Cabletron Systems GIGAswitch GSR-16, Руководство пользователя

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DIGITAL GIGAswitch/Router User Reference Manual 299
WAN Overview
the “no history” option. If the compression statistics do not improve or show a ration of
less than 1, then compression should be disabled altogether.
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.