he Cable repairs (dB) integrating receivers. Taking into consideration all these factors, Dispersion penalty (dB) state of the art fibre systems used codes of very low redundancy for data management purposes. These low redundancy data sequences Total system margin (dB) are scrambled to minimise the requirements on the clock extraction Section loss capability (dB) subsystem in the receiver. Block codes are not error correction Section loss (dB) codes and in principle error correction can be used in optical systems especially where an error floor is introduced. Again, the Additional system margin (dB) complexity and speed requirements of the electronics are such that error correction at the fibre bit rate is hardly used. Error correction is used at the low capacity tributary channels where, due to the The additional system margin can be used in a variety of ways. relative low speed, high levels of integration are possible. For example if the dispersion penalty increases to 3dB the disper- sion limited section length increases to 19.2km. This represents an 53.5.2 Design of multimode systems additional total loss of 3.65dB reducing the margin to 5.25dB. This example indicates the limitations of multimode transmission Consider as the design target the maximisation of the section length for high bit rates and low loss fibre. Had the dispersion been of a digital system operating at 140Mbit/s and using a directly negligible the section loss capability would have been equivalent to modulated laser with (OdB) output power and a spectral width 46.6km and this big difference is the fundamental reason for the under modulation of 3nm. introduction of single mode transmission. Since we want to maximise the length the obvious choice of wavelength is the 1550nm window. At 1550nm the fibre loss is 53.5.3 Design of single mode systems around 0.3dB/km with an intermodal dispersion of 120ps/km and a intramodal dispersion of 17ps/nm.km. With a 3nm spectral width As a design example for a single mode system consider the problem the tot