Stac (LZS)

A data compression algorithm used in cisco devices, developed by STAC Electronics (now Hi/fn, Inc.) and based on the Lempel-Ziv algorithm. LZS is in Cisco's Link Access Procedure, Balanced (LAPB), HDLC, X.25, and Frame Relay. The Cisco IOS uses an optimized version of LZS that provides pretty good ratios but requires manic-hamster CPU cycles.

LZS searches the input data stream for redundant strings and replaces them with a shorter token. It creates dictionaries of the string matches and tokens that are pointers into the previous data stream. The dictionary is used to replace the redundant strings found in the new data streams.

This compression is not just available in Cisco routers. In fact, most any router will support this type of compression, albeit that they may require an expansion card.

It is also available for Linux on ISDN connections. The source code is in the isdn4linux CVS tree (www.isdn4linux.de).
Not all ISPs support this technology, but the speed increase is very noticeable. Expect anything in between 15 to 75 kb/s on just one ISDN channel (64 kilobit) for uncompressed data (text or html for example).