 an even the data field, the recommended practice is for bits 9 to 20 to specify bit parity check of the whole 64 bit code word. The overall parity the addressee's identity, bits 21 to 32 to specify the addressor's bit ensures that all odd numbers of errors can be detected, SO that the identity, and bits 33 to 48 to be used for data. overall 64 bit code guarantees that up to 5 bit errors can be detected. At the receiver, each code word can be checked for errors by recalculating the check bits for the received information bits. Any differences between the received check bits and the recalculated BIT NO. check bits indicates that the received code word contains errors. NO. of BITS 46.8.5 Packet data communication Data communication networks employing error correcting proto- USER's ID ADDRESS AND DATA CHECK BITS cols have been used non-commercially for some time now, and are currently starting to become employed for data transmission over PMR systems. Packetised data with automatic requests for re-trans- mission may be used to overcome effects encountered in typical Figure 46.12 MPT1317 address code word structure PMR usage such as Rayleigh fading in a mobile environment, as well as allowing the shared use of a single r.f. channel. Intercon- nected packet nodes may be linked to radio base stations operating 46.8.3 Data code word on h.f., v.h.f., u.h.f. and s.h.f. to form an international network for data store and forward operations, using both terrestrial routing and An arbitrary number of data code words as shown in Figure 46.13, orbiting digital communications satellites for inter-continental may follow an address code word, as required to accommodate the message routing. message. AX.25 (Arrl, 1984) a derivative of X.25, is commonly used Bit 1 is always binary 0 to distinguish the word from an address together with level 2, 3, and 4 node systems. code word, bits 2 to 48 are used for the data, and bits 49 to 64 are User access employs 1200 baud FFSK operation using either Bell used as check bits. 202 tones or MPT1