rt Layer simply places the packet into a buffer while it waits for the other ten packets to arrive. It will wait as long as it has to; it knows that it cannot 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 Chapter 2 deliver a partial message because the higher layers are not smart enough to figure out the missing pieces. Once it has received all eleven packets, the Transport Layer reassem- bles the original message and passes it up to the Session Layer, which examines the Session header created by the transmitter and notes that this is to be handed to whatever process cares about logical channel number seven. It then strips off the header and passes the message up to the Presentation Layer. The Presentation Layer reads the Presentation Layer header created at the transmit end of the circuit and notes that this is an ASCII message that has been encrypted using PGP and compressed using BTLZ. It decompresses the message using the same protocol, decrypts the mes- sage using the appropriate public key, and, because it is resident in a mainframe, converts the ASCII message to EBCDIC. Stripping off the Presentation Layer header, it hands the message up to the Application Layer. The Application Layer notes that the message is X.400 encoded, and is therefore an e-mail message. As a result, it passes the message to the e-mail application that is resident in the mainframe system. The process just described happens every time you hit the SEND button. Click. Other Protocol Stacks Of course, OSI is not the only protocol model. In fact, for all its detail, intricacy, and definition, it is rarely used in practice. Instead, it serves as a true model for comparing disparate protocol stacks. In that regard, it is unequalled in its value to data communications. The most commonly deployed protocol stack is that used in the Inter- net, the so-called TC