t required before transmission, the line coding function will either immediately precede the modulator or be incorporated within the modulation process. The relationship between line coding and other Here redundancy is added for synchronisation purposes. Once cor- channel coding functions is shown in Figure 28.10. From this we see rect alignment is assured, any variation from the known frame that the line coder may have to operate at the transmitted symbol alignment word can again be attributed to transmission errors. As rate, in which case a reasonably simple design will usually be with line codes, care has to be taken to distinguish between word essential, especially for high speed systems. misalignment conditions and transmission errors. The decision al- gorithms are often quite complicated but nevertheless can give a 28.3.2 reasonable estimate of transmission error performance. Frame Purpose of a line code alignment words usually remain associated with the transmitted information over more than a single link and SO can offer the A line code provides the transmitted symbol sequence with the opportunity for error monitoring over a greater portion of a connec- necessary properties to ensure reliable transmission and subsequent tion. detection at the receiving terminal. In order to achieve this the following have to be considered: 1. Spectrum at low frequency. Most transmission systems are 28.2.6.3 Error control coding easier to design if a.c. coupling is used between stages. This means that decoding must not rely on receiving a d.c. compo- It may not always be viable to provide an end-to-end level of error nent. Furthermore, the coupling components (capacitors or performance which is compatible with the most demanding digital transformers) will introduce a low frequency cut-off and this service likely to be supported. In such circumstances error control will put a limit on the permissible low frequency content of the coding (detection and then correction) over certain problem links or line code if long term inte