 1300 nanometres and works well for distances of two miles or less and can work adequately at lower data rates over ten miles or more. It is best suited to local area network applications with bit rates of 10 to 100 Mbit/s Single-mode fibre is normally used at 1310 or 1550 nanometres and has a light-carrying core of 8 to 10 micrometres that largely eliminates the dispersion limitations of multimode, thus providing a significantly better capacity distance product. However, a narrow spectral width laser is needed that is typically two to four times the cost of a multimode light source. For radio communication systems capacity generally increases with frequency but dis- tance reduces or looking at it the other way round attenuation at longer wavelengths/lower frequencies is lower but SO is capacity. So, for instance, the distance product for long wave radio is high, the capacity product is low. In the optical domain a similar relationship exists in that longer wavelengths/lower fre- quencies exhibit lower propagation loss but also there are particular windows with minimum dispersion and minimum attenuation characteristics that determine the optical band plan as shown in Table 2.2. The windows are determined by two phenomena, Rayleigh scattering, which reduces as wavelength reduces and hydroxide anion absorption, also referred to as the water absorption peak, which occurs at 945 nm, 1240 nm and 1380 nm. Avoiding these peaks produces a window at 800 to 900 nm that has fairly high loss and is only ever used for short-distance transmission (usually with LEDs for cost reasons - see above). The second and third telecom windows are at 1300 nm and 1500 nm, but are subdivided as follows. A nanometre is a billionth of a metre. Fingernails grow a nanometre every second or 86400 nanometres in a day htp://www.sciencelearn.org.nz/Contexts/Nanoscience/Science-Ideas-and-Concepts/Nanometres-and-nanoscale. 13 Light introduced into a fibre has to be guided by the core and has to be launched at a critical angle in order for transmissio