chy, ITU-T Rec. G.708, ITU Helsinki, 1993. 7. Synchronous Multiplexing Structure, ITU-T Rec. G.709, ITU Geneva 1993. 8. Digital Hierarchy Bit Rates, CCITT Rec. G.702, Fascicle III.4, IXth Plenary Assembly, Mel- bourne, 1988. 9. G. Held, Dictionary of Communications Technology, Wiley, Chichester, UK, 1995. ASYNCHRONOUS TRANSFER MODE 20.1 EVOLVING TOWARD ATM Frame relay (Section 12.5) began a march toward an optimized digital format for multimedia transmission (i.e., voice, data, video, and facsimile). There were new con- cepts in frame relay. Take, for example, the trend toward simplicity where the header was notably shortened. The header was pure overhead, SO it was cut back as practically possible. The header also signified processing. By reducing the processing, delivery time of a data frame is speeded up. In the effort to speed up delivery, frames were unacknowledged (at least at the frame relay level); there was no operational error correction scheme. It was unnecessary because it was assumed that the underlying transport system had excellent error performance (BER better than 1 X 10-7).2 There was error detection for each frame, and a frame found in error was thrown away. Now that was something that we would never do for those of us steeped in old-time data communication. It is assumed that the higher OSI layers would request repeats of the few frames missing (i.e., thrown away). These higher OSI layers (i.e., layer 3 and above) were the customer's responsibility, not the frame relay provider. Frame relay also moved into the flow control arena with the BECN and FECN bits and the CLLM. The method of handling flow control has a lot to do with its effectiveness in preventing buffer overflow. It also uses a discard eligibility (DE) bit, which set a type of priority to a frame. If the DE bit was set, the frame would be among the first to be discarded in a time of congestion. DQDB (distributed queue dual bus) was another antecedent of ATM. It was developed by the IEEE as a simple and unique network access scheme. 