The software architecture in this paper has evolved through a sequence of research projects at AT&T Bell Laboratories. We review the architecture and describe an experimental reimplementation. Calls from each device attached to a virtual-circuit switch are managed by a software process, called a line process; the line process translates call requests from the device into a uniform device-independent internal protocol. Host computers and trunks can have numerous lines multiplexed over a single physical link. This process-per-line architecture leads to a profusion of specialized processes, most of them idle, with very simple contexts. Previous implementations used machinedependent code to manage tile processes. The experimental implementation does basic call processing and was completed largely by three people in three months. It is portable. The architecture was reused, but the code was not. The relatively small effort supports the belief that the architecture is suited to call processing and dispels the myth that the complexity of control software prevents limited experiments.