Cabletron Systems SSIM-R8-02 Spécifications télécharger pdf (Page 377)

Enterasys Xpedition User Reference Manual 357

WAN Overview

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:

The following command line displays an example for PPP:

frame-relay set payload-compress ports se.3.1.300

ppp set payload-compress port se.4.2

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Cabletron Systems SSIM-R8-02 Spécifications Page 377