n between source and destination (this refers to urgency of traffic): required or not required. Bit rate: constant or variable. Connection mode: connection-oriented or connectionless. When we combine these parameters, four service classes emerge as shown in Figure 20.8. Examples of the services in the classes shown in Figure 20.8 are as follows. Class A: constant bit rate such as uncompressed voice or video. Class B: variable bit rate video and audio, connection-oriented synchronous traffic. ASYNCHRONOUS TRANSFER MODE Service Parameters Class A Class B Class C Class D Timing Required Not Required Compensation Bit Rate Constant Variable Connection Connection-oriented Connectionless AAL3/4 AAL3/4 AAL Types or AAL5 or AAL5 Examples DS1, E1, Packet Frame relay IP, SMDS X 64-kbps video, emulation audio Figure 20.8 Services classifications of AAL. (Based on Refs. 2, 8, and 10.) Class C: connection-oriented data transfer, variable bit rate, asynchronous traffic. Class D: connectionless data transfer, asynchronous traffic such as SMDS. Note that SMDS stands for switched multimegabit data service. It is espoused by Bellcore (Telcordia) and is designed primarily for LAN interconnect. 20.6.3.3 AAL Categories or Types. There are five different AAL categories. The simplest of AAL-0. It just transmits cells down a pipe. That pipe is commonly a fiber-optic link. Ideally, it would be attractive that the bit rate here be some multiple of 53 X 8 bits or 424 bits. For example, 424 Mbps could handle 1 million cells per second. AAL-1. AAL-1 is used to provide transport for synchronous bit streams. Its primary application is to adapt ATM cell transmission to typically E1/DS1 and SDH/SONET cir- cuits. AAL-1 is specifically used for voice communications (POTS-plain old telephone service). AAL-1 robs one octet from the payload and adds it to the header, leaving only a 47-octet payload. This additional octet in the header contains two major fields: sequence number (SN) and sequence number protection (SNP). The principal purpose of these t