(Software) Modem @@@@@@@@ User B (External) In Figure 7-5, another device is attached on the other end of the cloud. Now the magic of the packet-switching world occurs. As packets are generated through the network from user device A to B across the network, several things happen, as shown in Figure 7-6. The data is sent serially from the DTE to the PAD (which acts as the Data Circuit-Terminating Equipment [DCE] for the computer terminal). The PAD breaks the data down into smaller pieces and add the necessary overhead for delivery. The PAD then routes the packets to the network PSE. The receiving PSE (PSE 1) sees the packet coming in on a logical channel, SO it remembers where the packets are coming from. After analyzing the data (performing a CRC on the packet) and verifying that it is all right, the PSE sends back an acknowledgment to the originating device (which is the DCE). The PSE then sends the packet out across an outgoing channel to the next downstream neighbor (PSE 2). This establishes a logical connection between the two devices (PSEs) from the out channel to an in channel at the other end. The logical channel is already there; it is used for the transfer of these specific packets. A virtual connection is 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 Chapter 7 Figure 7-6 User A The packet sequence. Port 202 Port 12 Port 196 Port 17 4. Packets are sent across virtual circuit to B. Port 5 3. Packets sent 1. Serial data is Port 27 sent to the PAD. to network. User B Modem Modem 2. PAD creates 00000000 packets. 00000000 also created, allocating the time slots for the packets from device A to B to run on the virtual circuit. Back at PSE 1, the next packet is sent down from the originating PAD. This is analyzed and acknowledged once again, and then passed along. At the same time this is happeni