ternative link. relationship between the various performance parameters. Some Errors are monitored by adding redundancy to the information subtleties of the terminology are to be found in the full specification signal. This added redundancy may also be used for other purposes (CCITT, 1988a). (such as with line coding and frame alignment) or specifically for Total time Available time Unavailable time Error-free seconds Errored seconds Degraded Severely minutes errored seconds Figure 28.9 Relationship between error performance parameters Line codes 28/9 Low-pass channel Information to Line coder be transmitted Error control/ coding for security/ Modulator Band-pass channel multiplexing etc. Channel coding Figure 28.10 Relationship between line coding and other channel coding functions error detection (as with error control coding). These topics are dealt blocks of data for example, are monitored to a high degree of with in more detail in following sections, and are briefly introduced confidence. below. 28.3 Line codes 28.2.6.1 Line coding 28.3.1 Definition Codes such as HDB3, 7B8B and the Manchester code can provide a monitoring capability on a per link basis. These codes have well A line code defines the equivalence between sets of digits generated defined coding rules which can be verified at a regenerator or in a terminal and the corresponding sequence of symbol elements subsequent link terminating point. Once any requisite code word transmitted over a channel. It must be chosen to suit the charac- synchronism has been obtained, any violations of the coding rules teristics of the particular channel (Cattermole, 1983; Waters, 1983). can be attributed to transmission errors and SO the error performance For metallic cables which have an essentially low-pass frequency of the link can be estimated. characteristic it will usually be the last coding function performed before transmission. For band-pass channels such as optical fibres, radio and the analogue telephone network, where modulation is 28.2.6.2 Frame alignmen