s, cations. An AHYC message requests the mobile to transmit an indicating the number of available message 'slots' available. FFSK data message containing the unit identity and its individual Mobiles transmit randomly timed messages using FFSK to coincide Electronic Security Number (ESN), a non-registered or incorrect with the available slots, in an effort to overcome data collisions. user receiving a CLEAR message from the TSC. Throughout com- Upon receipt, the base station sends an acknowledgement to the munication on the channel, periodic identification fields from the calling mobile thus confirming correct reception. base may be sent, an incorrect field received causing the mobile to A queuing system is used where the network operates on a 'first clear. This ensures the required privacy from other users during in, first out' basis together with call time limitation thus ensuring co-channel reception from other cells' areas. 46/8 PMR and Trunked Radio Systems 46.4.6 Data messaging on Band III transmitting beam is significant and is kept as small as possible by choosing transmitting and receiving sites so as to have an unob- Status messages (SDM1) and short data messages (SDM2) employ structed view of the horizon. use of the control channel only, the ACK protocol combined with As such, tropospheric scatter is normally limited to point to point automatic data re-transmission providing integrity of transmitted links for medium to long distance communication. A common use data reception. SDM1 may be linked for automatic status trans- is that over a sea path between a land based station on a cliff top and mission of vehicle activity for fleet communication (GEC, 1990), an offshore platform to provide telephony and other communica- and/or rapid digital messaging for fleet communication. 32 RQQ tions services. status codes are provided for, with RQQ (00000) and RQQ (11111) used for 'off hook' and 'on hook' status signalling. 46.5.3 Signal levels SDM2 may be used for dispatcher originated short data messages entered vi