ximum frequency re-use with closely defined cover- the mobile to move its operational channel to that used by the age in a given urban area supplementing that in rural and semi-rural neighbouring cell. Here the base station in use polls adjacent cell areas. sites to examine the signal strength being received from the mobile, and if traffic channels are available on the relevant cell site the 46.3.6 Mobile roaming mobile is instructed to handoff accordingly. During the communi- cation, periodic 'maintenance messages' may also be processed to ensure correct channel utilisation and communication takes place. Mobile units may roam throughout the area covered by the network of the trunked radio system base station sites, the mobile receivers being commanded to hunt for a control channel with sufficient signal strength in the area of operation. An RSSI (Radio Signal 46.4 Band III Strength Indicator) circuit in the mobile unit constantly monitors the strength of the permanently enabled control channel base transmit- 46.4.1 Interface specification ter, alternatively or also a bit error check is performed on received data, to establish a usable control channel. Commonly called 'Band III' due to its intrinsic use in the U.K. An optional variation in a regional cellular trunked system for trunked PMR system operating in the 174MHz to 225MHz Band III more efficient utilisation of a limited number of channels for com- (HMSO, 1984) range of the spectrum, the MPT1327 signalling bined control and traffic utilisation is that of a 'time shared' control specification (DTI, 1988b) for trunked mobile use chosen as the channel. Here a dedicated signalling channel is time shared amongst mandatory protocol is also currently in use by several countries a number of control transmitters operated in a cyclic manner, operating on Band III and other v.h.f./u.h.f. bands. Although subject mobiles thus respond to the individual base transmitter giving to intellectual property rights, an agreement has been achieved for adequate reception strength