Space division multiplexing
Space division multiplexing (SDM) is nothing more than the provision of multiple fixed bandwidth channels by multiple physical paths (i.e., pairs of wires or optical fibers). A good example of SDM is the use of a 25-pair cable to carry the conversations of 25 individual users from the customer's premises to the local telephone company’s central office location.
SDM is not the most efficient technique from the standpoint of outside plant resources, but it does play a role in all carrier networks. A given copper or fiber facility has a finite capacity for information. When that capacity is exhausted, SDM is the only alternative.
By some arguments, SDM is not a multiplexing scheme at all, since it does not support multiple communication channels on a single medium. However, the concept is an important one because it occurs both in the deployment of transmission facilities as well as the internal architecture of some switches.