APCO Project 25

From The RadioReference Wiki

APCO Project 25 (P25) Trunked Radio Systems (TRSs) are systems that follow the open APCO Project 25 Standard for Public Safety TRSs. Multiple vendors make and sell Project 25 systems and compliant radios. Audio on these systems is exclusively digital using the APCO-25 Common Air Interface (P25 CAI) standard.

There are several "subsystem" features defined as part of the Project 25 Standard to enhance interconnectivity & interoperability and allow equipment from various manufacturers to work together.

These standards include
  • P25 Common Air Interface (P25 CAI)
    • Over-the-air modulation (digital audio)
  • P25 Inter-RF Subsystem Interface (ISSI)
    • The Inter-RF Subsystem Interface (ISSI) standard allows P25 systems from different manufacturers to be directly interconnected at the controller level, allowing seamless cross-system intercommunication, and system-to-system roaming for same-band systems.
  • P25 Console Subsystem Interface (CSSI)
    • The Console Subsystem Interface (CSSI) allows dispatch consoles from different manufacturers to be connected to the controller/core of other manufacturers' systems. For example, a Zetron console could be connected to a Motorola system, or a Harris console could be connected to a Tait system.

Different Types of Project 25 Trunking/Modulation

Project 25 uses either MBE vocoder. IMBE stands for Improved Multi-Band Excitation or AMBE Advanced Multi-Band Excitation, and both were developed & licenced by DVSI Inc. Numerous vendors have produced Project 25 capable subscriber equipment, including EF Johnson, Motorola, M/A COM, Racal, Uniden, and others. There are both Conventional and Digital Trunk solutions which are able to use P25 Digital Voice using a compatible transmitter, transceiver and/or receivers.

  • Currently, Motorola's implementation of P25 digital data & voice is marketed as "ASTRO-25". both older Motorola Type II and Phase I systems for two of their types of trunking solution, they utilize the P25 IMBE vocoder, with their newer AMBE or AMBE2 radios are backwards compatible with systems that use the IMBE packaging.


Motorola ASTRO IMBE is a Motorola digital solution which is not compliant with the Project 25 standard. It is also called the "ASTRO Digital CAI (Common Air Interface) Option". This is a proprietary trunking solution that uses the Project-25 vocoder as its digital voice solution on top of a standard Motorola Type II Smartnet/Smartzone system.

Project 25 Digital Trunking

This is the Project 25 (P25) Digital voice & data trunking solution, it is one that is vendor independent and designed around the Project 25 Digital Trunking standards. Phase I(PI) is 4800 symbols per second - where each symbol encodes two bits of data for a raw bit rate of 9600 bps. Phase II(PII) is 6000 symbols per second where each symbol encodes two bits of data for a raw bit rate of 12000 bps and utilizes the AMBE vocoder.

Project 25 Phases

Project 25 Phase I "FDMA"

Phase I FDMA consists of C4FM modulated signal or a CQPSK modulated signal. Both fit in a 12.5 kHz channel. Subscriber equipment transmits using C4FM. Site equipment may transmit in C4FM or CQPSK. Simulcast uses CQPSK modulation, however older Motorola ASTRO equipment used C4FM simulcast in a special mode called "WIDE pulse" which is not P25 compliant. P25 CQPSK Linear Simulcast Modulation is P25 compliant and is referred to as LSM. LSM is defined in the P25 standards.

Motorola "X2-TDMA"

Prior to the final Phase II standard being approved, Motorola developed and implemented their own TDMA protocol known as "X2-TDMA" uses the same modulation as Phase 1.

Project 25 Phase II "TDMA"

The Phase II standard is a 2-slot TDMA signal that fits inside a 12.5 kHz wide channel, providing two 6.25 kHz-equivalent channels. Fixed site output modulation is H-DQPSK with subscriber units using H-CPM on the input. This allows existing 12.5 kHz wide license holders to double call capacity by upgrading their infrastructure to Phase II. The Phase II standard was finalized and approved in November 2010 [1], and Motorola has begun shipping Phase II systems as of August 2011 [2] [3].

Motorola ASTRO-25 Phase II systems can also have an optional feature known as Dynamic-Dual-Mode (DDM), which will seamlessly revert a whole talkgroup (TGRP) to FDMA operating mode if a Phase I-only radio affiliates with a Phase II TDMA TGRP, and only go back to TDMA once all Phase I-only are unaffiliated with said TGRP.

Scanner Support FDMA and TDMA

Scanner Support for FDMA

The following scanners can only decode P25 Phase I, but not Phase II


1 Does not cover or will not properly track the 700 MHz band.

Other Receivers

Scanner Support for TDMA

The following scanners can decode P25 Phase I, Phase II, and Motorola X2-TDMA systems.


