 are paying for 1Mbps, but you're actually allowed to transmit at 2Mbps for short periods of time. However, note that the contracted-for 1Mbps has a dis- card eligibility of zero (DE = 0), ensuring that these frames do not get dropped. DTE (FRAD or Router) SP = Switching Point = PVC Figure 7.15 PVCs in a Frame Relay network Chapter 7 Wide Area Networking 1.024Mbps CIR on PVC 1.544Mbps Carrier Access Line Site 3 Point of Presence 2.048Mbps Access Line Carrier Point of Carrier Point of Presence Router 256Kbps CIR Presence Site 4 512Kbps Access Line Carrier Point of Site 1 Router Site 2 Presence 256Kbps Access Line Access Line Figure 7.16 Frame Relay bandwidth-on-demand When you burst above your CIR, everything else is DE = 1, which means those frames could get dropped. If you try to keep transmitting at your burst rate over a sustained period, the network will do one of two things. It might start dropping frames, which is another reason voice and video might suffer over Frame Relay. Or there might be a software mechanism that allows the excess traffic to be captured so that you can be billed for overtime. But the carrier is banking on the fact that not everybody is making use of the CIR at all times. Again, LAN traffic is quite unpredictable, SO there are lulls in the day when you're not transmitting anything and other times when you need twice your CIR, and, ideally, at the end of the day it all balances out. But the carrier is playing the same gamble, assuming that not everybody is going to try to exercise their CIR at the same time. If they do, whether you still experience your CIR depends on the integrity of the Frame Relay provider's engineering. In other words, if the provider oversubscribes a PVC and if everyone attempts to burst at the same time, somebody is not going to have capacity available. This is a big issue in terms of vendor selection. Frame Relay networks are much less expensive than other options because the operators save on how they carry the traffic. With SVCs, the connections are established 