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Frame Relay

Frame Relay is beginning to decline but is still worthy of inclusion here, especially when you consider regions of the globe outside the United States.

Frame Relay sites are interconnected using virtual circuits (VC). So a single router interface can have multiple VCs. Frame Relay is a Layer 2 technology, and a router uses locally significant identifiers for each VC. These identifiers are called data-link connection identifiers (DLCI). Because DLCIs are locally significant, DLCIs at the different ends of a VC do not need to match (although they could).

If a VC is always connected, it is considered to be a permanent virtual circuit (PVC). However, some VCs can be brought up on an as-needed basis, and they are referred to as switched virtual circuits (SVC).

Unlike a dedicated leased line, Frame Relay shares a service provider’s bandwidth with other customers of its service provider. Therefore, subscribers might purchase an SLA (previously described) to guarantee a minimum level of service. In SLA terms, a minimum bandwidth guarantee is called a committed information rate (CIR).

During times of congestion, a service provider might need a sender to reduce his transmission rate to the CIR. A service provider can ask a sender to reduce his rate by setting the backward explicit congestion notification (BECN) bit in the Frame Relay header of a frame destined for the sender that needs to slow down. If the sender is configured to respond to BECN bits, it can reduce its transmission rate by as much as 25 percent per timing interval (which is 125 ms by default). CIR and BECN configurations are both considered elements of Frame Relay Traffic Shaping (FRTS). A device that does packet shaping is referred to as a packet shaper.

Another bit to be aware of in a Frame Relay header is the discard eligible (DE) bit. Recall that a CIR is a minimum bandwidth guarantee for a service provider’s customer. However, if the service is not congested, a customer might be able to temporarily transmit at a higher rate. However, frames sent in excess of the CIR have the DE bit in their header set. Then, if the Frame Relay service provider experiences congestion, it might first drop those frames marked with a DE bit.

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