APCO Project 25

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APCO Project 25 Trunked Radio Systems are so named because they follow the open APCO Project 25 standard for public safety Trunked Radio Systems. 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 (CAI) standard.

Motorola's implementation of the Project 25 digital trunking standard is marketed as "ASTRO-25."

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:

1. P25 Common Air Interface (CAI)

  • Over-the-air modulation

2. 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.

2. 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 the IMBE vocoder. IMBE stands for Improved Multi-Band Excitation, and was developed by DVSI Inc. Numerous vendors have produced Project-25 capable subscriber equipment, including Motorola, M/A COM, EF Johnson, Uniden, Racal, and others. There are conventional voice solutions as well as trunking solutions available for this digital voice solution.

Currently, there are two trunking solutions available that use the P25 IMBE vocoder:


This is a P25 non-compliant Motorola digital solution, and 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 Digital trunking solution that is vendor independent and designed around the Project 25 Digital Trunking standards. The State of Michigan, the State of Colorado and the State of Illinois (STARCOM21) are three systems that use this format.

  • Project 25 Trunking uses a 4800 baud, 9600bps control channel.
  • All radios on a Project-25 Digital trunking system must use digital voice - no analog trunking capability is provided.

Project 25 Phases

Phase I is 4800 symbols per second - where each symbol encodes two bits of data for a raw bit rate of 9600 bps. Phase II is 6000 symbols per second where each symbol encodes two bits of data for a raw bit rate of 12000 bps.

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 transmit in 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". X2-TDMA was implemented on the following systems:

Search the database for others.

X2 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. 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 approved in November 2010 [1], and as of August 2011 Motorola has begun shipping Phase II systems [2] [3].

These systems can also have an option enabled known as Dynamic Dual Mode (DDM) which will seamlessly revert any given talkgroup to FDMA operating mode if a Phase I only capable radio affiliates with a Phase II capable talkgroup.

Scanner Support for TDMA

There are a few options for decoding Phase II and X2-TDMA signals:

  1. The Uniden BCD436HP and BCD536HP will copy both X2 and Phase 2 transmissions;
  2. GRE's PSR-800 can copy these modes, with updated firmware. On October 13, 2012, GRE first released beta test firmware for Phase II TDMA support on the PSR-800, which was followed by the release of an updated beta firmware on October 22, 2012. The beta firmware was confirmed as working on a true Phase II TDMA system soon after it was released.
  3. Whistler WS1080 and WS1095.

Software Based Decoders

DSDPlus decodes P25 Phase 1 and X2-TDMA.

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.


Misc Information

  • Project 25 Interest Groups Homepage - General information on the Project 25 User Group Homepage. Downloadable PDFs and more.
  • Project 25 Discussion List - Yahoo Groups mailing list dedicated to Project 25 information.
  • 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


  • AMBE, IMBE, DVP, DES etc need linked to Wiki Pages.
  • WACN - Wide Area Communication Network Identifier