elivery systems. Fibre began as single wavelength then progressed to multiple wavelengths and more recently has progressed to multimode, three generations of optical technology evolution. Single mode is used for backhaul and can support data rates of 40 g/bits or above. It has a loss of about 0.37 dB per km at 1310 nanometres (229 THz). It is expensive when compared with multimode fibre and requires expensive connectors and exotic laser diodes. Multiplexing in the frequency and time domain, for example means that a single optical fibre pair can support 160 channels at 10 Gbit/s or 40 Gbit/s over distances of 4000 km. As stated earlier these long-distance links are cost efficient because a high level of traffic aggregation can be achieved. Multimode is used in final-mile applications. In a multimode fibre, if the angle of the light entering the fibre is changed then that light 'channel' will follow a certain reflective path. This means it will have travelled a different distance to other light channels introduced at a different angle and will therefore arrive at a different time at the receiver. This is the basis for time division multiplexing and allows for up to 64 'channels' to be supported. Multimode fibre is relatively low cost when compared to single-mode fibre and does not require expensive high-precision connectors. However, it has a relatively high propagation loss, about 0.8 dB per km at 1310 nanometres (229 THz) and is relatively low bandwidth when compared with single mode. Newer multimode fibre also supports shorter-wavelength transmission at 1510 nanometres (199 THz). Attenuation losses reduce to about 0.23 dB per km, but chromatic dispersion is worse. Multimode fibre is used extensively in passive optical networks. The ITU-T G983 specifi- cation defines the downstream at 1490 nm (213 THz) at 622 Mbits/s, a 155 Mbits upstream at 1310 nanometres (229 THz) and an RF video overlay at 1550 nanometres (193 THz). 17 17 http://www.precisionphotonics.com/converter.asp Frequency is equal to the speed of light