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Scanning Trunked Systems
Scanning
Trunked Systems
What is a Trunked System?
This explanation would be impossible
without first mentioning conventional radio systems. Simplified, on
conventional two-way radio systems each channel has a dedicated
frequency. For
example, the Delaware County Sheriff
Department has channels as follows: Dispatch 155.655,
or 3-Way (on a set Frequency). When an officer switches
channels, he is tuning the radio to a
different frequency or channel. On a trunked radio system a channel can
be on
virtually any frequency in the system. A trunked radio system usually
consists
of 5 or more frequencies, and normally not more than 30. One of the
frequencies
is used as a control channel and in some cases switches
automatically every
twenty-four hours. Each radio in the trunked system monitors the
control
channel to determine what frequency is currently assigned for the
active talkgroup (channels) on the radio. That frequency may
be any frequency in the trunked radio
system, except the control channel, so in a 8 frequency trunked system,
there
will be 7 frequencies worth scanning.
Why Use a Trunked System?
Spectrum efficiency is the short
answer, but it's not the only reason. Since channels on a trunked
system are
"virtual" compared to a conventional system, there can be more and
flexible channels. For example, the City of Muncie operates on
a trunked system that
has several frequencies, but dozens, if not hundreds, of possible
talkgroups (channels). Virtual channels (talkgroups) provide
spectrum efficiency. It's unlikely that all frequencies will be in use
at the
same time. When that happens, lower priority users get a busy
signal.
Other benefits of trunked systems
include the ability to disable individual radios in the system that may
have
been lost or stolen. Another aspect of individual radio ids is the
ability for
dispatchers to track whose radio is transmitting. That's a big safety
feature,
especially when an officer may not be able to talk on the radio. There
is also
an emergency button on most public safety trunked radios. On shared
trunked
systems like the one in Muncie, it's easy for different departments
within a
city and even between different cities to share a channel. With
conventional
radios that was often a problem because different users might have been
on
different frequencies bands which would have required a more expensive
or
different radio. There are other benefits, but those are some of the
most
important.
What Frequencies Are Used?
Trunked
radio systems started
out on 800 Mhz frequencies and are now showing up in the 900 Mhz range,
and the
federal government is even going trunked in their 400Mhz band for some
departments.
Who Uses Trunked Systems?
Almost
everyone. Public safety,
business, transportation, federal government, public works, the list
continues.
City and state government systems are usually on separate trunked
systems, however with The Indiana Safe-T system it
is shared among agencies
statewide. Smaller businesses
typically do not have their own trunked system, but are members of a
trunked
system with many other businesses. Some reports even claim that the
federal
government is buying time on business trunked radio systems.
Who Makes Trunked Systems?
The big players in trunked radio
systems are Motorola,
Ericsson
(formerly GE),
Uniden,
and Johnson LTR.
The last two I'm not certain about. The Ericsson systems are the worst
for us,
because each transmission ends with a data bursts that make scanning
very
difficult. Almost all public safety trunked systems are one
of
several Motorola types.
How to Scan a Trunked System
Why is Scanning a Trunked System
Different than a Conventional System?
Remember
virtual channels from
above? Each time someone uses a radio it may be on a different
frequency, even
though the user has not changed channels on the radio. Channel does not
equal
frequency in a trunked system.
Conventional Scanners
After
determining what
frequencies a trunked radio system includes, program them into their
own bank. Once you've programmed all of the frequencies, make
sure they are
all unlocked. Hit the scan button until it stops on a frequency that
has a
continuos data sound. Lock out that channel in the scanner, and you're
ready to
go. Normally, you will have to lockout a new control channel everyday.
DO NOT
FORGET TO UNLOCKOUT THE PREVIOUS CONTROL CHANNEL! Note:
There are
lots of other data channels floating around on some trunked systems, so
make
sure you only lock out the frequency with the continuous data, most
other data
sounds are short bursts, and will alternate among the different
frequencies. Now it's just a
matter of trying to follow a conversation. For
instance, an officer calls the dispatcher.
| Transmission
|
Channel
On Police
Radio |
Frequency
Used in
Trunked System |
| Officer: "325
Radio" |
Dispatch |
854.9875
|
| Dispatcher:
"325 Go ahead" |
Dispatch |
855.2375
|
| Officer:
"Send my that call downtown" |
Dispatch |
866.0375
|
| Dispatcher:
" 325 Clear " |
Dispatch |
854.9875
|
Notice the channel never changes
for the dispatcher and officer, but the frequency, which they are not
concerned
with, does change. All of the radios set to channel Dispatch switched
to the
current frequency for Dispatch because the radio monitors the data
channel the
entire time checking to see what frequency is currently assigned to the
channel
that is set on the radio.
So following the conversation
above you would have had to hit scan a few times to tune to the correct
frequency, and you probably would have stopped on other frequencies
that were
being used by other channels on the trunked system. You have to listen
more
closely to follow the conversation, otherwise you'll be listening to
several
different conversations mixed together. No, it's not as easy to follow
as
staying tuned to a single frequency, but it can be done for most
trunked
systems, provided you know how units are numbered and can recognize
voices and
topics of conversations.
Trunk Tracking Scanners
Orginally there
had not
been a way for the average scanner user to really scan trunked radio
system
channels like conventional systems. Finally, Uniden has released their
new line
of Trunk Tracker scanners that would scan Motorola (the most
popularly used for
public safety), GE/Ericsson EDACS, and LTR trunked systems. This is a
HUGE
step for the scanner hobbiest!
Trunked Radio
If you have
lots of money and
really good connections, you may be able to buy a trunked radio and
have it
programmed with the users you want to hear (receive only, no
transmitting). For
the average scanner hobbiest, don't get your hopes up. This is how the
media
does it though, and I suspect that there are some individuals out there
who
have trunked radios that technically they should not have. Motorola has
been to
court a time or two against people who make this possible.
Motorola
Trunking Training
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