after all, pretty far down in the primordial protocol ooze of the OSI Model. Con- nectivity is critically important to network people-it represents their lifeblood. Customers, on the other hand, are typically only aware of the criticality of connectivity when it isn't there for some reason. No dial tone? Visible connectivity. Can't connect to the ISP? Visible connectivity. Dropped call on a cell phone? Visible connectivity. Interoperability, however, is something that customers are much more aware of. Interoperability is the process of guaranteeing logical connec- tivity between two communicating processes over a physical network. It's wonderful that the lower three layers give a user the ability to spit bits back and forth across a wide area network. But what do the bits mean? Without interoperability, that question cannot be answered. For exam- ple, in our e-mail scenario, the e-mail application running on the PC and the e-mail application running on the mainframe are logically incom- patible with each other for any number of reasons, which will be dis- cussed shortly. They can certainly swap bits back and forth, but without some form of protocol intervention, the bits are meaningless. Think about it: If the PC shown on the left side of Figure 2-13 creates an e-mail mes- sage that is compressed, encrypted, ASCII encoded, and shipped across logical channel 17, do the intermediate switches that create the path over which the message is transmitted care? Of course not. Only the transmitter and receiver of the message that house the applications that will have to interpret it care about such things. The intermediate switches care that they have electrical connectivity, that they can see the Figure 2-13 Connectivity Madrid Marseilles 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 bits, that they can determine whether they a