ta rates in the region of tens of Tbits 10 but only over short distances and capacity reduces with distance, a relationship generically described as optical Moore's law. 11 Impairments in long-haul networks include noise, attenuation, nonlinear effects and disper- sion including polarisation mode dispersion (PMD). These impairments are less of a problem in local access networks though PMD becomes a dominant constraint at data rates over and above 10 Gbit/s. 10 A terabit is equivalent to 1000 gigabits. Optical Moore's law can also be applied to optical storage where storage versus density versus cost metrics apply rather than just capacity versus distance. This topic is revisited in later chapters. Making Telecoms Work Receiver noise is a product of the thermal noise introduced when the photocurrent generated by received photons is amplified. Noise is introduced due to the random motion of electrons. The photodetection process, the generation of a photocurrent in response to incident photons, also generates random electrons and therefore noise. Attenuation reduces the optical signal-to-noise ratio, which in turn has an impact on the bit error rate, the proportion of incorrect (error) bits received. Optical transmission systems have a target to achieve less than one error bit for every trillion bits received (1 in 10 12). Fixed wireless systems by comparison have a target to achieve 1 in 101 and mobile broadband networks 1 in 103, for reasons that will be examined later. A decrease in power can be offset by using optical amplifiers, but these also introduce noise. Multiplexers additionally introduce insertion loss. The ultimate capacity of an optical fibre system can be calculated using Shannon's law and expressed as information spectral density (ISD). ISD in the optical domain can be shown to be of the order of 4 to 6 bits per Hz. However, fibre is a nonlinear transmission medium, which means that as the signal power increases the number of bits per Hz increases to a maximum value and then starts decreasing. Nonli