he equipment verifying that the FAW occurs in the predicted position in each (approx. (10 bits) frame. If a misalignment condition is then detected, the mechanism 34Mbits/s) must transfer to the searching state and begin an inspection of all Fourth 111110100000 possible FAW positions. When the correct position has been found (approx. (12 bits) the search can be abandoned and the equipment allowed to revert to 140Mbits/s) the locked condition. In practice the alignment mechanism needs to be yet more com- plicated than that shown in Figure 28.20. This is because it is desirable to try and verify that the current state really is incorrect Ideally these features should be obtained with a minimum of before transferring to another state. For example, when the equip- added redundancy in the data stream (in the form of an FAW) and ment is in alignment and locked, transmission errors may occasion- with reasonably simple alignment equipment. ally mutilate the FAW in a given frame; it may be sensible to wait and check the next frame (or the next few frames) to minimise the 28.5.3 Searching techniques probability of setting off on an unnecessary search. Similarly, when in the searching state, a match to the FAW may be found which in fact is an imitation caused by the data bits. This condition is unlikely A simple bit-by-bit aligner is outlined in Figure 28.21. A test to be duplicated in the same position in successive frames (unless window, of length equal to the FAW, is shown to be correctly the FAW chosen is a bad one!) and SO again, a check on subsequent aligned with the incoming signal. That is, a comparator (or pattern frames may be sensible before reverting to the locked condition. matcher) indicates that there is agreement between the received Thus in practice, a series of check states are inserted between the FAW and a locally stored version. This causes a bit counter to count locked and searching states of Figure 28.20 to reduce the probability through the known frame length and SO locate and verify the posi- tion o