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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: * 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. 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.〔Andrew S. Tanenbaum, David J. Wetherall (2011, Fifth Edition. International Edition), "Computer Networks". page 361 ISBN 978-0-13-255317-9〕 ==Layer 4 virtual circuits== Connection oriented transport layer datalink protocols such as TCP〔RFC 793〕〔RFC 1180〕 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,〔〔RFC 955〕〔RFC 1644〕 since TCP includes segment numbering that allows reordering on the receiver side to accommodate out-of-order delivery. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「virtual circuit」の詳細全文を読む スポンサード リンク
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