 and control information. Thus the choice of frame align- causes for transition from one state to another are given in the ment word (FAW) and the design of reliable alignment detection, diagram and it can be seen that they relate directly to the conditions misalignment detection and searching algorithms is crucial. Figure (1) to (3) listed above. Misalignment Imitation Out of alignment continues of FAW but locked undetected Misalignment is detected Searching Bit slips for alignment FAW found Searching for FAW continues FAW continues Mutilation In alignment to be of FAW by and locked recognised transmission errors Figure 28.20 A state transition diagram for frame alignment Frame alignment 28/17 Before considering the mechanisms involved in such an align- ment process there are two variations to the above which can be Table 28.4 CCITT Recommendations for frame alignment, found in practice. The FAW may not be transmitted in the bunched for the 30 channel multiplexer hierarchy format shown in Figure 28.18. If a system is known to be prone to short bursts of transmission errors, some protection against them Multiplexer CCITT Frame Frame Number of may be afforded by distributing the FAW in known positions throug- level length alignment frame checks hout the frame. Whichever method is adopted, the alignment prin- (bits) before ciples are similar. Secondly, some systems use a unique FAW, that changing state is, the data bits are encoded so that in the absence of transmission Lock- Search errors the FAW can never be imitated (e.g. the frame marker flag toseard 01111110 in CCITT Recommendation X.25). For a given level of confidence in the alignment process, this will simplify the requisite algorithms but at the expense of some redundancy in coding the data Primary 0011011 3 or 4 bits. (approx. (7 bits) 2Mbits/s) 28.5.2 The alignment process Second 11100110 (approx. (8 bits) 8Mbits/s) When a system is operating correctly the aligner should reside in the Third 1111010000 in alignment and locked state of Figure 28.20, with t