ver, ACKQ - Call queued with many PMR systems having 'private' PABX and PSTN access ACKX - Message rejected facilities from within their own systems, together with trunked PMR ACKV - Called Unit unavailable offering intra-fleet communication and PABX/PSTN access fa- ACKE Emergency cilities, dependent upon the system in use. ACKB Call-back 46.10.4 Radio unit requests 46.9.1 Business use RQS Simple PMR is thus generally used within a business environment for RQD Data inter-unit communication within the business fleet, e.g. for rapid RQT Divert dispatcher to delivery vehicle communication. Through the use of RQE Emergency carefully tailored base station aerial sites, communication may be RQR Registration usefully achieved within an area where public access systems are RQQ Status unable to provide a service, for example within an office or building RQC Short Data site environment, or on a larger scale along a dedicated network of permanent ways such as the British Railways trunked PMR system 46.10.5 TSC Ahoys now in use. Due to the one to one communication scenario, PMR is generally AHY - General availability check considered unsuitable for private citizen use where PSTN access, AHYQ Status message either one way or two way, is often required. Here, cellular tele- AHYC - Short Data Invitation phony, CT2, and PCN are likely to be of greater benefit. Fees levied by national cellular system operators for network calls however often make this unsuitable for users with frequent but often short 46.11 References calls, e.g. taxi and delivery firms operating in a given locality, where PMR normally offers a more economic solution if frequency assign- ments exist for this. ARRL (1984) Packet Radio Link Layer protocol, AX25, December. Armstrong, W. (1989) Trunked PMR in British Airways, IBC Trunked Private Mobile Radio Conference, November. 46.9.2 PMR growth Bye, K.J. (1989) Handover criteria and control in cellular and microcellular systems, IEE Fifth International Conference on Mobile Radio and Personal Communications, Decemb