From: (Brian Kantor)
Newsgroups: rec.radio.amateur.misc
Subject: Re: How does 'Trunked radio' work?
Date: 15 Jun 1992 04:30:37 GMT

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