ey"); and in ATM it is called a virtual circuit identifier as well. Each of these will be described in greater detail later in the book. When device D generates its message to device K for transport across the network, the Transport Layer "packages" the data for transmission. Among other things, it assigns a logical channel that the ingress switch uses to uniquely identify the incoming data. It does this by creating a unique combination of the unique logical address with the shared phys- ical port to create an entirely unique virtual circuit identifier. When the message arrives at the first switch, the switch enters the logical channel information in a routing table that it uses to manage incoming and outgoing data. There can be other information in the rout- ing table as well, such as quality of service indicators; these details will be covered later, when we discuss the Network Layer (layer three). The technology that a customer uses in a switched network is clearly not dedicated, but it gives the appearance that it is. This is called virtual circuit service, because it gives the appearance of being there when, in fact, it isn't. Virtual private networks (VPNs), for example, give a 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 customer the appearance that they are buying private network service. In a sense they are: They do have a dedicated logical facility. The differ- ence is that they share the physical facilities with many other users, which allows the service provider to offer the transport service for a lower cost. Furthermore, secure protocols protect each customer's traffic from interception. VPNs are illustrated in Figure 2-27. As you may have intuited by now, the degree of involvement that the Transport Layer has varies with the type of network. For example, if the network consists of a single, dedicated, po