 be larger (as much as 512 bytes). Following the information field is the Cyclic Redundancy Check (CRC), a 16-bit sequence that will be used for error detection and/or correction. Using a CRC-16, the error detection capability is approximately 99.99 per- cent. This is to ensure the integrity of the data; rather than having to deliver information over and over again, the concept is to deliver reliable data to the far end. The closure of the packet is the end-of-frame flag. In the HDLC frame format, this denotes the end of the frame so the switches and related equip- ment know that nothing follows. The switches then calculate all of the error detection and accept or reject the packet on the basis of the data integrity. The Packet Network Figure 7-2 shows a typical network layout. The cloud in the center of the drawing represents the network provided by one of the carriers. From our discussions in earlier chapters, the obvious network configuration is that each of the designated carriers has its own cloud. Actually, each cloud inter- Figure 7-2 The X.25 network. Packet Switching Exchange 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. X.25, Internets, Intranets, and Extranets X.25, Internets, Intranets, and Extranets connects to the clouds provided by other carriers SO that transparent com- munications can take place. Once the cloud is established, the next step is to provide the packet-switching systems (called packet-switching exchanges [PSEs]). These are nothing more than computers that are capable of read- ing the address and framing information. The PSEs then route the packet to an appropriate outgoing port to the next downstream neighbor. In many cases, these PSEs are connected to several other PSEs. The packet switches can select the outbound route to the next downstream neighbor on the basis of several variables. The selection process c