rates. Polarisation mode dispersion (PMD) happens because light waves are made up of two orthogonal components, one travelling along the X plane and one along the Y plane. Im- perfections in the fibre will mean that these two components will have a different refractive index, which means they will travel at different speeds. PMD also varies with temperature SO Physical Layer Connectivity compensation can be quite complex and expensive and for that reason is only usually deployed in long-haul systems. Mitigation of all these impairments over time increases the distance product and indirectly the capacity product, which is also a function of the bit error rate or to use an internet term, 'good put'. Capacity product increases are also due to improved fibre and to the use of laser sources that can generate a range of optical frequencies that are chosen on the basis of their propagation loss and dispersion characteristics, gallium arsenide laser sources for example working at wavelengths around 800 nanometres¹ 12 (374 THz), indium gallium arsenide laser sources at 1300 nanometres (230 THz) and indium gallium arsenide phosphide laser sources at 1500 nanometres (199 THz). These are either laser diodes or lower-cost LED (light-emitting diodes). Light emitting diodes are electroluminescent and emit incoherent light with a relatively wide spectral band- width of 30 to 60 nanometres. The LED devices are relatively inefficient typically only converting about 1% of their input power to launched power but similarly priced devices known as vertical cavity surface emitting lasers have better power and spectral properties. Both couple effectively into multimode fibre. Multimode fibre has a large light-carrying core of between 50 and 100 micrometres, com- monly 62.5 micrometres in the US. The wider spectral bandwidth makes the optical signal more susceptible to dispersion as the light is dispersed into multiple paths as it travels along the path causing pulses to overlap over longer distances. Multimode is normally used at 850 or