Cisco 2600 SERIES 8MB COMPRESSION MODULE Specification Guide

This document answers the frequently asked questions (FAQs) about WAN compression. This document
includes the Compression Overview, Implement Compression in Cisco Routers, and Troubleshoot
Compression sections.

A. Data compression works by the identification of patterns in a stream of data. Data
compression chooses a more efficient method to represent the same information.  Essentially,
an algorithm is applied to the data in order to remove as much redundancy as possible.  The
efficiency and effectiveness of a compression scheme is measured by its compression ratio,
the ratio of the size of uncompressed data to compressed data.  A compression ratio of 2:1
(which is relatively common) means that the compressed data is half the size of the original
data.

There are many different algorithms available in order to compress data.  Some algorithms
are designed to take advantage of a specific medium and the redundancies found in them.
However, they do a poor job when applied to other sources of data. For example, the Motion
Picture Experts Group (MPEG) standard is designed to take advantage of the relatively small
difference between one frame and another in video data. It does an excellent job in
compression of motion pictures, but does not compress the text well.

One of the most important ideas in compression theory is that there exists a theoretical limit,
known as Shannon's Limit. This limit tells you how far you can compress a given source of
data.  Beyond this point, it is impossible to reliably recover compressed data.  Modern
compression algorithms coupled with the fast processors available today allow users to
approach Shannon's Limit. However, they can never cross it.

Refer to these documents for more information on on Shannon's Limit:

Shannon's Law

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Non−Parametric Approach and Shannon's Superresolution Limit

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Cisco − WAN Compression FAQs