ies the X.400 format of the message. The X.400 message with its new header is then passed down to the Presentation Layer, which encodes it as ASCII, encrypts it using PGP, and compresses it using a British Telecom Lempel-Ziv compression algo- rithm. After adding a header that details all this, it passes the message to layer five. The Session Layer assigns a logical session number to the message, glues on a packet header identifying the session ID, and passes the steadily growing message down to the Transport Layer. Based on net- work limitations and rule sets, the Transport Layer breaks the message into eleven packets and numbers them appropriately. Each packet is given a header with address and quality-of-service information. The packets now enter the chained layers, where they will first encounter the network. The Network Layer examines each packet in turn, and, based on the nature of the underlying network (connection- 4 specifically note "human user" here because some protocols do not recognize the existence of the human in the network loop. In IBM SNA environments, for example, "user" are devices or processes that "use" network resources. There are no humans. 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. Protocols Protocols oriented? Connectionless?) and congestion status, queues the packets for transmission. After creating the header on each packet, they are handed individually down to the Data Link Layer. The Data Link Layer proceeds to build a frame around each packet. It calculates a CRC, inserts a Data Link Layer address, inserts appropriate control information, and finally adds flags on each end of the frame. Note that all other layers add a header only; the Data Link Layer is the only layer that also adds a trailer. Once the Data Link frame has been constructed it is passed down to the Physical Layer, which encodes the incoming