analog system, Continuous Tone Carrier Sub-Audible Squelch (CTCSS) and Digital Continuous Squelch (DCS) are used to to handle access control. With P25, this is done through the use of a Network Access Code (NAC) and is roughly equivalent to a DCS. With some analog signaling formats (MDC1200, FleetSync, etc) additional features such a peer to peer calling, status functions, paging and emergency status are supported. These functions are also present in P25 (though now standardized across multiple vendors unlike proprietary signaling formats).
