2101828-A is referenced by 11 patents.
In a complex electronic system, such as a telephone exchange, inter- rack cabling is a substantial part of the system's cost. To reduce this cabling, inter rack communication, at least in respect of information, uses beams of free space electro-magnetic radiation. In the present case infra-red beams are preferred because they are not very susceptible to external interference, and also do not cause much interference with other equipment. In some cases care is taken that the beams are directional, while in other cases such as the distribution of clock pulses some measure of non- directionality may be useful.