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 fibers 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 signaling 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 through to them. The TSC (Trunking System Controller) gains this information by mobiles "registering" on a control channel. The MPT1327 standard describes several registration mechanisms, aimed at limiting the load on the control channel caused by mobiles roaming. Registrations may be explicit or implicit. An explicit registration may be initiated by the control channel demanding that a mobile issues a registration request; or initiated by a mobile which has roamed to a new registration area. It is also possible for a mobile to implicitly register, where the TSC will update its registration records when the mobile makes a call attempt. It is possible that the TSC's record does not match the mobile's location. This can occur, for example, where a mobile is switched off and then moved within coverage of a different site.
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 described within the standard 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 discipline is Slotted Aloha where the forward or downlink channel (that received by the mobiles) provides timeslots within which a mobile may transmit a request in the uplink channel. In general, a mobile may only transmit on the control channel if invited to by TSC. This invitation may be explicitly addressed to a mobile or a group; or it may be random access. Random access timeslots will be used when a mobile user initiates a call, or when a mobile registers on the TSC. On a heavily loaded control channel, it is likely that two or more mobile radio units will try to transmit at the same time on the same random access timeslot. This is detected by mobile, when the expected reply from the TSC is not received within a certain timeout. The mobile may then retry its request in another random access slot. The timeouts and number of retries is configured in the mobile when it is set up for the network.
Signaling 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 synchronization 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, both the 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.
Acknowledgments — 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
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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