APCO Project 25
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."
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:
Motorola ASTRO IMBE
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 it's digital voice solution on top of a standard Motorola Type II Smartnet/Smartzone system. Some agencies using Motorola ASTRO IMBE systems include the City of Baltimore, the State of South Dakota, the State of Ohio, the Metro Washington Airports Authority, Fairfax County VA, and Sarasota County FL.
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 of the few systems that use this format. This is becoming a more popular trunking format for new systems.
- Project 25 Trunking uses a 9.6 Kb control channel.
- All radios on a Project-25 Digital trunking system must use digital voice - no analog trunking capability is provided.
Project 25 Phases
Both formats are 4800 symbols per second - where each symbol encodes two bits of data for a raw bit rate of 9600 bps.
Project 25 Phase I FDMA
Phase I FDMA consists of a C4FM modulated signal that fits in a 12.5 kHz wide channel or a CQPSK modulated signal that fits in a 6.25 kHz channel. Motorola has established an additional de-facto standard for simulcast with a Phase I compatible variation called LSM. LSM is a CQPSK modulated 12.5 kHz wide signal.
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:
Project 25 Phase II TDMA
The Phase II standard is a 2-slot TDMA signal that fits inside a 12.5 kHz wide channel. 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 , and as of August 2011 Motorola has begun shipping Phase II systems  .
These systems can operate in a mixed-mode configuration of Phase I FDMA and Phase II TDMA. If a Phase I only capable radio affiliates with a Phase II talkgroup, these systems are configured to automatically use Phase I modulation on the frequency.
Scanner Support for Motorola X2-TDMA and P25 Phase II TDMA
Motorola's X2-TDMA protocol can be monitored with the GRE PSR-800 EZ-Scan Digital Scanner. The PSR-800 is currently the only scanner model with the hardware to support any form of TDMA decoding (as of August 14th 2011).
On August 13th 2011, GRE stated that the PSR-800's support for P25 Phase II TDMA cannot be confirmed until it is tested live on a system that uses the final Phase II TDMA standard.
Uniden has confirmed that none of their current models support TDMA in any form, nor do they have the requisite hardware to do so, therefore cannot be made TDMA compatible via a firmware update. 
X2-TDMA can also be monitored with DSD.
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.
- Project 25 Interest Groups Homepage - General information on the Project 25 User Group Homepage. Downloadable PDF's and more.
- Project 25 Overview - PDF Document with general Project 25 information.
- Project 25 Discussion List -Yahoogroups 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 none-the-less. Interesting Reading!
- 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