In: Categories » Electronics and communication » Network security » Frame Relay Virtual Circuits
| 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). SVCsThe 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:
NOTE When an SVC remains in an idle state for a defined period of time, the SVC can be torn down and the call 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. PVCsThe 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:
NOTE Because a PVC connection has no call setup or termination, it is not terminated under any circumstances when in an idle state, unlike the SVC idle state. SVCs encounter startup delays after an idle period. 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.
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