10.3
Optimizing Traffic Flow with Data Compression
10.3.2 Link compression over a point-to-point connection
Link compression (also known as per-interface compression) involves compressing both the header and payload sections of a data stream. Unlike header compression, link compression is protocol independent.

The link-compression algorithm uses Predictor or STAC to compress the traffic in another link layer, such as PPP or LAPB, to ensure error correction and packet sequencing (Cisco HDLC uses STAC compression only):

  • Predictor --Predicts the next sequence of characters in the data stream by using an index to look up a sequence in a compression dictionary. It then examines the next sequence in the data stream to see if it matches. If it does, that sequence replaces the looked-up sequence in a maintained dictionary. If it does not, the algorithm locates the next character sequence in the index, and the process begins again. The index updates itself by hashing a few of the most recent character sequences from the input stream.
  • STAC -- Developed by STAC Electronics, STAC is a Lempel-Ziv (LZ)-based, compression-based algorithm. It searches the input data stream for redundant strings and replaces them with what is called a token, which turns out to be shorter than the original redundant data string.

Note: Predictor is memory intensive and Stacker is CPU intensive.

If the data flow transverses across a point-to-point connection, use link compression. In a link-compression environment, the complete packet is compressed and the switching information in the header is not available for WAN-switching networks. Thus, the best applications for link compression are point-to-point environments with a limited hop path. Typical examples are leased lines or ISDN.