 dialup line. The UNI then leads to the Frame Relay switch, which is basically a statistical multiplexer. Based on the type of sub- scription in place, the traffic is sent over either a permanent virtual circuit (PVC) or a switched virtual circuit (SVC). Recall from Chapter 1 that a PVC is analogous to a leased line. It is predetermined, and it is manually configured and entered into a network management system SO that it stays between two locations until it is Packet-Switched Networks reprogrammed. SVCs, on the other hand, are like the dialup scenario; they are dynamically provisioned via signaling on an as-needed basis. (The reality is that PVCs are predominant, and SVCs are rare.) Figure 7.15 illustrates the use of PVCs. When a packet goes through the inter- face in the DTE (probably a router or a FRAD), it is put into the LAPD format, and then the LAPD frame is passed to the switching point. The switching point looks at the DLCI and then looks it up in its table to determine over which particular cir- cuit or virtual circuit to send the message. Subscribers specify the port speed and the committed information rate (CIR) in a Frame Relay network. Port prices are based on bandwidth, which determines the speed of the interface into the network. The PVC charges are based on the CIR and the distance. (The CIR generally refers to the PVC's minimum bandwidth under normal conditions. Generally, the CIR is less than the access rate into the network, and the access rate into the network determines the maximum amount of usable bandwidth.) Figure 7.16 illustrates the concept of bandwidth-on-demand mentioned earlier in this chapter. Say you have an access line, an E-1, that allows 2.048Mbps to your carrier's switching point. Between these two locations of the network, you have contracted for a PVC that is essentially 1Mbps. In this environment, bandwidth- on-demand works like this: You are allowed to burst above your PVC's CIR of 1Mbps, up to the rate of your access line, or port speed, which is 2Mbps. In other words, you