lying physical network over which the message is being transmitted, including the following characteristics: Class of service required Data transfer requirements User interface characteristics Connection management requirements Specific security concerns Network management and reporting status data There are two basic network types: dedicated and switched. We will examine each in turn before discussing Transport Layer protocols. Dedicated networks are exactly what the name implies: an always-on network resource, often dedicated to a specific customer, which provides 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 very high-quality transport service. That's the good news. The bad news is that dedicated facilities tend to be expensive, particularly because the customer pays for them whether they use the facility or not. Unless they are literally using it 100 percent of the time, the network is costing them money. The other downside of a dedicated facility is susceptibility to fail- ure: Should a terrorist backhoe driver decide to take the cable out, there is no alternative route for the traffic to take. It requires some sort of intervention on the part of the service provider that is largely manual. Furthermore, dedicated circuits tend to be inflexible, again, because they are dedicated. Switched resources, on the other hand, work in a different fashion and have their own set of advantages and disadvantages to consider. First and foremost, they require an understanding of the word "virtual." When a customer purchases a dedicated facility, he or she literally owns the resources between the two communicating entities as shown in Figure 2-25. Either the circuit itself is physically dedicated to them (com- mon in the 1980s), or a timeslot on a shared physical resource such as a T-Carrier is dedicated to them. Data is placed on 