From: brian@ucsd.edu (Brian Kantor)
Newsgroups: rec.radio.amateur.misc
Subject: Re: How does 'Trunked radio' work?
Date: 15 Jun 1992 04:30:37 GMT
Organization: The Avant-Garde of the Now, Ltd.
NNTP-Posting-Host: ucsd.edu

There are several kinds of trunked radio systems; I'll describe the
Motorola one since that's the kind I was trained on several years ago.
The others aren't too different, I'm told.

Typically there are at least 4 channels in a trunking system.  They
are really repeater pairs; most often in a small system, all the
repeaters are at a single location on a good hill or tall building.
One channel is the control channel and continously transmits digital
codes (3600 baud, if I remember right).  In a Motorola system, the
control channel shifts between the repeaters in the system every 12
hours to even out the wear on the equipment.  Larger systems (like San
Diego's 14-channel trunking system) will have multiple control
channels, usually spread around different sites for better coverage.

When a mobile (or portable) is first turned on, it scans all the
channels in its system until it finds the control channel.  (If it
can't find it, you'll get an error indication.)

When you key the radio to make a transmission (after an idle period),
the radio sends a quick digital request containing your unit id and
the 'call group' of who you want to talk with to the trunking
controller, which is listening on the input of the repeater that is
the control channel at that time.

The controller selects an idle repeater channel, and then transmits a
command that tells your radio to switch to that frequency.  If all
channels are busy, you get an error indication.  All the other radios
in the selected talk group will also switch to that frequency.

(A talk group may be everyone on your system, just a few units, or one
specific unit.  It's completely programmable.)

While the conversation continues, you'll all stay on that frequency.
After a configured idle period (which can be short or long), you'll
all return to the control channel, freeing up the repeater.

Whilst you're on the talk channel, a continuous subaudible digital
code (63 baud? don't remember!) is sent to confirm that you're on the
right channel and that you should stay there.  It can also instruct
you to switch to another talk channel or to return immediately to the
control channel.

MDT (Mobile Data Terminals), alphanumeric pagers, and mobile status
indicators (in service, out of service, man down, emergency, hijack,
etc) can operate on the digital control channel without needing to
allocate a talk channel, although in a busy police system, it's likely
that MDTs have their own channel.

It is possible to poll a mobile to see if it's in service; it's even
possible to reconfigure the mobile remotely in the more advanced
systems.  I see no reason to believe that it is impossible to remotely
command a mobile to transmit, but I don't think that would be
practical as an eavesdropping technique, since typically the
microphone is out of the circuit unless the button on the mic is
pushed, and because most mobiles have blab-off timers to prevent a
wedged microphone from jamming a channel for long.  It could be done,
though.

The voice transmissions are normally standard 5 (or 3) kHz deviated
FM, but they can be digitally encrypted if the system is so equipped.
That's pretty much separate from the trunking function.  (As a side
note, the Motorola digital encryption is done by digitizing the signal
using a CVSD coder doing the A/D/A conversion; as I recall, it's like
a 12kb/s code.  To me, it sounds really muffled - too low a sample
rate for my taste.  Obviously they had to do that to fit it into
standard channels.)

I understand that the Johnson and G.E. systems work similarly, but
don't leave the controller transmitting continuously.  They may use
different baud rates too.

To monitor a trunking system on a scanner, you pretty much have to
program all the frequencies into your radio.  In the US, you can get
them from the public copy of the station license.  However, if the
system gets busy, and the idle homing timer is set short, you'll have
a lot of trouble finding a specific conversation.

I think it would be really cool for someone to make a trunking decoder
that could be attached to a scanner that would instruct it to follow a
specified bunch of talk groups as the channel assignments took place.
No doubt it'll cost a fortune.

In the mean time, the only technique that occurs to me is to get a
used trunking radio and pursuade a radio tech to program it the same
as one of the official radios.  Whatta pain!

	- Brian


>From noiro.acs.uci.edu!usc!sdd.hp.com!mips!darwin.sura.net!Sirius.dfn.de!Urmel.Informatik.RWTH-Aachen.DE!dfv.rwth-aachen.de!alf Wed Jun 17 08:38:45 PDT 1992
Article: 20183 of rec.radio.amateur.misc
Newsgroups: rec.radio.amateur.misc