 with the best possible accuracy. In practice, the presence of noise and other propagation impairments cause errors to occur in the received datastream and an aim of systems design is to ensure that the number of errors is kept to an acceptable level and at an acceptable cost. The factors that affect the bit error rate are therefore discussed in this section. The Receiver In this section we assume the use of "direct detection," in which there is no retention of phase information in the detected optical signal. In this case, the role of an optical receiver is to convert a photonic datastream into an electric current and interpret it to decide whether the individual bits are logical 1s or Os. The detector in the front end of the receiver is a semiconductor diode and, for operation in the 1250- to 1650-nm range, is normally based on the InGaAs system. The two main types are p-i-n ("positive-intrinsic-negative" layers) (Taguchi, 2002) and Avalanche Photodiodes (APDs) (Kobayashi and Mikawa, 2002). The p-i-n detector is normally lower cost and relatively low noise, but the APD offers an internal current gain, Mapd, through the process of impact ionization, leading to greater sensitivities. Unfortunately, the APD imposes an additional noise term. Optical receivers also include electronic amplifiers and filters, together with clock recovery and decision cir- cuits. The clock recovery unit (Ellis, Smith, and Patrick, 1993; Philips et al., 1998) isolates a frequency component at f = B, the optical bitrate, from the received signal in order to syn- chronize the decision process. When using direct-detection optical systems the transmitted pulse stream is converted into a time-varying electric current, which is proportional to the incident optical power, Pinc, via the relation Ip(t) = RPinc(t). R is the detector's "responsivity," given by R = nq/hvs, in which q is the charge of an electron and n is the (semiconductor device-dependent) photon- to-electron conversion efficiency, which can often be close to unity. Typical values