xpense of increased terminal complex- ity, but overall it yielded cost effective systems. With the advent of Transmitter output (dBm) fibre communications the issue of bandwidth lost its importance Receiver sensitivity (dBm) -30.0 because the bandwidth of single mode fibre is very large. For this Available section loss capability (dBm) reason, in the initial stages of the introduction of fibre communica- tions, coding was used to ensure that the transmitted signal had Transmitter penalties sufficient clock content so that low Q clock extraction circuits could Implementation penalties (dB) be used. In principle of course that was not necessary. The requirement for a code to perform a function of primary Temperature effects (dB) importance emerged with the use of integrating optical receivers. Ageing effects (dB) Since the dynamic range of an integrating receiver is limited there Total transmitter penalty (dB) is the need for codes with small digital sum variation (DSV). Such codes turn out to be invariably block codes. In a block code a Receiver penalties number of information digits, say N, are mapped through determin- Implementation (dB) istic rules into M output digits with M > N. This will lead to a slight increase of the bit rate in the fibre and in error extension, but the Temperature effects (dB) small penalty in receiver sensitivity is worth paying. Block codes Digital regenerator (dB) such as 5B6B and 7B8B have been used in optical systems up to 565MBit/s. Ageing (dB) As the information capacity increased it became progressively Total receiver penalty (dB) more difficult to design such coding schemes because of the limita- System margins tions in the speed of the electronics. In addition the requirements for information transparency forced the design of optical receivers Operating margin (dB) capable of handling signal formats with very high DSV. These Connectors (pair) (dB) receivers offer sufficient sensitivity to make possible cost effective systems even if their sensitivity is not as good as that of t