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Virtual Circuits

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أستاذ المادة وسام سمير عبد علي بهيه       29/01/2016 10:28:14
Virtual Circuits

The connection through a Frame Relay network between two DTEs is called a virtual circuit (VC). The circuits are virtual because there is no direct electrical connection from end to end. The connection is logical, and data moves from end to end, without a direct electrical circuit. With VCs, Frame Relay shares the bandwidth among multiple users and any single site can communicate with any other single site without using multiple dedicated physical lines.


There are two ways to establish VCs:
SVCs, switched virtual circuits, are established dynamically by sending signaling messages to the network.
PVCs, permanent virtual circuits, are preconfigured by the carrier, and after they are set up, only operate in DATA TRANSFER. Note that some publications refer to PVCs as private VCs.



In the figure, there is a VC between the sending and receiving nodes. The VC follows the path A, B, C, and D. Frame Relay creates a VC by storing input-port to output-port mapping in the memory of each switch and thus links one switch to another until a continuous path from one end of the circuit to the other is identified. A VC can pass through any number of intermediate devices (switches) located within the Frame Relay network.


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