Figure 64 Frame Relay requires the establishment of logical pairs of data link connections

Balanced against this minimization of access facilities cost is a reduction in performance relative to point-to-point leased lines. Frame Relay introduces a significant amount of overhead in terms of framing and protocol, which is added to the overheads of the point-to-point leased line. The rule of thumb that guides engineering the DLCI and CIRs on a Frame Relay connection is to subscribe a maximum of 1.024 Mbps of the 1.544 Mbps of available bandwidth. This guarantees that each DLCI receives its committed information rate, and that there is a margin of extra bandwidth for temporarily bursting beyond this preset rate. Frame Relay supports bursting beyond the CIR, up to the amount of unused bandwidth currently on the transmission facility.

It is possible to define a series of DLCIs with a cumulative CIR that is greater than the bandwidth available on the transmission facility. Continuing the example of T-1-based Frame Relay, one could configure 2.048 Mbps of CIRs over the 1.544 Mbps facility. This is known as oversubscription. Oversubscription is, from the consumer's perspective, an undesirable practice. It assumes that not all the DLCIs will be active and, therefore, will not be consuming their entire CIR at any given time. This is not an unreasonable assumption, but will most likely result in the sporadic degradation of service during peak usage periods.

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