Virtual circuit (VC)
A virtual circuit (VC) is a means of transporting data over a packet-switched network in such a way that it appears as though there is a dedicated physical link between the source and destination end systems of this data. The term virtual circuit is synonymous with virtual connection.
Before a connection or virtual circuit may be used, it must be established between two or more nodes or software applications by means of call setup. 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 Protocol data units.
Many virtual circuit protocols, but not all, provide reliable communication service through the use of data retransmissions invoked by error detection and automatic repeat request (ARQ).
Comparison with circuit switching
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:
- varying packet queue lengths in the network nodes,
- varying bit rate generated by the application,
- varying load from other users sharing the same network resources by means of statistical multiplexing, etc.
Virtual call capability
In telecommunication, a virtual call capability, sometimes called a virtual call facility, is a service feature in which:
- a call set-up procedure and a call disengagement procedure determine the period of communication between two DTEs in which user data are transferred by a packet switched network
- end-to-end transfer control of packets within the network is required
- data may be delivered to the network by the call originator before the call access phase is completed, but the data are not delivered to the call receiver if the call attempt is unsuccessful
- the network delivers all the user data to the call receiver in the same sequence in which the data are received by the network
- multi-access DTEs may have several virtual calls in progress at the same time.
- An alternative network configuration to virtual calls is connection less communication using datagrams.
Layer 4 virtual circuits
Connection oriented transport layer protocols such as TCP 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, 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.
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