Emulation a dedicated physical link over a packet-switched network
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A virtual circuit (VC) is a means of transporting data over a data network, based on packet switching and in which a connection is first established across the network between two endpoints. The network, rather than having a fixed data rate reservation per connection as in circuit switching, takes advantage of the statistical multiplexing on its transmission links, an intrinsic feature of packet switching.
A 1978 standardization of virtual circuits by the CCITT imposes per-connection flow controls at all user-to-network and network-to-network interfaces. This permits participation in congestion control and reduces the likelihood of packet loss in a heavily loaded network.[1][2] Some circuit protocols provide reliable communication service through the use of data retransmissions invoked by error detection and automatic repeat request (ARQ).
Before a virtual circuit may be used, it must be established between network nodes in the call setup phase. Once established, a bit stream or byte stream may be exchanged between the nodes, providing abstraction from low-level division into protocol data units, and enabling higher-level protocols to operate transparently.
An alternative to virtual-circuit networks are datagram networks.
^"X.25 - Interface between Data Terminal Equipment (DTE) and Data Circuit-terminating Equipment (DCE) for terminals operating in the packet mode and connected to public data networks by dedicated circuit". ITU-T. October 1976.
^Rybczynski, A; Wessler, B; Després, R; Wedlake, J (1976-06-07). "A new communication protocol for accessing data networks: The international packet-mode interface". In AFIPS (ed.). Proceedings of the June 7-10, 1976, national computer conference and exposition on - AFIPS '76. p. 477. doi:10.1145/1499799.1499869. S2CID 8790311.
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