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Virtual circuit
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<p>A virtual circuit (VC) is a means of transporting data over a data network, based on <a href="/facts/Packet_switching/olDlDP1R">packet switching</a> 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 <a href="/facts/Circuit_switching/fJ3vdmAB">circuit switching</a>, takes advantage of the <a href="/facts/Statistical_multiplexing/rLGcaGHV">statistical multiplexing</a> on its transmission links, an intrinsic feature of packet switching. </p><p>A 1978 standardization of virtual circuits by the <a href="/facts/CCITT/9nJwkPYx">CCITT</a> imposes per-connection <a href="/facts/Flow_control_(data)/qcdKcLon">flow controls</a> at all user-to-network and network-to-network interfaces. This permits participation in <a href="/facts/Congestion_control/9ohkRMso">congestion control</a> and reduces the likelihood of <a href="/facts/Packet_loss/6fuIcFHk">packet loss</a> in a heavily loaded network. Some circuit protocols provide <a href="/facts/Reliability_(computer_networking)/v3kGvfvs">reliable communication service</a> through the use of data retransmissions invoked by <a href="/facts/Error_detection/o2rH8e3n">error detection</a> and <a href="/facts/Automatic_repeat_request/1bR2urmF">automatic repeat request</a> (ARQ). </p><p>Before a virtual circuit may be used, it must be established between <a href="/facts/Network_node/LBp2vNcJ">network nodes</a> in the <a href="/facts/Call_setup/CDULq334">call setup</a> phase. Once established, a <a href="/facts/Bit_stream/Pf9oGYyY">bit stream</a> or <a href="/facts/Byte_stream/Pf9oGYyY">byte stream</a> may be exchanged between the nodes, providing abstraction from low-level division into <a href="/facts/Protocol_data_units/UJMVQEt9">protocol data units</a>, and enabling higher-level protocols to operate transparently. </p><p>An alternative to virtual-circuit networks are <a href="/facts/Datagram/TMHVctMz">datagram</a> networks. </p>