Virtual circuit
Virtual circuits are logical connections established between two devices. Depending on how they are established, virtual circuits include the following types:
Permanent virtual circuit (PVC)—A PVC is manually configured or dynamically learned through the LMI negotiation. The availability of a PVC must be detected before the PVC can be used.
Switched virtual circuit (SVC)—An SVC is dynamically established between two devices through calls. The network provides data transmission services on established SVCs. The terminal users can terminate an SVC through clearing the call.
Unlike SVCs, PVCs rarely break or disconnect. PVCs are used more than SVCs.
On a DTE device, the state of a PVC is determined by the DCE device. On a DCE device, the state of a PVC depends on the way that the DCE device is connected to the DTE device.
When the DCE device is directly connected to the DTE device, they use LMI to negotiate the state of a PVC.
When the DCE device is connected to the DTE device through a Frame Relay network, the state of a PVC is determined by the LMI negotiation result and the virtual circuit state in the Frame Relay network.