In telecommunications and computer networks, a virtual circuit (VC), synonymous with virtual connection and virtual channel, is a connection oriented communication service that is delivered by means of packet mode communication. After a connection or virtual circuit is established between two nodes or application processes, a bit stream or byte stream may be delivered between the nodes; a virtual circuit protocol allows higher level protocols to avoid dealing with the division of data into segments, packets, or frames.
Virtual circuit communication resembles circuit switching, since both are connection oriented, meaning that in both cases data is delivered in correct order, and signalling overhead is required during a connection establishment phase. However, circuit switching provides constant bit rate and latency, while these may vary in a virtual circuit service because of reasons such as:
Many virtual circuit protocols, but not all, provide reliable communication service, by means of data retransmissions because of error detection and automatic repeat request (ARQ).
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Connection oriented transport layer datalink protocols such as TCP[1][2] may rely on a connectionless packet switching network layer protocol such as IP, where different packets may be routed over different paths, and thus be delivered out of order. However, a virtual circuit[2][3][4] is possible since TCP includes segment numbering and reordering on the receiver side to prevent out-of-order delivery.
Network layer and datalink layer virtual circuit protocols are based on connection oriented packet switching, meaning that data is always delivered along the same network path, i.e. through the same nodes. Advantages with this over connectionless packet switching are:
Examples of transport layer protocols that provide a virtual circuit:
Examples of network layer and datalink layer virtual circuit protocols, where data always is delivered over the same path:
Switched virtual circuits (SVCs) are generally set up on a per-call basis and are disconnected when the call is terminated; however, a permanent virtual circuit (PVC) can be established as an option to provide a dedicated circuit link between two facilities. PVC configuration is usually preconfigured by the service provider. Unlike SVCs, PVC are usually very seldom broken/disconnected.
A switched virtual circuit (SVC) is a virtual circuit that is dynamically established on demand and is torn down when transmission is complete, for example after a phone call or a file download. SVCs are used in situations where data transmission is sporadic and/or not always between the same data terminal equipment (DTE) endpoints.
A permanent virtual circuit (PVC) is a virtual circuit established for repeated/continuous use between the same DTE. In a PVC, the long-term association is identical to the data transfer phase of a virtual call. Permanent virtual circuits eliminate the need for repeated call set-up and clearing.