This article first appeared in the June 2000 issue of Monitoring Times.


In January 1982, during a snowstorm, Air Florida flight 90 crashed into the 14th Street bridge in Washington, D.C. Half an hour later a Metrorail accident occurred just a few miles away. Responding rescue personnel from federal, state, and local public safety agencies quickly discovered that coordinating their efforts was extremely difficult because radios from each agency used different frequencies and signaling techniques. On-scene commanders were forced to borrow radios from one another to coordinate their crew activities.

More recently, the Oklahoma City bombing further emphasized the need for interoperability. More than a dozen search and rescue teams arrived, each with at least fifty personnel and their own communications system. The systems, for the most part, could not communicate with each other. Two-way radio was the only way to relay information back to dispatchers and request specific support, since wireline and cellular phone lines were damaged or overloaded. At one point it became so bad that one agency had to resort to sending runners with messages.

Major natural disasters such as hurricanes, earthquakes, and floods are typically handled by several different public safety agencies where the ability to communicate between agencies is also a necessity.

Project 25

To address the problem of interoperability as well as make better use of scarce radio frequencies, in 1989 the Association of Public Safety Communications Officials International (APCO) established Project 25 (P25). Representatives from Federal, state, and local governments began an effort to develop a set of common technical standards for land mobile radio systems. An additional benefit of a common standard would allow any number of manufacturers to produce compatible equipment, thus increasing competition and lowering prices. P25 promised to avoid locking customers into a proprietary system from a single manufacturer.

Equipment manufacturers control most standards processes. In contrast, P25 documents were developed by the Telecommunications Industry Association (TIA) based on user community needs, then approved by the APCO Project 25 Steering Committee. Phase I of P25 is nearly complete, with 30 of 32 standards documents now available, totaling more than 1800 pages.

P25 is not a single standard but really a number of individual protocols that can be mixed and matched. A "Project 25 compliant" system may really use only a few of the many standards. For instance, a P25 system may be conventional or trunked, use encryption or transmit in the clear, and carry voice, data, or both.

Common Air Interface

P25 systems use what is called the Common Air Interface (CAI). This standard specifies the type and content of signals transmitted by compliant radios. One radio using CAI should be able to communicate with any other CAI radio, regardless of manufacturer.

At present, most public safety channels are 25 kHz wide. Current P25 radios are designed to use 12.5 kHz wide channels, allowing two conversations to take place where only one used to fit. Eventually, P25 radios will use 6.25 kHz channels, allowing four times as many conversations compared to analog.

P25 radios must also be able to operate the old way, in analog mode on 25 kHz channels. This is called backward compatibility, and allows agencies to gradually transition to digital while continuing to use older equipment.

P25 transmissions may be protected by encryption. The standards specify the use of the U.S. Data Encryption Standard (DES) algorithm, but other algorithms may be used. There is an additional specification for over-the-air rekeying (OTAR) to deliver new encryption keys to radios.

P25 channels that carry voice or data, called traffic channels, operate at 9600 bits per second (bps). These channels are protected by a substantial amount of forward error correction, which helps receivers to compensate for poor radio frequency conditions and improves useable range.

P25 also supports data transmission, either piggybacked with voice (so-called slow data), or in several other modes up to the full traffic channel rate of 9600 bps.

Digitized Voice

The most important difference to scanner listeners is the fact that voice transmissions are now digital rather than analog. P25 uses a specific method of digitized voice called Improved Multi-Band Excitation (IMBE). The IMBE voice encoder-decoder (vocoder) listens to a sample of the audio input and only transmits certain characteristics that represent the sound. The receiver uses these basic characteristics to produce a synthetic equivalent of the input sound. IMBE is heavily optimized for human speech and doesn't do very well in reproducing other types of sounds, including dual-tone multifrequency (DTMF) tones.

The IMBE vocoder samples the microphone input every 20 milliseconds and produces 88 bits of encoded speech, or said another way, the vocoder produces speech characteristics at a rate of 4400 bits per second. Error correction adds another 2800 bps, and signaling overhead brings the total rate to 9600 bps. P25 standards specify exactly how that information is structured and transmitted.

Project 25 Manufacturers

Only a handful of manufacturers have demonstrated P25 mobile and portable radios, and all of them have been non-trunked. These companies include Motorola, Transcrypt International/EF Johnson, Racal, RELM and IDA. However, it appears most agencies have chosen to purchase Motorola radios, specifically the Spectra mobiles, ASTRO portables and the XTS-3000 portable.

Project 25 versus Motorola Astro

There is some confusion regarding the similarities and differences between Project 25 and Motorola's ASTRO product line.

