Data compression in the V.42bis modems

A first international data compression standard became available when the CCITT adopted the V.42bis. standard in 1990. The standard is now used by more than 50 modem manufacturers and is rapidly,spreading into Local and Remote Area Networks (LANs, WANs). Follow-on standards now being developed (such as V.34 or V.fast) use the same data compression method in different communications protocols. British Telecom developed the V.42bis standard by combining a variety of inventions and ideas into a standard called BLTZ (British Telecom Lempel Ziv). Data compression is based on tree libraries invented by Holtz in 1974, which Welch modified in 1984 into the LZW codes. British Telecom added “Delayed Innovation” and a limited recycling library. A built-in microprocessor monitors the compression ratio, switching compression on or off using IBM´s “Escape Character” sequences. The output codes are stuffed into 8 bit octets and combined into packets which include error checking codes. Defective packets are re-transmitted automatically, ensuring error free data transmission