We continue the listing of APCO Project 25 (P-25) systems in the United States.
These listings are based on Federal Communications Commission (FCC) license records, and thus do not include military installations or systems operating in other countries. If you have additional information, corrections, updates or just a question or two, please send me an e-mail at firstname.lastname@example.org.
Two-way radio systems can operate in one of two ways. The first is conventional, where a conversation takes place on a pre-assigned pair of radio frequencies. All participants in a conversation always transmit on one frequency and receive on another frequency. The frequency pairs are commonly assigned according to department or organization. For instance, a small or medium-sized town may have licensed three pairs of frequencies from the FCC. The first pair may be assigned to the police department, the second pair to the fire department, and the third pair as a "tactical" channel for occasional use by either department. What this means for the scanner listener is that when there is activity on a particular frequency, it's a safe bet that the activity will always be from the same group of users. In our example, when a listener hears transmissions on the second frequency pair, it's going to be related to the fire department.
The second method of operating is trunked, where a set of radio frequency pairs is shared among more than one group of users. These groups of users are identified with a number known as a talkgroup. When a member of group wants to communicate with other members, he or she presses the push-to-talk button on the radio. The radio then transmits a quick digital request to a control computer, identifying itself with the talkgroup number and asking for a channel (a frequency pair) assignment. The control computer checks all of the available, licensed frequencies and picks one that is not currently in use. It then sends a digital broadcast message out to everyone saying, basically, "Talkgroup X is now active on channel Y." All the radios that hear the message and are part of talkgroup X then tune to channel Y. In APCO P-25 systems, each of those digital messages - the request and the broadcast response - occur on what's called a control channel.
These control channels come in two flavors. The first is an older format, first developed by Motorola, which transmits digital information at a rate of 3600 baud. Older systems with analog radios typically use the 3600-baud control channel, since that's all the analog radios will support. The second flavor is a newer format, spelled out in the Project 25 specifications, which operates at 9600 baud. Every radio on the system has to be able to understand the format of the control channel, so 9600-baud systems are only found with systems that have gone totally digital. Such systems include the states of Colorado, Michigan, and Minnesota, as well as several counties and cities.
So, to summarize, there are four different combinations for a P-25 system. It can operate conventional or trunked, and it can either be all-digital voice or a mix of digital and analog voice.
According to Uniden information, the handheld BC250D and the base/mobile BC785D are able to follow three of the four possible types of APCO Project 25 systems. The units are not able to track trunked transmissions in those systems that use a 9600 baud control channel.
There are currently a handful of P-25 installations that use 9600-baud control channels, including systems in Colorado, Michigan, Minnesota, Texas and Virginia. In these cases the scanner will decode the audio but will not be able to follow a trunked conversation.
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