ctionless, CL Information AAL protocol AAL3/4 field for AAL3/4 user data 47 octets Service classes and examples: Class A: Constant bit rate service, ordinary video or voice Class B: Variable bit rate service with timing information, for variable bit rate video or voice Class C: Variable bit rate service, ordinary data transmission Class D: Variable bit rate service with connectionless service, data transmission of very short messages (no connection establishment) Figure 6.63 Service classes and adaptation layer of ATM. Introduction to Telecommunications Network Engineering 6.8.5.2 Class B/AAL2 AAL2 provides the support for VBR traffic that requires maintenance of tim- ing information during the call. Timing information is transmitted in the adaptation layer header. Examples of this type of traffic are variable-bit-rate voice and video applications in a LAN environment. 6.8.5.3 Class C/AAL5, AAL3/4 AAL3/4 is a complicated protocol and it provides both connection-oriented service of class C and connectionless service of class D. AAL3/4 was found to be too complex and inefficient for ordinary LAN traffic. AAL5 evolved after AAL3/4 and it was designed to be simple and effi- cient. It supports only variable-bit-rate traffic, like burst data of LANs, with no timing relationships. AAL5 does not provide enhanced services and thus consequently it does not require much overhead for protocol information. It is the primary AAL used to provide LAN interconnections over ATM networks. 6.8.5.4 Class D/AAL3/4 This class supports variable-bit-rate traffic that requires no timing informa- tion. It supports connectionless service that does not require connection establishment. Class D is suitable for datagram transmission in which only a small amount of data is transmitted during one connection. 6.8.6 Applications and Future of ATM ATM is used as a technology for high-data-rate backbone networks of some telecommunications network operators. ATM was expected to be a major backbone technology and some access technologies, such as ADSL