ASTRO equipment is capable of operating using the P25 CAI, transmitting and receiving IMBE digital voice at 9600 bps. Depending on configuration, ASTRO equipment may also use a different method of digital speech called Vector-Sum-Excited Linear Prediction (VSELP), which is also used in some digital cellular systems but is not compatible with Project 25.

ASTRO systems may also use an "analog" control channel (usually Motorola Type II format) operating at 3600 bps rather than the P25 trunking standard at 9600 bps. This is commonly done to support older analog radios that can only understand the 3600 bps control channel.

Many public safety agencies are moving to P25 systems, switching their voice traffic from analog to digital IMBE.


The State of Michigan claims their Public Safety Communications System is the first APCO Project 25 compliant statewide radio system. The Motorola 800 MHz ASTRO SmartZone digital trunked communications system complies with P25 standards for common air interface, trunked operation, and encryption. All seven State Police districts are part of the system, as well as a number of other public safety agencies, including park rangers, highway workers, county and municipal police and fire departments, and 9-1-1 dispatch centers. The complete system is scheduled to be in operation by the spring of 2002 serving a total of more than 14,000 mobile and portable radios.


The Florida Highway Patrol shares a large 800 MHz P25 system in central and southern Florida with a number of other state agencies including the Florida Department of Law Enforcement, Alcohol and Tobacco, Fish and Wildlife Conservation, and Motor Carrier Compliance. The system has recently experienced some problems, described as a "glitch" that occasionally disables the system for it's 3,000 users. Technicians are working with Motorola to identify and correct the problems in the $350 million system, but have not conclusively fixed the glitch.

Frequencies include 853.9625, 854.0125, 854.0375, 854.0875, 854.1125, 854.1375, 854.1875, 854.2375, 854.2625, 854.2875, 866.4500, 866.9375, 866.9625, 866.9750, 866.9875, 867.4375, 867.450, 867.4750, 867.9375, 867.9500, 867.9625, 867.9750, 867.9875, 868.4500, 868.4625, 868.4750, 868.4875, 868.9375, 868.9500, 868.9625, 868.9750, and 868.9875 MHz.


Last December the State of Connecticut announced the activation of their $47 million wireless voice and data system. Motorola sold them an 800 MHz ASTRO SmartZone trunked voice system, including equipment for a dozen dispatch centers and more than 2,000 P25-compliant digital radios.

In addition, a RD-LAP wireless data communications system connects patrol car laptops and global positioning system (GPS) receivers to the nearest dispatch center, providing in-car access to state and federal criminal information databases. The data system operates on a different set of frequencies than the voice network.

41 towers provide coverage for nearly all areas of the 5,000 square mile state. The system is expected to be in full operation by mid-2000.

Mesa, Arizona

The city of Mesa, Arizona, recently approved a $15 million contract with Motorola for a new 800 MHz digital trunked radio system for police, fire, and other city workers. Nearby municipalities of Gilbert and Apache Junction will share the system.

Fairfax County, Virginia

Fairfax County, Virginia is in the process of replacing their 20 year old analog system with a twenty channel, 800 MHz P25 trunked radio system that will use IMBE voice. Although scanner listeners won't be able to hear the 800 MHz transmissions, county Fire and Rescue have promised to simultaneously broadcast dispatch information on 460.575 MHz.

Eight repeater sites will be located in Butts Corners, Fair Oaks, Great Falls, Lorton, Merrifield, Mount Vernon, Reston, and Springfield.

Baltimore, Maryland

Last fall the City of Baltimore switched to a digital system using an IMBE vocoder for all voice communications. Control channels are reportedly still operating at 3600 bps.

Active frequencies include 866.2250 866.3500, 866.6625, 866.6250, 866.8250, 866.8500, 866.8750, 866.9000, 867.1500, 867.1750, 867.2125, 867.4000, 867.4375, 867.4625, 867.8250, 867.9000, 867.9250, 867.9875, 868.1000, 868.1250, 868.1500, 868.3000, 868.4500, and 868.7000 MHz.

Project 25 scanners

Although the APCO Project 25 standards are expensive for non-governmental agencies (more than $2000 for the full set), they are open and available. It is certainly possible to produce a scanner, or an add-on box to an existing scanner, that could decode the IMBE voice portion of P25 traffic channels. Stay tuned to the column for further developments along this line.

One possible stumbling block to a hobby P25 scanner is the fact that the IMBE vocoder is covered by patents assigned to Digital Voice Systems, Inc. DVSI has licensed IMBE for use in P25, it is not clear whether they would do so for a scanner application.

Both Motorola and IFR manufacture communications analyzers that will decode P25, but are priced well above the price range of an average scanner listener.

That's all the space I have for this month. I welcome comments, questions, frequency lists, talkgroups, and general updates via electronic mail to dan @ My web page at http:// also has a variety of radio-related subjects.

Comments to Dan Veeneman

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