Trunked Radio Systems
 One type of
radio system is known as "Trunked" this is quite different to the normal radio
communications that we are used to following. The way a trunked system works
is by employing multi frequencies, whereas most radio communications take place
on one frequency, (e.g. You tap in one frequency on your scanner and all comms
from this user will take place on that frequency) whereas with a trunked system
you have to scan all the frequencies used in the network.
Database All trunked systems have a computer
controlled database that controls what users are allowed to use the system,
what Talkgroups users are currently affiliated to and the allocation of radio
frequencies for use
as and when required.
At least one frequency will be used as a "Data Channel". The purpose of the
data channel is to control the radios telling them which frequency to monitor
for incoming/outgoing transmissions. The whole point of using trunking is to
save on the frequency spectrum, rather than assigning each user their own assigned
frequency, multiple users can share just a few frequencies as the radios are
not in constant use by everyone at the same time. When a user uses his radio
he presses the PTT (Push to talk button) he is assigned a frequency to use
by the data channel (Computerised) and comms will take place on that frequency.
OK that makes sense? Now what happens if you have three different users on
the same radio network wanting to communicate with their colleagues at the
same time? Well when the PTT button is pressed the computer will check all
frequencies available, frequency one is being used by Fred's Skips, the next
available frequency is being used by Bloggs's Couriers, but the next frequency
is available so all radios on the network are told to go to that frequency
and then the communications can take place. At the end of a conversation (or
when the system times out) a data burst is sent from the radio which releases
the frequency in use. This is all done in a fraction of time so although it
sounds long winded it is in fact very quick. The data channel can be identified
by a continuous warbling data stream which can be heard on a receiver. The
most common trunking system used in the UK is the MPT1327 protocol.
MPT1327
This can be heard on many frequencies, typically in the band
3 ( The old TV Band) which is split into two sub-bands. Sub band 1. 176.5 MHz
- 184.5 MHz This band is used by Fleetcomm a nationwide
MPT1327 network plus councils , large factories and Universities have their
own trunked systems in this band. Sub band 2 - 201.5 MHz - 207.5 MHz This band
is used by NB3 a nationwide
MPT1327 network plus smaller regional trunked networks such as Zycall. In addition
this band is used by public transport companies trunked systems
and Railtrack's national wide radio system along with other users. The radio
frequency allocation is performed automatically on a trunked system, MPT1327
systems use a logical approach - sequentially stepping through channel numbers.
All trunked systems are able to detect problems within the system, and remove
active frequencies from the 'active' list if they become unusable, also if
the data/Control Channel goes down, another is selected by the system and all
the system radios are able to carry on as usual.
NOTE: At the time of writing band 3 is due to
be closed down. However the RA are planning on opening the band up to other
MPT1327 users such as regional networks and councils.
Indeed a search in this area shows a marked decline in activity in this band.
The public services are likely to remain on this band for the foreseeable future.
There are others on 162MHz. But use is not limited to these frequencies,
indeed in the West midlands there has been an upsurge of users in the 440MHz
area mainly used by councils and retail radio networks. Gatwick and Heathrow
airport also uses a trunked system in this area of the frequency spectrum.
Also airport trunked radios systems on 455 and 453MHz The data channel on an
MPT1327 system can be identified by a continuous warbling sound. The radio
transmissions used also have the "Warble" at the beginning and end of each
transmission. Typical users of this system are Bus companies, Skip hire companies,
Couriers, Railtrack. In fact just about anyone is likely to be using this system.
With MPT1327 typically following a conversation can be quite hard depending
on the number of users on the system but not impossible. To follow a conversation
with a scanner you will first have to find all the frequencies used, program
them into memory, scan through the frequencies you have put into memory and
listen for your required user. This will require plenty of button pressing
as you stumble on other users and to be honest it is quite a pain to use and
one method that I have quickly given up on on many occasions. This method can
be quite easy though if there are only a few users on the network, one which
I like to use has about five users but only 3 users use their radios to any
extent, also there is only about five frequencies in use so following the intended "Target" is
very easy to follow and is in fact a lot better reception than the single frequency
that they used to use. Is there software available to follow this trunking
protocol? The good news is yes, at this time I only know of one product readily
available and that is FTrunk, but I know that there is another project under
construction.
For more information on Trunk tracking MPT1327 click
Here
Motorola Smartzone
Although not used as extensively in the UK as the MPT1327 system,
the Motorola Smartzone radios are used mainly in isolated areas of the UK.
