me is preceded by 16 bit reversals, followed by the start flag, the U.K. a small number of such systems are licensed for national address field, control field, a network protocol identifier (PID), coverage, employing nodes interlinked via. landlines. Uses of simi- information field, frame check sequence (FCS) and an end flag. lar systems for Electronic Funds Transfer are also in hand. ADDRESS CONTROL INFORMATION 01111110 14/70 OCTETS 1 OCTET VARIABLE LENGTH 2 OCTETS 01111110 DESTINATION SOURCE DIGIPEATER ABCD12 EFGH34 10000001 OSSIDRRO ISSIDRRH ASCII RESERVED HAS BEEN FOR LOCAL REPEATED 0- HAS NOT REPEATED HDLC EXTENDER BIT SET TO o EXCEPT FOR LAST OCTET Figure 46.15 AX25 packet frame format Applications 46/13 46.8.7 Satellite land mobile data 46.10 Appendix 46.1 communication MPT1327 sample command structure Low Earth orbiting satellites (Clark, 1988) allow the use of store and forward data communication to be achievable using low power 46.10.1 Channel allocation transportable and mobile mounted equipment on v.h.f. and u.h.f. employing omni-direction aerials (AMSAT, 1991). GTC - Go To Channel As well as one to one AX.25 usage, a 'broadcast' protocol may also be employed from the satellite where continuous bulletin infor- mation is transmitted for reception by permanently installed land 46.10.2 Aloha invitations terminals linked to computer controlled disk based storage, with individual repeat request messages only being transmitted by the ALH General ground station to complete 'holes' in the received packetised data. ALHS - Standard Data Excluded ALHD - 'Simple' Calls excluded ALHE - Emergency only 46.9 Applications ALHR - Registration or Emergency Private Mobile Radio is identified thus to distinguish it from PAMR, 46.10.3 Acknowledgements Public Access Mobile Radio. PMR normally is used by individual fleets of users with inter-fleet communication, whereas PAMR ACK General covers public access communication such as those with PSTN ACKI Intermediate access, e.g. Cellular, CT2 etc. A degree of overlap occurs howe