317 tones modulating an FM carrier, with oper- Several messages may be sent in one transmission as shown in ating frequencies on v.h.f. and u.h.f.. TDMA is used with terminal Figure 46.14, i.e. concatenated messages. In this case it is not node controllers automatically re-transmitting packets of between necessary to send the preamble at the start of every message. 32 and 255 bytes when an ACK is not received within a given time. 46/12 PMR and Trunked Radio Systems MESSAGE 1 MESSAGE 2 MESSAGE 3 ADDRESS CODE ADDRESS CODE ADDRESS CODE AMBLE WORD 1 WORD 2 WORD 3 SHORT MESSAGES MESSAGE 1 MESSAGE 2 MESSAGE 3 ADDRESS CODE ADDRESS CODE DATA CODE ADDRESS CODE AMBLE WORD 1 WORD 2 WORD 3 MIX OF SHORT AND ARBITRARY LENGTH MESSAGES Figure 46.14 MPT1317 concatenated messages Inter-node routing and satellite uplink/downlinking commonly uses The information field may be of a variable length, this may be set 9600 baud FSK with Manchester coding (Miller, 1988) with opera- to any integral number of octets of information up to a maximum of ting frequencies in the u.h.f. regions. Coverage in many areas is such 256. This allows the length of the transmitted packets to be control- that hand portable data terminals may be successfully used, the led, thus allowing customisation to account for interpolation with automatic retry protocol linked to suitable packet data lengths for other overhead messages such as the periodic maintenance mess- the intended use combating signal fading to a large degree. ages found on MPT1327 systems. Together with a six unit alpha- numeric identifier string, a seventh identifier, the SSID (Secondary 46.8.6 Frame format Station Identifier) may be used to allow up to sixteen streams of individual data connections to be conducted from a given ID. Link layer packet transmissions are sent in a number of frames, each Commercial packet network usage is now commencing using frame subdivided into fields as in Figure 46.15. The transmission of proprietary protocols linked to data encryption for security, e.g. in a fra