Those computer applications which involve rapid response to events at distant points create special problems in digital communication. Such applications are increasing in number, and could increase more rapidly if better solutions existed to the communication problems. The present-day methods for communication of data in rapid-response systems employ 'private wires' for the transmission paths or, where the available data rate and reliability is sufficient, employ voice channels from the switched telephone network. Given these rather arbitrary transmission facilities the user adds the terminal equipment necessary to make a communication system and sometimes integrates a number of paths into a private network.