A Little Trunking Theory
Trunked radio received its name from the 'trunk line' which is used in commercial telephone communications. Put simply, a 'trunk' is a communication path between two or more points, typically between the telephone company central office and one or more users. The trunk line is time-shared by several different users, but users of the telephone service do not need to be aware of this sharing. One party places a call to another party and the call is completed; the internal working of the telephone system is transparent to the users.
Radio communication over a trunked system is quite similar to such telephone systems. The transmitting and receiving radio units can be thought of as the calling and receiving parties, and a trunked radio system can be thought of as the telephone company equipment. Instead of telephone lines, the radio system uses radio channels to place calls. As with the telephone system, the radio users are not aware of which particular radio channel they are communicating over. All that is apparent is that a communication path has been established between the users.
Trunking a multi-channel radio system increases the efficiency of the radio system by dynamically managing the use of a radio channel. This is accomplished by the computer control of radio channels and the virtual elimination of the delay experienced by field units in obtaining a clear radio channel.
Every trunked radio system uses two types of radio channel designations: control (or data) and traffic (or voice) channels. One control channel must be designated at each site and the remaining channels are used as traffic channels. The control channel is used to send digital information between the radio units and the computer equipment controlling the operation of the system.
Traffic channels are used to send the actual communications (voice or data) between radios. A simplified exchange between the radio unit and the site equipment may be described as follows:
The radio unit continually listens to the control channel waiting for instructions.
When a call is to be placed, the user presses his "push-to-talk" (PTT) switch; the radio then sends a short digital message over the control channel and tells the site equipment that it needs a channel to communicate.
The site equipment hears the request for a channel and assigns an available working channel by sending a return digital message over the control channel.
The radio unit receives the working channel assignment and sets its transmit and receive frequencies to the new channel.
The radio unit and the working channel perform a high speed "handshake".
The radio audibly signals to the operator that a channel has been assigned and that communications can start.
This procedure may be repeated several times during a communications sequence, and subsequent transmissions during communications sequence may be made over any one of the available working channels. All that is apparent is that a clear channel is available for communications. The user does not have to be aware of which particular radio frequencies the communication takes places.
A unique address code is allocated to each radio. This code identifies a calling radio to the site equipment, and allows the site equipment to call specific radio units. The radio units can be also partitioned into 'talk groups' so that radio users with similar communication requirements can communicate with each other.
If you are interested in the technical details, the Motorola SmartNet trunking system is theoretically using up to 760 channel frequencies, equally spaced at 12.5kHz or 25kHz. Transmission channels are in the lower half of the total channel number, and receiving channels are in the upper one. On the control (data) channel the base station transmits 84 bits frames at 3600 bit/s with direct frequency modulation of the carrier using Frequency Shift Keying (FSK). These frames contain different information as: system ID, current data and time, index of signalling channel, command for radio affiliation to groups and groups working frequencies. The frame format is quite complex because it needs to ensure correct decoding at the destination point and a high noise immunity level. It encapsulates data interleaving, a convolutional error-correcting method and a CRC (cyclic redundancy check) for final error detection, if any left after the first error-correcting algorithm. The Motorola SmartNet is currently the most commonly used trunking radio system in North America. It is also widely used in Europe, Australia and other countries.
The MPT1327 standard for trunked private land mobile radio systems is used mainly in Europe, Australia, Canada and other countries. It accommodates up to 1024 channels, 12.5kHz or 25kHz apart. The protocol relies on control channel signalling at 1200 bit/s using Fast Frequency Shift Keying (FFSK) subcarrier modulation, with a simple 15-bit CRC error detection. The binary '0' frequency is 1800Hz and the binary '1' is 1200Hz. Control channel time is divided into slots of duration 106.7 ms (128 bits), and one signalling message can be sent in each slot.