l distribution (as opposed to long-haul and core trans- mission, which is also implicitly point-to-point). As stated earlier, WDM systems are usually deployed in single-mode fibre and come in three densities, coarse, conventional and dense. A course system typically provides 8 channels on a single-mode fibre across the entire frequency band between 1310 and 1550 nm, which contains a minimum dispersion window and a minimum attenuation window. The advantage is that transceiver designs are relatively inexpensive. A conventional system provides eight channels in the third transmission window at 1550 nm. A dense system packs more channels into the same bandwidth, for example 40 channels at 100 GHz spacing, 80 channels with 50 GHz spacing or 100 channels at 25 GHZ spacing (ultradense wavelength division multiplexing). In the 1990s increases in data rates were achieved by adopting techniques that mitigated impairments such as dispersion (dispersion compensation) and by implementing forward er- ror detection (covered in Chapter 3). Further increases in bit rates in local access optical networks are therefore dependent on the development of low-cost high-performance temperature-independent optical components, photonic integrated circuits and DSP processing capability, for example for channel equalisation. Delivery cost economics are not, however, just a function of data throughput but are de- termined by how efficiently that throughput can be used. Core transmission and long-haul transmission systems support large numbers of simultaneous users and can achieve substantial multiplexing gain. 15 In a local network only a small number of users or a single user may be supported and the dominant cost becomes the transceiver equipment, fibre material cost and installation cost. So for example shorter distances and lower data rates (<10 Gbit/s) mean lower-cost Fabry-Perot10 light sources can be used rather than DFB lasers. Fabry-Perot lasers are less temperature sensitive making them more suitable for use in customer premises equ