Compression

Compression is the act of making data smaller, thus saving space on a network. By default, Cisco routers transmit data across serial links in an uncompressed format, but by using Cisco serial compression techniques, you can make more efficient use of your available bandwidth. It’s true that any compression method will cause overhead on the router’s CPU, but the benefits of compression on slower links can outweigh that disadvantage.

Three types of compression are used in a Cisco internetworking environment:

Header compression Cisco uses the Van Jacobson algorithm to compress the headers of IP packets before sending them out onto WAN links. This method leaves the data intact, compressing only the header information, and can be used for applications (e.g., Telnet) and HDLC or X.25 encapsulation. However, it doesn’t allow for protocol independence.

Payload compression The payload compression approach compresses the data, but leaves the header intact. Because the packet’s header isn’t changed, it can be switched through a network. As a result, this method is the one generally used for switching services such as X.25, SMDS, Frame Relay, and ATM.

Link compression The link compression method is a combination of both header and payload compression, and for you to be able to use it, the data must be encapsulated in either Point-to-Point Protocol (PPP) or Link Access Procedure, Balanced (LAPB). Link compression allows for protocol independence.

Software compression can significantly affect router CPU performance, and the Cisco rule of thumb is that the routers’ CPU load must not exceed 65 percent when running software compression. If it does, you’re better off just disabling any compression method.