Police Scanner

mogie said:

Do your homework. Do you know what trunk tracking is? It's basically a method of sharing a limited number of radio frequencies among a large number of users.

Motorola

Motorola Type I and Type II systems are the most common trunked radio systems used by public safety agencies today. Type I systems are older and make use of a "Fleet Map" to organize talkgroups. Type II systems are newer and have more capability than Type I. Every trunk-tracking scanner on the market is able to follow analog voice traffic on both types. Some Type II systems have a mixture of analog and digital voice traffic.

Programming a scanner for these systems requires only entering the control channel frequencies, since control channel messages include the traffic channel frequency. Each repeater site may have a maximum of 28 radio frequencies, with at least one and as many as four of those frequencies used as a control channel.

EDACS

Enhanced Digital Access Communication System (EDACS) is another popular public safety radio system. Each repeater site will have at least one dedicated control channel and as many as 23 traffic channels. In addition to analog voice, EDACS can carry different proprietary digital voice formats, which system operators may also encrypt. Traffic on EDACS control channels may also be encrypted through an optional product known as ESK (EDACS Security Key). Scanners on the market today cannot decode the digital voice formats and will not work properly if the control channel is encrypted.

EDACS requires that the system frequencies be entered into the scanner in a specific order. Each radio frequency is assigned a Logical Channel Number (LCN), and the LCN should correspond to the scanner's channel number.

Logic Trunked Radio

LTR systems are not as common in public safety but are often used for industrial and business applications. LTR systems do not have a separate control channel but use a technique called subaudible signaling to carry talkgroup and frequency information on the voice channel.

APCO Project 25

The Association of Public-Safety Communications Officials (APCO) created a set of standards for digital public safety radio. These standards are collectively referred to as Project 25 and were intended to inject competition into the public safety radio market by allowing agencies to purchase compatible equipment from different manufacturers.

Because APCO Project 25 (P25) is a set of standards, there are systems in operation that use some standards but not others. P25 has a Common Air Interface (CAI) and a specific format for digital voice, as well as a standard for trunking.

There are conventional P25 systems that do not use any trunking but do use P25 digital voice.

There are hybrid systems that mix analog and P25 digital voice traffic on a Motorola Type II control channel. You may find this on systems that are transitioning from older analog technology to fully digital but during the interim want to save money by continuing to use their old radios.

There are also "pure" P25 networks that use all digital voice and the P25 control channel standard for trunking.

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