New 800 MHz. trunked radio systems are rapidly replacing "conventional" radio systems in municipalities which can afford them. The benefits of using trunking radios are numerous; some are:
Also, most systems in use today are owned by the vendor which installed them, thus the users might pay only to lease the radios and dispatch consoles and don't need to maintain their own radio shop. The principal vendors offering trunked systems currently are:
There is a myth among some users of these systems that their communications are "un-monitorable". Although most systems do support DES encryption, this feature is rarely used (except on federal systems), and most communications are sent in the clear where they can easily be received. An experienced scanner user can follow any particular unit or dispatcher through the system. It just takes a good ear for voices and a quick finger on the "scan" button.
The FCC licenses channels for the Specialized Mobile Radio Service (SMRS) in blocks, in multiples of 5 discrete frequencies. The frequency assignments are public record, and are available from a variety of sources. The channel assignments (whether for police, fire, ambulance, etc.) are pretty much up to the user. Channels are combined into "talk groups" which are rotated among the various frequencies, and any specific channel assignment may change each time the repeater drops carrier.
Only the first frequency in the block is required to be identified with the user's call sign. The identification is required once per hour.
Motorola systems can be recognized by the presence of a "control channel" which has a very distinctive roar. The control channel is used to tell the radios which frequency they should receive and transmit on. The radios always transmit 45 MHz. below the frequency they receive on.
To scan these channels, program them with no delay and lock out the control channel. This control channel will periodically move to another frequency (usually at midnight), so you'll continually need to lock out a new control channel and unlock the old one.
For more information about trunked radio systems from a scanner-buff's perspective, see Warren Whitby's TRS User's Page.
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