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Datapac is the TransCanada Telephone System's public packet-switched network, based on the Northern Telecom SL-10 Packet Switching System. The primary packet-switched communications facility offered by the SL-10 in Datapac is a virtual circuit service for which several types of customer interfaces are available. The virtual circuit service relies on a datagram facility for basic internodal communications. This paper characterizes the virtual circuit and datagram communications layer of Datapac and presents the routing, flow, and congestion control techniques currently in place for these two layers. Also, the additional measures used to control international virtual circuits are discussed.