dwidth that frame-relay service providers absolutely guarantee on a PVC-by-PVC basis, is called the committed information rate (CIR). In addition to CIR, service providers will often support an excess information rate (EIR), which is the rate above the CIR they will attempt to carry, assuming the capacity is avail- able within the network. However, all frames above the CIR are marked as eligible for discard, which simply means that the network will do its best to deliver them but makes no guarantees. If push comes to shove, and the network finds itself to be congested, the frames marked discard eligible (DE) are immediately discarded at their point of ingress. This CIR/EIR relationship is poorly understood by many customers because the CIR is taken to be an indicator of the absolute bandwidth of the cir- cuit. Whereas bandwidth is typically measured in bits per second, CIR is a measure of bits in one second. In other words, the CIR is a measure of the average throughput that the network will guarantee. The actual transmission volume of a given CIR may be higher or lower than the CIR at any point in time because of the bursty nature of the data being sent, but in aggregate the network will maintain an average, guaranteed flow volume for each PVC. This is a selling point for frame relay. In most cases, customers get more than they actually pay for, and as long as the switch loading levels are properly engineered, the switch (and therefore the frame-relay service offering) will not suffer adversely from this char- itable bandwidth-allocation philosophy. The key to success when selling frame relay is to have a very clear understanding of the applications the customer intends to use across the link so that the access facility can be properly sized for the anticipated traffic load. Managing Service in Frame-Relay Networks Frame relay does not offer a great deal of granularity when it comes to QoS. The only inherent mechanism is the DE bit described earlier as a way to control network congestion. However, the DE bit is 