Modern business requires effective communication facilities. The communication requirements of a small business may be met by a single telephone, and probably a fax machine, or a few telephones served by a key system. A larger business requires a private automatic branch exchange (PABX) to serve a larger number of extension lines and provide connections between them and the public switched telephone network (PSTN). A business which operates on several sites will have a PABX at each. If extension users generate sufficient traffic between these PABXs, it may be economic to join the PABXs by a private network, rather than route traffic between them over the public network. Organisations also have a need for data communication which may justify a private data network.