One reason for Frame Relay's popularity is its capability to logically create multiple connection-oriented data link layer communication paths between two devices across a single physical interface. These VCs provide you with a bidirectional communications path that can exist between a single pair of equipment, commonly called a point-to-point connection, or between multiple pairs of equipment, also known as a partial or full mesh. Each of these VCs is identified by a unique data link connection identifier (DLCI) that differentiates the communications between different devices.

VCs can be mapped across any service provider's Frame Relay network without regard for the number of hops the connection will cross. A VC is not limited to three devices, two DTEs, and a DCE when traveling from source to destination. Just remember that each hop adds to your circuit's overall delay because of the processing that each device needs to do to read the packet and send the packet toward its destination.

VCs can be divided into two separate categorizesswitched virtual circuits (SVCs) or permanent virtual circuits (PVCs).

SVCs

The SVC gives you a way to automatically create temporary connections between DTE devices in the Frame Relay network that can be used in on-demand situations, such as those requiring only sporadic data transfer. An SVC's communication component consists of the following four operational states:

• Call setup Indicates that the establishment of the SVC between two Frame Relay DTE devices is currently being negotiated.

• Data transfer Indicates that data is being transmitted between DTE devices over an SVC.

• Idle Indicates that the SVC between DTE devices is still active, but no data is currently being transferred.

• Call termination Indicates that the SVC between DTE devices is being terminated.

After the termination of an SVC is complete, if additional data needs to be transmitted between the DTE devices, a new SVC is established. Cisco devices use the same signaling protocols used by ISDN to establish, maintain, and terminate SVCs.

PVCs

The PVC, unlike the SVC, establishes a permanent connection between your DTE devices. This type of circuit is typically used for frequent and consistent data transfers across the Frame Relay network. Because PVC establishment does not require call setup or termination, it is always up. PVCs have only two operational states:

• Data transfer Indicates that data is currently being transmitted between the DTE devices over the PVC.

• Idle Indicates that the connection between DTE devices is active, but no data is currently being transferred between DTE devices.

As soon as the PVC is established, your DTE devices may transfer data whenever they are ready, without the delay associated with the establishment of an SVC.