ed upon a virtual circuit address. A virtual circuit is simply a connection that gives the user the appearance of being dedicated to that user, when in point of fact the only thing that is actually dedicated is a time slot. This technique is generically known as label-based switching, and is accomplished through the use of routing tables in the ATM switches that designate input ports, output ports, input addresses, output addresses, and QoS parameters required for proper routing and service provisioning. As a result, cells do not contain explicit destination addresses, but rather con- tain timeslot identifiers. There are two components to every virtual circuit address, as shown in Figure 7-9. The first is the virtual channel (VC), which is a unidirec- tional conduit for the transmission of cells between two endpoints. For example, if two parties are conducting a videoconference, they will each have a VC for the transmission of outgoing cells that make up the video portion of the conference. The second "level" of the ATM addressing scheme is called a virtual path (VP). A VP is a bundle of VCs that have the same endpoints and that, when considered together, make up a bidirectional transport facil- ity. The combination of unidirectional channels that we need in our two- way videoconferencing example makes up a VP. ATM Services The basic services that ATM provides are based on three general char- acteristics: the nature of the connection between the communicating sta- tions (connection-oriented VS. connectionless), the timing relationship between the sender and the receiver, and the bit rate required to ensure proper levels of service quality. Based on those generic requirements, 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. Transport Technologies Chapter 7 both the ITU-T and the ATM Forum have created service classes that address the varying 