d. 4. Allocations are made for the implementation penalties of sub- systems. 5. The impact of environmental factors is assessed in terms of performance impairment. 6. The system operational margins are introduced. 7. On the basis of (3) to (6) the expected performance is estimated and compared to the required one from (1). 8. If the requirements are not satisfied then a design iteration begins, covering steps (2) to (7). The design therefore is in both principle and practice an iterative process and some of the parameters entering the iteration depend on Figure 53.30 Digital modulator for LEDs and lasers existing practices and understanding rather than scientific prin- ciples. This particularly applies to steps (4) and (6) above. A methodology of system design as summarised in steps (1) to (8) current through the constant current source at the tail of the switch. can be treated completely in the abstract in terms of system theory, In addition to the actual modulator an optical transmitter is re- but it will be far easier to describe system design by analysing a quired to maintain a constant output optical power. This is far more concrete case. Since the approach applies to both analogue and important for lasers which are very sensitive to temperature. A digital systems only digital system shall be discussed. However, Detector Laser Figure 53.31 Mean optical power controller Optical system design 53/23 before system design examples are discussed it is important to 30dBm. The various impairments and margins introduced reflect examine the question of coding in optical fibre systems. current practice and they are expected to change as better under- standing is achieved. The system power budget is summarised in 53.5.1 Coding in optical fibre systems Table 53.7. The coding of the transmitted information has been used in coaxial Table 53.7 System power budget systems to reduce the bandwidth and make it possible to increase the section length and as a result minimise the number of repeaters in a system. This was at the e