A communication system is presented which consists of a switch (Datakit [Fraser 1979]) with associated control and interface hardware and software. The switch offers virtual circuit service which is internally implemented through packet switching. The users of the communication system are operating systems; using our communication system, we have implemented a network of UNIXTM systems and a new distributed operating system derived from UNIX. Our work concentrated on the performance of the subscriber-switch interface; on the reliability and recoverability of the switching service; and on the exploitation of the circuit concept for operating system design.