From: alf@dfv.rwth-aachen.de (Ralf Crumbach)
Subject: Re: How does 'Trunked radio' work?
Organization: Communication Networks
Date: Wed, 17 Jun 1992 12:50:32 GMT

geertj@ica.philips.nl (Geert Jan de Groot) writes:

>I heard that last week, the local authorities here have switched their
>old single-channel radios to 'trunked' ones. This as preparation to
>a police force for a much greater area (region instead of city).

>Shure enough, while looking for the signals, I found that something
>has changed. It looks like the rigs change frequency 
>every time they make a new call. People complain that they have to
>wait for 1 sec after they have given PTT, apperently to give the rig
>time to tune.

>I would imagine that such a new system would be all digital, keeping
>out the scanner freaks. However, while listning in, I found out that 
>the transmissions are clear NBFM. It seems that they also have an
>encryption possibility, sometimes noise (like a G3RUH modem) is heard.
>Why isn't encryption always used? I found that sometimes, people are
>unable to produce a relyable signal (mobile flutter). Would that 
>be the reason that encryption is only used on demand? 

>Some channels are very loud, some have much less signal strength.
>yet, they seem to be connected somehow. Sometimes a transmission
>is heard on 3 frequencies. Assuming that it is routed somehow,
>how would the installation know to whom a message should be relayed?

>I also found out that the base station can read the ID of the rig
>transmitting. However, I don't hear a startup signal on each
>transmission, so it must be sent elsewhere.

>I think the new units are made by Motorola, but I am not shure.

>Does anybody recognize this system? If true, do you have some literature
>on how it works? Thanks in advance for any information!

Hi out there,

Jan Geert described in his article a trunked mobile radio system. In
my opinion it is an implementation of the MPT 1327. This standard was
defined by the British Ministry of Post and Telecommunication (MPT),
Department of Trade and Industry, Radiocommunications Division in
1988. The standard was overtaken by several European states, at least
France and Germany. In Germany the "Deutsche Bundespost Telekom' has
licensed in several regions (e.g. in the "Ruhrgebiet") trunked mobile
radio systems.

To the technical part of the system:
(I try to summarize the informations I have)
- Structure of the system: cellular; if necessary several cells are combined to
   a big system ofr covering an area.
- Diameter of a cell: 10 - 25 km for big areas
                       5 km for industrial zones
- separate channel for signalling data; e.g. connection establishment, exchange
   of ID's ...
- Speech transmission analog
- Data transmission: 2400 bit/s, FFSK ("0" - 1800 Hz; "1" - 1200 Hz)
- Signalling data is transmitted digital with 1200 bit/s
- Transmitted power: < 15 W erp (in Germany)
- Frequency range: 410 - 430 MHz (in Germany); the standard allows a freq range
   from 50 to 500 MHz
- Access to the Signalling channel: S-ALOHA
- possible call types: point-to-point, 'group'-call, broadcast, conference, call
   redirection, priority calls, emergency calls
- possible access to the telephone system (with tradeoffs caused by the half
   duplex in TMR)

There are a lot of different manufacturer for these units, the
following list is a summary of the manufacturer I know and is not ment
as an advertising in any sense.

Bosch, SEL/Alcatel, Nokia, Motorola, Rohde & Schwarz (for base stations [?]) 
 and surely some other ...


I hope this informations helps a bit. The informations I got are mostly taken
out of "Funkschau" a newspaper on radio communication (in German).

Bye es 73
   Ralf, DD2KZ


-- 
Ralf Crumbach, DD2KZ                                     |     C   O   M
Communication Networks, Technical University of Aachen   | --v-^-v-^-v-^-v--
  Phone: +49-241-807919           Fax: +49-241-84964     |   N   E   T   S
  Email: alf@dfv.rwth-aachen.de                          | RWTH Aachen, Germany


>From noiro.acs.uci.edu!usc!cs.utexas.edu!uunet!mcsun!news.funet.fi!cc.tut.fi!tut!benjamin Thu Jun 18 07:55:10 PDT 1992
Article: 20233 of rec.radio.amateur.misc
Newsgroups: rec.radio.amateur.misc