requirements of the most common forms of trans- mitted data. ITU-T Service Classes The ITU-T assigns service classes based on three characteristics: con- nection mode, bit rate, and the end-to-end timing relationship between the end stations. They have created four distinct service classes based on the model shown in Figure 7-10. Class A service, for example, defines a connection-oriented, constant bit rate, timing-based service that is ideal for the stringent requirements of voice service. Class B, on the other hand, is ideal for such services as variable bit rate video, in that it defines a connection-oriented, variable bit rate, timing-based service. Class C service was defined for such things as frame relay, in that it provides a connection-oriented, variable bit rate, timing-independent service. Finally, Class D delivers a connectionless, variable bit rate, timing-independent service that is ideal for IP traffic as well as SMDS. In addition to service classes, the ITU-T has defined AAL service types, which align closely with the A, B, C, and D service types described previously. Whereas the service classes (A, B, C, D) describe the capabil- ities of the underlying network, the AAL types describe the cell format. They are AAL1, AAL2, AAL 3/4, and AAL 5. However, only two of them have really survived in a big way. AAL1 is defined for Class A service, which is a constant bit rate envi- ronment ideally suited for voice and voice-like applications. In AAL1 cells, the first octet of the payload serves as a payload header that con- Class A Class B Class C Class D Figure 7-10 ITU-T ATM AAL Type 5,3/4 service Connection Connection- Connection- Connection- Connectionless definitions oriented oriented oriented Bit Rate Constant Variable Variable Variable Timing Required Required Not required Not required Relationship Service Types Voice, video VBR voice, Frame relay video Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies. All rights 