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So What is a P25 System?














APCO Project 25 Radio System (P25 Phase 1 and 2): 























Project 25 Standard for Public Safety APCO's (TRSs) because they follow the open Trunked Radio SystemsAPCO Project 25 (P25) 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



















Advanced Multi-Band Excitation, and both were developed & AMBE stands for Improved Multi-Band Excitation or IMBE. vocoderProject 25 uses either IMBE licences 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 Conventional and or Digital Trunk solutions, both available to use P25 Digital Voice via compatible transmitter, transceiver and/or receivers.

  • Currently, Motorola's implementation of P25 digital data & voice is marketed as "ASTRO-25", for 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.


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 (P25) Digital voice & data trunking solution, 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 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" uses the same modulation as Phase 1. X2-TDMA was implemented on the following systems which may have been upgraded to Phase II since the standard was finalized in late 2013. 


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 a 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 


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 TDMATGRP, and only go back to TDMA once all Phase I-only are unaffiliated with said TGRP.


Here's a video on the Benefits of a P25 Trunked Radio System

























To learning more about trunked radio systems, click here to view. 


Also, See the difference between Digital and Analog and FDMA and TDMA, click here to read more. 


For More Information on P25 System, Please click here for an article that helps better explain how trunking works and how digital radio system actually works. 


Also, feel free to visit Project 25 page to get more information on what agencies use the P25 system throughout the United States.


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