f the next FAW, and SO on. If an FAW is not found in the of erroneous transitions between states. Of course a compromise is necessary, if a genuine transition to another state is required, it is expected position the control circuit will initiate a bit-by-bit search equally undesirable to waste time making unnecessary checks. through the frame until a new alignment position is found. Unfortunately the equipment cannot instantaneously distinguish As discussed in the previous section, it will usually also be between necessary and erroneous transitions. Table 28.4 gives some necessary to guard against erroneous action being taken when an examples of practical frame alignment arrangements, the last two FAW is mutilated by transmission errors or when an imitation of an columns detail the number of frame checks recommended before FAW occurs within the data region. We have seen that this can be changing state. done by checking an appropriate number of frames before entering or leaving the search state. A further sophistication is that of off-line To summarise, when in alignment, a good frame alignment pro- searching in which extra equipment commences a search as soon as cess should exhibit a low probability of losing alignment and, when, a misalignment condition is suspected. This takes place in parallel out of alignment, a high probability of fast recovery to the aligned with the normal receiver processing and offers the considerable condition. To achieve these objectives the following features are advantage that if the search proves to have been unnecessary no required: damage will have been done. It also benefits the true out of align- ment condition in that an early start will have been made on the 1. Reliable verification of the in alignment condition. searching process. 2. An efficient search procedure which ensures rapid location and The bit-by-bit searching technique can be relatively slow in estab- verification of the position of the FAW. lishing the aligned condition when there are random FAW imitation- 3. Rap