Predominantly in London where it is employed by the Metropolitan police, known
as Metradio . Again just like MPT1327 this system uses multi
UHF frequencies (However MetRadio system uses VHF for one of the cells, this
is for the helicopters, they use 3 frequencies at 152MHz, one Control channel
and two voice, from there they can select any Divisional primary channel and
work officers on the ground on UHF.)
which are shared by neighbouring divisions (Only police activity on this system
no other users). The way this system works is very similar to the MPT1327 protocol
but with a few differences. MPT1327 is an open protocol, so most makers do
a range of radio models to work on MPT1327 systems.
Motorola SmartZone is an in-house protocol, so no other makers get a look in
at supplying radios to these systems.
You do not get the warble sounds that you get with MPT1327 but
the radios do give a data burst at the beginning of a transmission this data
burst serves the same purpose as before (Basically it is saying this is who
I am I want to communicate, assign me a channel, talkgroup etc.) and is used
for assignment of channels. This data burst is not heard over the radio system
only by the operator (Unlike mpt1327 where data is heard by the recipient and
the operator). The data Channel sounds different to an mpt1327 system also,
it sounds like an old bus engine. On MPT1327 systems the channel is only freed
up at the end of a conversation, on SmartZone systems it is release at the
end of every
over.
Again channel assignment depends on activity on other channels
and when transmissions cease the channel becomes vacant for another division,
(Or indeed the same division) but whereas mpt1327 steps through the available
frequencies sequentially SmartZone systems use a random sequence.
Again the idea is to save on frequency spectrum (Also the old metropolitan
police radio system had been going for years and was in need of an upgrade.)
By assigning a few frequencies on a trunked system is far more efficient than
giving each division their own individual frequency, not to mention all the
other channels that they would need for say crowd control, surveillance and
other events, some of these channels would lie unused for months on end, a
complete waste of frequencies that could be freed up and used by another user.
Talkgroups
Although most trunking radios have channel selectors, they have no control
on what frequencies are used, and serve to select what other radio ID's contact
is requested with, a group of radio ID's that communicate together is called
a Talkgroup. Also with this system talkgroups can be patched onto other talkgroups
so that users from another division could monitor what is going on on another
users talkgroup. This system has been in activity for some years in the US
now and was seen as the death for emergency service listeners, (In the US
they see it as their goddam right to listen to Police/Fire and other emergency
communications) and quickly realising that this would be a major money loss
for themselves Uniden developed the first generation Trunktracking scanner
and everyone was happy again. To follow this system using a conventional
scanner is quite hard but not impossible again a lot of button pressing to
bypass the unwanted transmissions is required. As previously stated in the
United States the problem was rectified by employing a Trunktracking scanner
and these scanners are now readily available in the UK, none more so than
the Uniden Bearcat UBC780XLT. This scanner works by monitoring the control
channel (Again Data) you can search for active trunk Talkgroups ( Each user
is on a talkgroup as opposed to a channel as is the case when conventional
scanning) and enter these active talkgroups into the scanners memory. Just
like conventional scanning you can scan these programmed talkgroups just
like you would scan active channels in conventional scanning, or monitor
one talkgroup at a time and the Trunktracking scanner will "Sit" on the control
channel waiting to be told what frequency to go to (This works just the same
as the actual radios). Another user of this system is military airbases such
as RAF Mildenhall and Lakenheath. There is another option of monitoring this
system and that is a freeware program called trunker .
This requires the use of two scanners, a simple interface and maybe a modification
to access a discriminator output on your scanner and of course a P.C.
CLICK HERE To join the METRADIO Yahoo
discussion group
Staffordshire Starnet
Is a name given to the Police and fire services trunked network
in Staffordshire. This system was so named because originally it was going
to involve the ambulance service, a road company and another unknown user making
five users in all ( Five points of a star!) but they dropped out of the scheme
leaving just fire and police. This is the same system as the Metradio but is
a bit more complex, this system originally employed "Scanner traps" these came
in the form of continuous recorded messages "Scanner trap 1, Scanner trap 1,
Scanner trap 1"... etc. " Scanner trap 2, Scanner trap 2"... etc. The idea
of this is that when trying to scan this system with a scanner not only would
you have to contend with different users ( Remember when they release the PTT
button another division or fire brigade could start transmitting on that frequency
) but also these "scanner traps" causing your scanner to get... well trapped
on that broadcast! This is very clever and does the job extremely well making
monitoring this system extremely difficult. Nowadays the recorded messages
have disappeared but in their place is another recorded what sounds like encrypted
messages, again doing the job just as efficiently this all works by assigning
the "Scanner traps" their own talkgroups. Another way it differs from the Metradio
system is that it uses VHF as opposed to UHF. To monitor the Starnet system
is as already stated very difficult with a conventional scanner, and indeed
normal trunktracking scanners could not handle it due to the different setup
of the system. (For a start it uses VHF 12.5KHz steps and the available trunktrackers
at the time could only resolve 5KHz steps on VHF) but with the introduction
of the Bearcat 780/UBC780 scanners this has now changed and monitoring is again
within the grasp of scanning enthusiasts. Again the trunker software will give
good results with this system.
