Virtual circuit

A virtual circuit (VC) is a means of transporting data over a packet switched computer network in such a way that it appears as though there is a dedicated physical layer link between the source and destination end systems of this data. The term virtual circuit is synonymous with virtual connection and virtual channel. Before a connection or virtual circuit may be used, it has to be established, between two or more nodes or software applications, by configuring the relevant parts of the interconnecting network. After that, a bit stream or byte stream may be delivered between the nodes; hence, 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 a constant bit rate and latency, while these may vary in a virtual circuit service due to factors such as:

Many virtual circuit protocols, but not all, provide reliable communication service through the use of data retransmissions because of error detection and automatic repeat request (ARQ).

An alternate network configuration to virtual circuit is datagram.[1]

Layer 4 virtual circuits

Connection oriented transport layer datalink protocols such as TCP[2][3] 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, it is possible to use TCP as a virtual circuit,[3][4][5] since TCP includes segment numbering that allows reordering on the receiver side to accommodate out-of-order delivery.

Layer 2/3 virtual circuits

Datalink layer and network 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 protocols that provide virtual circuits

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:

Permanent and switched virtual circuits in ATM, frame relay, and X.25

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.

See also


  1. Andrew S. Tanenbaum, David J. Wetherall (2011, Fifth Edition. International Edition), "Computer Networks". page 361 ISBN 978-0-13-255317-9
  2. RFC 793
  3. 1 2 RFC 1180
  4. RFC 955
  5. RFC 1644
  6. ITU-T, B-ISDN ATM Adaptation Layer specification: Type 3/4 AAL, Recommendation I.363.3 (08/96), International Telecommunication Union, 1996, p5.
  7. ITU-T, B-ISDN ATM Adaptation Layer specification: Type 5 AAL, Recommendation I.363.5 (08/96), International Telecommunication Union, 1996, p5.
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