provides data communication capabilities among a comparatively small and closed user group covering a very limited geographical area, a WAN not only has the potential of covering the entire world and outer space, but also has the capability of reaching an extremely large and diverse user group (e.g., the INTERNET). With these facts in mind, what are the really key essentials that we must understand that will make such a system provide us the capabilities we would expect? Let's brainstorm and prepare a short list of requirements for a data network to communicate data messages: We must first assume that a data message is made up of one or more data frames or packets. See Section 10.10.1 and Figure 10.25. Fundamentals of Telecommunications, Second Edition, by Roger L. Freeman ISBN 0-471-71045-8 Copyright © 2005 by Roger L. Freeman ENTERPRISE NETWORKS II: WIDE AREA NETWORKS 1. Data messages should have a high probability of reaching (a) recipient(s) intact and comparatively error-free. 2. There may be an issue of urgency. Here we mean how soon after transmission will a data message reach its recipient. 3. The recipient(s) must be prepared to receive the message and "understand" its contents. 12.1.1.1 A Data Message Must Reach the Recipient(s). How is a data message routed such that the indicated recipient(s) receive the message? With conventional tele- phony, signaling carries out this function. It sets up a circuit (Chapter 7), maintains the connectivity throughout the call, and then takes the circuit down when one or both sub- scribers go on-hook. With data communications these same functions must be carried out. One form of data connectivity is called connection-oriented, where indeed a circuit is set up, traffic is passed, and then the circuit is taken down. There is another form of data communications where a frame is launched by the originator, and that frame must find its own way to the destination. Isn't this what happens using the postal service? A letter is dropped in an outgoing box, and it finds its way (