9
UNDERSTANDING CTCSS/DCS/NAC
Continuous Tone Coded Squelch System (CTCSS) and Digital Coded Squelch (DCS) are two
methods used to prevent interference by other radio communications. Your scanner can receive
transmissions that use these codes (or more commonly referred to as tone codes).
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 requests an unused frequency when that user presses their PTT
(push to talk) button. The trunking system automatically assigns the call to 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 that other users (and
scanners) can follow the comm to the correct frequency. 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. Trunk Tracking scanners 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 trunktracks the following types of systems:
•
Motorola Type I
•
Motorola Type II
•
Motorola Type IIi Hybrid
•
Motorola Type II Smartnet
•
Motorola Type II Smartzone
•
Motorola Type II Smartzone Omnilink
•
Motorola Type II VOC
•
LTR Standard
•
Project 25 Standard
•
P25 X2-TDMA
•
P25 Phase I and Phase II
•
EDACS Standard (Wide)
•
EDACS Narrowband (Narrow)
•
EDACS Networked (Wide/Narrow)
•
EDACS SCAT
•
EDACS ESK
When you program Motorola/P25 and EDACS system frequencies into the scanner, one
frequency is the control (or data) channel, and the rest are voice frequencies shared by all the
users. There may be 3 or 4 frequencies assigned as (primary or alternate) control channels but
only one control channel will be active at a time. These scanners will allow you to program just
Motorola/P25 control channels into the trunking system and the voice channels will automatically
