id detection of a genuine out of alignment condition caused S within the data region. This arises because each time an imitation by bit slips. is found the search will stop and a check made for an FAW one 4. Robust performance in the presence of FAW limitations and frame later. On failing to find the requisite pattern, the search will transmission errors. resume. This temporary halt in the searching process will occur 28/18 Digital transmission Data bits window Enable Control test window Comparator Bit counter Stored version of FAW Figure 28.21 Basic configuration for a frame aligner every time an FAW imitation is found. A faster search can be 28.5.4 Choice of frame alignment word achieved by using more memory. For example, the location of sequences which match the FAW can be simply recorded and the Some recommended patterns have been given in Table 28.4. By way search allowed to continue. Those which do not repeat at one-frame of example, Figure 28.22 takes the first pattern in the Table (for intervals are ignored. This process continues until the location of the 30-channel pcm) and plots the mean number of bit matches from the true FAW becomes apparent. The aligner can then be made to jump alignment comparator for different window positions. When the test directly to this new position. Thus, there is a trade-off; faster window is in the correct position (and there are no transmission searches are possible but at the expense of equipment complexity. errors) the comparator will register seven agreements. If the posi- 001101 X = Data (assumed random) Comparator For each bit: agreement = 1 disagreement = 0 0011011 (Stored version) Mean number of agreements Window in random data region -7 -6 -5 -4 -3 -2 -1 0 1 2 3 4 5 6 7 Window position error Figure 28.22 Mean number of bit agreements for different window positions References 28/19 tion of the window is in error by 1 bit (in either direction), then it Bylanski, P. and Ingram, D.G.W. (1987) Digital Transmission Sys- can be seen that three agreements and three disagr