What is the X in X-mas?
A system or facility for transfering data between persons and equipment. The system usually consists of a collection of individual communication networks, transmission systems, relay stations, tributary stations and terminal equipment capable of interconnection and interoperation so as to form an integrated whole. These individual components must serve a common purpose, be technically compatible, employ common procedures, respond to some form of control and generally operate in unison.
["Communications Standard Dictionary", 2nd Edition, Martin H. Weik].