lished in the Bell System Technical Journal should be consulted (in particular the papers by Bennett and the one by Rowe, all in Vol 37, 1958 and also the paper by Manley in Vol 48, 1969). On jitter in digital networks, (Kearsey and McLintock, Searching 1984) is a helpful paper which relates closely to the CCITT recom- for alignment alignment mendations. Alignment lost Frame alignment 28.5.1 The task Figure 28.19 The alignment task In addition to clock extraction, most signal formats used for digital transmission will entail some further structure which must be re- The frame alignment word may, in most systems, be tempo- liably extracted at the receiving terminal. In particular, when several rarily imitated in random positions within the data region. information streams are combined before transmission using time 2. The frame alignment word may be mutilated by transmission division multiplexing, a frame structure will be involved. The boun- errors. daries between frames are usually marked by inserting a carefully In some systems bit slips can occur, that is, frames may tempo- selected frame alignment word (also called a marker, flag or frame rarily lose or acquire erroneous bits. sync pattern) which the receiver has to locate before demultiplexing can be performed (Figure 28.18). To deal with these problems the 2-state model of Figure 28.19 has The reliable frame alignment of such multiplexed signals is essen- to be extended to at least the 3-state model of Figure 28.20. This tial for the proper functioning of many digital communication model distinguishes between two locked (not searching) states; the systems. A loss of alignment will result in the failure to correctly desirable in alignment and locked state and the highly undesirable identify the received bits and SO will cause the disorientation of the out of alignment but locked state. In this latter state a misalignment demultiplexing process. This leads to a catastrophic loss of both condition has occurred but the system has not yet detected it. The message