Understanding Trunking - Uniden BCD325P2-AU Owner's Manual

CTCSS and DCS systems all use some form of coded squelch. Coded squelch
techniques involve the transmission of a special 'code' signal along with the audio
of a radio transmission. A receiver with coded squelch only activates when the
received signal has the correct 'code'. This lets many users share a single frequency,
and decreases interference caused by distant transmitters on the same channel. In
all major metropolitan areas of the United States, every available radio channel is
assigned to more than one user. Public safety radio systems on the same frequency
are usually set up at a distance of 40 miles apart, or more. This means that you may
hear transmissions from a distant system when your local system is not transmitting.
By programming the CTCSS tone for a local channel the scanner will not stop on
transmissions from the distant system. With few exceptions, such as the VHF Aircraft
and Marine bands, almost every other VHF or UHF radio system uses some form
of coded squelch. By far, CTCSS is the most popular mode among non-trunked
systems.
NAC, similar to CTCSS/DCS, has a 3 digit hexadecimal code that is broadcast along
with the digital signal being transmitted.

UNDERSTANDING TRUNKING

Trunking systems let a large group of 2-way radio users (or even different groups
of 2-way radio users) efficiently use a set of frequencies. Instead of selecting a
specific frequency for a transmission, the user's radio selects a programmed trunking
bank in the system when that user presses their PTT (push to talk) button. The
trunking system automatically transmits the call on the first available frequency, and
also sends (on a different frequency called a Data or Control channel) a code that
uniquely identifies that transmission as a talkgroup ID (or just ID.) So when you are
trunktracking a system, you are listening to active IDs transmitting in the system
(each using the first available frequency in the system.) Trunking systems in general
allocate and use fewer frequencies among many different users.
Since the trunking system might send a call and its response on different
frequencies, it is difficult to listen to trunked communications using a conventional
scanner. A trunktracker let you monitor the control channel frequency so you can
hear calls and responses for users and more easily "follow" conversations than with
a conventional scanner.
This trunking scanner tracks the following types of systems:
15