I would like to thank all the scanning community who contributed
the information I required and without their help this page would have been
a lot shorter.
2002
Note:The information here is provided as information
only. Listening to any broadcasts that you are unlicenced for in the UK
is illegal. Below is the RadioCommunications Agency's view as displayed on
their web site http://www.radio.gov.uk RECEIVE ONLY- SCANNERS
INFORMATION SHEET The purpose of this information sheet is to remove
any confusion regarding the reception of personal or business radio transmissions
by unauthorised persons or groups. It also gives guidance on the subject
by setting out the legal position with regard to the Wireless Telegraphy
Act 1949 (WT Act). How the legislation is applied will depend on the individual
circumstances of the offence. Anyone who intends to listen to radio transmissions
should be aware of the following: A licence is not required for a radio
receiver as long as it is not capable of transmission as well (The Wireless
Telegraphy Apparatus (Receivers) (Exemption) Regulations 1989 (SI 1989
No 123). The exception to this is that it is an offence to listen to unlicensed
broadcasters (pirates) without a licence. Licences are not issued for this
purpose. Although it is not illegal to sell, buy or own a scanning or other
receiver in the UK, it must only be used to listen to transmissions meant
for GENERAL RECEPTION. The services that you can listen to include Amateur
and Citizens' Band transmissions, licensed broadcast radio and weather
and navigation broadcasts. It is an offence to listen to any other radio
services unless you are authorised by the Secretary of State to do so.
There are two offences under law: Under Section 5(b) of the WT Act 1949
it is an offence if a person "otherwise than under the authority of the
Secretary of State or in his duty as a servant of the Crown", either: i)
uses any wireless telegraphy apparatus with intent to obtain information
as to the contents, sender or addressee of any message whether sent by
means of wireless telegraphy or not, which neither the person using the
apparatus nor any person on whose behalf he is acting is authorised by
the Secretary of State to receive; This means that it is illegal to listen
to anything other than general reception transmissions unless you are either
a licensed user of the frequencies in question or have been specifically
authorised to do so by the Secretary of State. or: ii) except in the course
of legal proceedings or for the purpose of any report thereof, discloses
any information as to the contents, sender or addressee of any such message,
being information which would not have come to his knowledge but for the
use of wireless telegraphy apparatus by him or by another person." This
means that it is also illegal to tell a third party what you have heard.
With certain exceptions, it is an offence under Section 1 of the Interception
of Communications Act 1985 if a person- "intentionally intercepts a communication
in the course of its transmission by post or by means of a public telecommunications
system." This means that it is illegal to listen to telephone calls, including
mobile phone networks which are designated as forming part of the public
telecommunications system. COMMON QUESTIONS ANSWERED Q. Am I breaking the
law by owning a scanner? A. No, but it is illegal to use one to listen
to frequencies other than general reception transmissions or those parts
of the radio spectrum which your transmitting licence, if you have one,
allows you to use. You could be prosecuted for this. Q. Can
I get a licence to use a scanner? A. No, there is no scanner licence. Q.
Could I get authority from the Secretary of State to listen to emergency
service transmissions, for example? I am interested and might be able to
help. A. No, the Secretary of State's authority is usually reserved for
people acting at his request and on his behalf. If you wish to listen in
to messages, you should obtain the permission of the person sending them.
Q. Isn't it all right to listen as long as I don't pass on what I hear?
A. No, using radio equipment to listen in, except as provided by section
5(b) of the WT Act, is an offence, regardless of whether the information
is passed on. Q. Isn't this all a bit heavy? A. No. No-one
likes their private or business conversations to be listened to. Parliament
has passed these laws to protect the privacy of radio users.
If you require more information on the law regarding receive-only radio
and scanners, please write to : RA3/Enforcement Policy Unit, Wyndham House
189 Marsh Wall London E14 9SX For further information on other
radio matters contact the Agency's 24 hr Enquiry Point on 020 7211 0211
or e-mail on library.ra@gtnet.gov.uk or visit the Agency website at www.radio.gov.uk
RA 169 (REV 4) May 1999 _______________________________________________
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