). The CIR is a guaranteed rate of throughput when using Frame Relay. The CIR is assigned to each of the permanent virtual circuits selected by the user. Each PVC is assigned a CIR that is consistent with the average expected volume of traffic to the destination port. Because Frame Relay is a duplex service (data can be transmitted in each direction simultaneously), a dif- ferent CIR can be assigned in each direction. This produces an asymmetri- cal throughput based on demand. For example, a customer in Boston might use a 64-Kbps service between Boston and San Francisco for this connec- tion; yet for the San Francisco-to-Boston PVC, a rate of 192 Kbps can be used. This provides added flexibility to meet the customer's needs for trans- port. However, because the nature of LANs is that of bursty traffic, the CIR can be burst over and above the fixed rate for two seconds at a time in some carriers' networks. This committed burst rate (Bc) is up to the access chan- nel rate, but many of the carriers limit the burst rate to twice the speed of the CIR. When the network is not very busy, the customer could still burst data onto the network at an even higher rate. The excess burst rate (Be) can be an additional speed of up to the channel capacity, or in some carriers' net- works, it can be 50 percent above the burst rate. Combining these rates, an example can be drawn as follows: CIR + Bc + Be = Total throughput Remember that the burst and the burst excess rates are for two seconds or less, depending on the carrier used. Some carriers do not allow any Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. PCUSN-to-SGSN Interface (Gb) Chapter 9 bursting across the network. Rather, they require that the maximum throughput be limited to the CIR. We are emphasizing that no standard offerings exist. Provisioning PVCs and SVCs The primary difference between PVCs