ng, PSE 2 is sending the first packet to PSE 3. Again, at each step of the way, the packets are opened and a CRC is performed before a packet is actually accepted. At each PSE along the way, the packet is buffered (at PSE 1) until the next receiving PSE accepts the packet and acknowledges it (PSE 2). PSE 1 flushes the packet only after it is acknowledged. Prior to that, the network node (PSE 1) stores it just in case something goes wrong. Don't forget what happens at the receiving end (device B), where the packets arrive in sequential order and are checked and acknowledged. Then the overhead is peeled away so that a serial data stream is delivered to the receiving DTE. This process continues from device A through the network to device B until all data packets are received. Every packet along every step of the link is sent, accepted, acknowledged, and forwarded until all get through. This is the guaranteed delivery of reliable data properly sequenced. The logical link that is established between devices A and B is full duplex. The two devices can be sending and receiving simultaneously. 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 You can see from the packet delivery process that the packet-switching process is beneficial. However, nothing is perfect. The overhead on the packet, the buffering of multiple packets along the route, the CRC perfor- mance at each node along the network, and the final sequenced data deliv- ery all combine to present the risk of serious delays. What you receive in integrity and reliability can be offset in delays across the network. You must always weigh the possibilities and choose the best service. Benefits of Packets The real benefit to this method of data delivery ties into the scenario pre- sented in the beginning of this cha