ay be continued Digit 1 as required to allow link establishment time in the case of simplex or half duplex operations. The following code word carries the Digit 2 user's identity together with address data and parity check bits to Digit 3 ensure correct reception, the message being re-transmitted as re- quired in case of corruption due to Rayleigh fading. Digit 4 Digit 5 Digit 6 Data transmission Digit 7 46.8.1 Characteristics Digit 8 Digit 9 Digital signalling is commonly operated in land mobile radio sys- items, for example data messaging on Band III systems as previously Digit 0 detailed. The logical progression from this is the transmission of Repeat other or longer data messages over any radio system. Typical appli- cations include status messaging, vehicle location, pre-programmed messages e.g. of 'bar code' information, progressing up to portable length and mobile data terminals employing printers and visual display screens. A format for the transmission of digital information over land mobile radio systems has been documented in the MPT1317 code of practice (DTI, 1981c), and common usage of this employs 1200 tronissches Industrie) and CCITT (International Telegraph and baud FFSK using phase continuous 1200Hz (binary 1) and 1800Hz Telephone Consultative Committee). To conform with 12.5kHz (binary 0). channel spacing where audio transmission is tailored to a 2.55kHz The data stream is in the format shown in Figure 46.10, with a maximum frequency, a modified form of ZVEI, named 'Depressed' minimum length of transmission of 96 bits. A preamble for remote or DZVEI, is used. receiver decoder bit synchronisation is used, consisting of a mini- mum of 16 bit reversals, i.e. 1010 to be sent once the transmit- 46.7.4 FFSK digital selective signalling ter has been enabled and is transmitting power. The length of the preamble may be extended to allow for link Fast Frequency Shift Keying is now commonly used on v.h.f. and establishment, the preamble always ending in a logic '0'. Each u.h.f. systems to achieve data addr