A virtual circuit (VC) is a logical path defined through a virtual circuit packet network. Unlike a circuit in a circuit-switched network, the virtual circuit does not dedicate bandwidth on each link through the network. It exists only in the logic of the packet switches and it consumes bandwidth on the facilities between the packet switches only when there is a packet in transit. Virtual circuits are identified by a number, which the transmitter is required to place in any packet it sends. The network nodes use this circuit number to guide the packet through the network, along the predefined path, to its destination. Examples of technologies that implement virtual circuits include frame relay and asynchronous transfer mode (ATM).
ATM provides a virtual circuit service, supporting both switched virtual circuits (SVC) and permanent virtual circuits (PVC). A third type of service offered today as an enhanced PVC service is the switched PVC (SPVC). Virtual circuits can be set up once, when the network is provisioned, or can be requested by end-devices as needed. The former are PVCs and the latter are called SVCs.
PVCs are full-period circuits, established at the switch by an administrator. Once created, these connections are ready for traffic. Only a failure or administrative intervention stops these connections.
Although SVCs (on-demand connections) have come to market slowly, users are beginning to demand SVC service because of PVC inflexibility. SVCs are set up and terminated using signaling messages defined by ITU-T recommendation Q.2931. When an SVC is set up, the virtual channel connection (VCC) or virtual path connection (VPC) attributes are negotiated, along with the destination address.
SPVCs offer the reliability of a PVC and ease of use of an SVC. The “S” portion of the PVC is between switches in the ATM network. If a switch-to-switch link fails, the switches reroute and reconnect the PVC automatically. SPVCs are also called soft PVCs, “soft” referring to the switched portion of the connection in contrast to the hard-coded (manual) PVC.
Since these virtual circuits can be established as VCCs or VPCs, the acronyms can be confusing; for example, “I just installed a PVC VPC.”
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