In this environment the data transfer rates to individual users are likely to be limited and can be met with spectrum that is available or is likely to be so. In a building where both highly mobile and fixed terminals may be present the situation is less clear. Mobile connections are available using the traditional cellular networks. The radio wireless LANs available at present use unregulated 'free' spectrum regions. At lower data rates RF is excellent at providing coverage, due to the scattering and the diffraction of the radio waves, and the sensitivity of the receivers that can be constructed.
Channels are robust to being blocked by obstacles and coverage can be achieved between rooms.
Higher data rates require higher frequencies where spectrum is available. At these frequencies the radio signal propagation becomes line of sight, and problems become similar to that using light. Optical LANs uses two approaches, outlined in Figure 1. Diffuse networks use wide angle sources and scatter from surfaces in the room to provide an optical 'ether' similar to that which would be obtained using a local radio transmitter. This produces coverage that is robust to blocking, but the multiple paths between source and receiver cause dispersion of the channel, thus limiting its bandwidth.