2 Requires latest firmware update.
3 Requires Whistler Official Upgrade (WOU).

Project 25 Phase II Capable Receivers

Software Based Decoders

See our Trunked Radio Decoders page for a listing of applications. Packages like Digital Speech Decoder, DSDPlus, can handle Phase 1 signals. SDRTrunk can handle Phase 1, Phase 2, and DMR, and DSDPlus also handles X2. In addition there are various decoding applications such as UniTrunker and Pro96Com.

Some like DSDPlus may also be used by Software Defined Radio (SDR) such as the RTL-SDR.

Using a PC running Linux' Ubuntu, it is possible to compile/build and run the software package OP25 to monitor Conventional and/or Trunked P25 Phase I & II systems. OP25 only requires one SDR device, whether it be an RTL-based or something more capable like a HackRF or Airspy. At the current time (Jan 18) OP25 only builds out and works properly using Ubuntu 14.04.x LTS; the current version is 14.04.5 and it is a Long Term Support (LTS) version, so there will be updates through at least April 2019.

In November 2019, OP25 was successfully built and run on a Raspberry Pi 3 under Raspbian Buster. Therefore, there is now an alternative to Intel based architecture under Ubuntu.

A cross-platform application called SDRTrunk written in Java can handle P25 Phase I, II and DMR with the latest build. It supports tuners such as Airspy, Funcube Dongle Pro, Funcube Dongle Plus, HackRF, RTL-2832 with Elonics E400, RTL-2832 with Rafael R820T or R820T2, and sound card(s) connected to scanner audio output.

Desktop Receiver Support

NOTE: No trunking support with these radio. Phase I only.

Project 25 Trivia

Conventional P25 systems don't support CTCSS tone or DCS code for access. Instead they use what is called a NAC. This is a 12 bit code that prefixes every packet of data sent (including voice packets).

For trunking, the control channel delivers an average of 40 trunking commands per second. These commands may carry caller or callee identifying information such as a radio id or talkgroup. Talkgroups are 16 bits - allowing over 65000 talkgroups. Radio ids are 24 bits - allowing over 16 million unique radios. To support roaming, radios are associated with two additional IDs - a system ID and a WACN. The system ID is 12 bits while the WACN is 20 bits - allowing for over 4 billion unique systems. Voice channels are identified in trunking commands by a 16 bit number. These 16 bits can be broken down into two pieces - a four bit identifier and a 12 bit channel number. The 4 bit identifier selects the appropriate bandplan. A bandplan is a simple algebraic formula for computing a frequency from a channel number.

Some P25 trunked systems, including many military 380 MHz systems, have WACNs that decode into a hint as to the system's name. The encoding of WACNs in these cases follows the "Guidelines to Assign Wide Area Communication Network and System Identities" document approved by the APCO Project 25 Steering Committee on April 6, 2001. For example, WACN 580A0 decodes to "NCR" (National Capital Region). A conversion application is available to decode WACNs and System IDs. The real intent of this encoding scheme is to generate unique WACNs and System IDs from a trunked system's license callsign.

Related Pages


Misc Information

  • Project 25 Interest Groups Homepage - General information on the Project 25 User Group Homepage. Downloadable PDFs and more.
  • The APCO International Web Site - This page is homepage for the Association of Public Safety Communications Officials. They are the group that defines standards for public safety communications in the United States
  • The APCO Project 25 Homepage - Here you can find white papers and technical documents regarding the APCO-25 digital public safety project - on the APCO International Web Site
  • Users Accelerate Move To Project 25 Systems, Technology - An APCO bulletin posted on the Motorola Web site. This outlines how many large Public Safety agencies across the U.S. have chosen APCO-25 compliant digital system for Public Safety communications. Gives examples of which agencies have made the switch, and which agencies are about to.
  • Motorola Encryption techniques - DVP, DES, Securenet, ASTRO, and Fascinator are all discussed here.
  • IFR's homepage Designed to communicate the latest information regarding APCO-25 Digital Testing technologies. Much of the information here centers around the IFR 2975 Project 25 Service Monitor. Lots of good reference information here.
  • Interesting post - Posted to Usenet regarding decoding APCO-25 digital signals. This was written by a college student who was developing an APCO-25 digital decoder as a class project. Although we never actually saw anything come out of this, the information posted is very useful.
  • Digital Source Coding of Speech Signals - Great information on vocoders, and the IMBE Vocoder standard developed by DVSI.
  • IMBE and AMBE Speech Compression - [PDF] - Article describing the scientific theory behind the IMBE and AMBE speech compression vocoders. From the Engineering Electronic Times.
  • Project 25 Data Representation
  • Digital Frequency Search website Search for P25 users on the FCC database


RR Glossary terms

useP25 ISSI