The 56 kbit/s modem has been proposed as the solution for bandwidth-starved Internet users, and they are likely to represent the fastest affordable option for some time to come. But how do they achieve this data rate? It is almost double the 28.8 kbit/s of V.34, which was claimed to be close to the fundamental limit of the telephone line. This article examines the problems facing the modem designer, and how they have been overcome in X2 and K56flex.
This article was written when the 56K standards process had just begun. As a result, it does not refer to the ITU deliberations, or the V.pcm proposal and V.90 standard. An update on the draft standard is now available.
Throughout the article, diagrams are presented in thumbnail form. To view the diagram at full-size, simply click on the thumbnail.
Author: Nick Chalk
Published: Archive 10.12 & 11.1, September-October 1997.