re the right bits, and whether they are the intended recipient of those bits-or not. Therefore, the end devices, the sources and sinks of the message, must implement all seven layers of the OSI Model, because they must not only concern themselves with connectivity issues, but also with issues of interoper- ability. The intermediate devices, however, care only about the functions and responsibilities provided by the lower three layers. Interoperability, because it only has significance in the end devices, is provided by the end-to-end layers-layers four through seven. Connectivity, on the other hand, is provided by the chained layers, layers one through three, because those functions are required in every link of the network chain -hence the name. Let me say this one more time: The chained layers are the historical domain of the telephone company. Layer by Layer The OSI Model relies on a process called enveloping, illustrated in Figure 2-14, to perform its tasks. If we return to our earlier e-mail example, we Figure 2-14 A schematic representation of enveloping 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 find that each time a layer invokes a particular protocol, it wraps the user's data in an "envelope" of overhead information that tells the receiv- ing device about the protocol used. For example, if a layer uses a partic- ular compression technique to reduce the size of a transmitted file and a specific encryption algorithm to disguise the contents of the file, then it is important that the receiving device be made aware of the techniques employed SO that it knows how to decompress and decrypt the file when it receives it. Needless to say, quite a bit of overhead must be transmit- ted with each piece of user data. The overhead is needed, however, if the transmission is to work properly. So, as the user's data passes d