Cabletron Systems SmartSwitch manual Average Packet Size, Nature of the Data, Link Integrity

Chapter 22: WAN Configuration Guide

Average Packet Size

In most cases, the larger the packet size, the better the potential compression ratio. This is due to the overhead involved with compression, as well as the compression algorithm. For example a link which always deals with minimum size packets may not perform as well as a link whose average packet size is much larger.

Nature of the Data

In general, data that is already compressed cannot be compressed any further. In fact, packets that are already compressed will grow even larger. For example, if you have a link devoted to streaming MPEG videos, you should not enable compression as the MPEG video data is already compressed.

Link Integrity

Links with high packet loss or links that are extremely over-subscribed may not perform as well with compression enabled. If this is the situation on your network, you should not enable compression histories (this applies only to PPP compressions; in Frame Relay compression, histories are always used).

Compression histories take advantage of data redundancy between packets. In an environment with high packet loss or over-subscribed links, there are many gaps in the packet stream resulting in very poor use of the compression mechanism. Compression histories work best with highly-correlated packet streams. Thus, a link with fewer flows will generally perform better than a link with many flows when compression histories are utilized.

The “no history” (max-histories = 0) option causes packets to be compressed on a packet- by-packet basis, thus packet loss is not a problem. Also, the number of flows is not an issue with this option as there is no history of previous packets.

Latency Requirements

The use of compression may affect a packet’s latency. Since the compressed packet is smaller, less time is needed to transmit it. On the other hand, each packet must undergo a compression/decompression process. Since the compression ratio will vary, the amount of latency will also vary.

Example Configurations

The following command line displays an example for Frame Relay:

frame-relay set payload-compress ports se.3.1.300

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