rsymbol interference is to be avoided end-to-end over the whole connection may be used. This is the most (Cattermole, 1983). direct method of error performance monitoring as the redundancy 2. Transmission bandwidth required. If transmission bandwidth is will usually have been specifically added, and so optimised, for the at a premium, a multi-level line code, in which each transmitted detection of errors. Of course certain error combinations may go symbol represents more than one bit of information, will enable undetected or only partially detected and this can lead to erroneous the transmitted symbol rate to be reduced. However, for a given action in a subsequent error corrector. However, this problem can bit error ratio, a multi-level line code will require a better usually be reduced to an acceptable level by using coding which is signal-to-noise ratio than a corresponding binary code. appropriate to the error characteristics of the particular transmission 3. Timing content. There must always be sufficient embedded link. It then becomes possible to ensure that transmission errors in timing information in the transmitted symbol sequence so that 28/10 Digital transmission the distant receiver (and intermediate repeaters if used) can symbol for transmission, as shown in Figure 28.11(a). No d.c. extract a reliable clock to time their decision making processes. component is generated because the polarity and amplitude of the This usually means ensuring that the line code provides an first half of each transmitted symbol is complemented by the second adequate density of transitions in the transmitted sequence. half. Also, the resulting signal transition in the centre of each 4. Error monitoring. By adding redundancy into the information symbol ensures an adequate clock content. The absence of such a stream a line code can provide a means of in-service monitoring transition indicates a transmission error and SO error monitoring is of the error rate of a transmission link. For example, a line coder also possible. Howe