From: PJML@ibma.nerc-wallingford.ac.UK (Pete Lucas, NERC Computer Services, Swindon)
Newsgroups: rec.radio.amateur.misc
Subject: Trunked Radio Systems.
Date: 11 Sep 91 15:47:23 GMT
Organization: UCSD Usenet Gateway


Steve Schallehn was asking about these; basically all the radios are
frequency agile; you do not know between successive mic-presses which
frequency you are going to transmit on. When 'idle', all the radios
listen on one common control channel; the base station or repeater
transmits a constant signal on this frequency (this signal contains
information about the base station, such as what frequencies are
available from it; the receiver can also use this signal to, for
example, measure the strength of signal from the base/repeater and so
decide whether the signal is good enough to give an acceptable service
quality).

When you hit the transmit switch, the repeater & mobile both agree on
a frequency to use, tune to it, and you get your link. When you de-key
the transmitter, the frequency you were using is released for use by
another person. If you are out of range of the repeater, you cannot
transmit. This has advantages where people drive from one area to
another; with normal CTCSS-type radios you can transmit on a frequency
allocated to someone else if you happen to move into a different
geographical area; with a trunked system, unless the repeater 'gives
you permission' to transmit, you do not get a slot and your TX is
disabled.

Its also possible for the mobiles to dynamically tailor their power
output on the basis of the signal received on the control channel;
this reduces 'spill-over' between regions and allows frequency re-use
on a finer scale.

This, and the dynamic frequency re-use, result in greater efficiency
of spectrum utilisation (something we must encourage, since it helps
to take some of the pressure off *our* bands!).

The 3 beeps are produced when your transmitter has moved to a
frequency under command of the repeater, and you are allowed to start
speaking.  Because its all digitally controlled, the control channel
can be used to broadcast paging-type information to the mobiles.
Theres also the ability to interleave voice and data; the control
channel may be used to send special alerts, group pages etc.

This is how the 'who called you' information can be passed. Its also
possible for a user to 'ping' each mobile to see if he has his radio
switched on or if he has switched off... Big Brother is watching.....!

Another big advantage to the users of such a service is that its
harder for people with scanners to listen in, since the frequencies in
use change between 'overs'. In order to follow such conversations, you
would have to have a receiver that could decode the digital squawks
and work out which frequency to tune to.

Finally, the best thing (from the perspective of the service-provider)
is that you can account for the time each registered user has spent
talking, and then invoice him for it every month.....!

Here in the UK, a trunked system has been established in the frequency
band 174-216MHz following the total closedown of VHF TV broadcasting
some years ago.

 Pete Lucas   PJML@UK.AC.NWL.IA    PJML%IA.NWL.AC.UK@UKACRL   G6WBJ.ampr.org
 'If we encounter any problems, fear not, for I still have the magic Fish'



>From usc!cs.utexas.edu!uunet!mcsun!news.funet.fi!cc.tut.fi!benjamin Thu Sep 12 09:12:15 PDT 1991
Article: 7143 of rec.radio.amateur.misc