PMR Trunking using MPT1327
A trunked version of the Private Mobile Radio (PMR) concept that is defined
under the standard MPT 1327 (MPT1327) is widely used and provides significant
advantages over the simpler single station systems that are in use. MPT1327
enables stations to communicate over wider areas as well as having additional
facilities.
In view of the very high cost of setting up trunked networks, they are
normally run by large leasing companies or consortia that provide a service to a
large number of users. In view of the wider areas covered by these networks and
the greater complexity, equipment has to be standardised so that suppliers can
manufacture in higher volumes and thereby reduce costs to acceptable levels.
Most trunked radio systems follow the MPT1327 format.
To implement trunked PMR a network of stations is set up. These stations are
linked generally using land lines, although optical fibres and point to point
radio are also used. In this way the different base stations are able to
communicate with each other.
In order to be able to carry the audio information and also run the variety
of organisational tasks that are needed the system requires different types of
channel to be available. These are the control channels of which there is one in
each direction for each base station or Trunking System Controller (TSC).
A number of different control channels are used so that adjacent base
stations do not interfere with one another, and the mobile stations scan the
different channels to locate the strongest control channel signal. In addition
to this there are the traffic channels. The specification supports up to 1024
different traffic channels to be used. In this way a base station can support a
large number of different mobile stations that are communicating at the same
time. However for small systems with only a few channels, the control channel
may also act as a non-dedicated traffic channel.
The control channels use signalling at 1200 bits per second with fast
Frequency Shift Keying (FFSK) subcarrier modulation. It is designed for use by
two-frequency half duplex mobile radio units and a full duplex TSC.
For successful operation it is essential that the system knows where the
mobiles are located so that calls can be routed trough to them. This is achieved
by base stations polling the mobile stations using the control channel.
To make an outgoing call the mobile transmits a request to the base station
as requested in the control channel data stream from the base station. The
mobile transmits its own code along with that of the destination of the call,
either another mobile or a control office. The control software and circuitry
within the base station and the central control processing area for the network
sets up the network so that a channel is allocated for the audio (the traffic
channel). It also sets up the switching in the network to route the call to the
required destination.
To enable the mobile station to receive a call, it is paged via the incoming
control channel data stream to indicate that there is an incoming call. Channels
are allocated and switching set up to provide the correct routing for the call.
There is no method to "handover" the mobile from one base station to the next
if it moves out of range of the base station through which a call is being made.
In this way the system is not a form of cellular telephone. It is therefore
necessary for the mobile station to remain within the service area of the base
station through which any calls are being made.
The control channel signalling structure has to be defined so that all
mobiles know what to expect and what data is being sent. Signalling on the
forward control channel is nominally continuous with each slot comprising 64 bit
code words. The first type is the Control Channel System Codeword (CSCC). This
identifies the system to the mobile radio units and also provides
synchronisation for the following address codeword. As mentioned the second type
of word is the address codeword. It is the first codeword of any message and it
defines the nature of the message. It is possible to send data over the control
channel. When this occurs, botht he CSCC and the address codewords are displaced
with the data appended to the address codeword. The mobile radio unit data
structure is somewhat simpler. It consists fundamentally of synchronism bits
followed by the address codeword.
There are a number of different types of control channel messages that can be
sent by the base station to the mobiles:
Aloha messages -- Sent by the base station to invite
and mobile stations to access the system.
Requests -- Sent by radio units to request a call to be set up.
"Ahoy" messages -- Sent by the base station to demand a response from
a particular radio unit. This may be sent to request the radio unit to send his
unique identifier to ensure it should be taking traffic through the base
station.
Acknowledgements -- These are sent by both the base stations and the
mobile radio units to acknowledge the data sent.
Go to channel messages -- These messages instruct a particular mobile
radio unit to move to the allocated traffic channel.
Single address messages -- These are sent only by the mobile radio
units.
Short data messages -- These may be sent by either the base station
or the mobile radio unit.
Miscellaneous messages -- Sent by the base station for control
applications.
One of the problems encountered by mobile signalling systems is that
of clashes when two or more mobile radio units try to transmit at the same time
on the control channel. This factor is recognised by the system and is overcome
by a random access protocol that is employed. This operates by the base station
transmitting a synchronisation message inviting the mobile radio units to send
their random access message. The message from the base station contains a
parameter that indicates the number of timeslots that are available for access.
The mobile radio unit will randomly select a slot in which to transmit its
request but if a message is already in progress then it will send its access
message in the next available slot. If this is not successful then it will wait
until the process is initiated again.
Although the data is transmitted as digital information, the audio or
voice channels for the system are analogue, employing FM. However some work has
been carried out to develop completely digital systems. The main systems are by
Motorola, by Ericsson (EDACS) and Johnson (LTR). These systems have not gained
such widespread acceptance.
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