Avaya Configuring Data Compression Services Manuel d'utilisateur télécharger pdf (Page 25)

Data Compression Overview

1-7

Data Compression Parameters

The following sections describe data compression parameters, and give

information about decisions you must make to use compression effectively on

your network.

Compression Mode

You can compress data in one of two modes:

• Continuous Packet Compression (CPC) maintains compression history across

packets. CPC yields a higher compression ratio

than does Packet-by-Packet.

• Packet-by-Packet Compression (PPC) creates a new history for each packet.

PPC yields a lower compression ratio than does CPC.

In most circumstances, you should select CPC to maximize compression.

Select PPC only for links that drop a very large number of packets. Be aware that

under these circumstances, implementing data compression may offer marginal or

no advantages.

If either side of the link speciﬁes PPC, both sides of the link use PPC.

History Size

Each side of the link maintains both a

compression

and

decompression history

and

lookup table

. The compression and decompression histories maintain a record of

data that has already traveled across the network. The lookup tables maintain

pointers to redundant strings and the offset and length tokens that replace each of

those strings.

You can specify either 8 KB or 32 KB of local memory to maintain a compression

history.

If the link uses software compression with a history of 8 KB, each end of the link

allocates 8 KB of memory for compression, 16 KB for the compression lookup

table, and 8 KB for decompression, or 32 KB. If the link uses a compression

history of 32 KB, each end of the link allocates 32 KB of memory for

compression, 64 KB for the compression lookup table, and 32 KB for

decompression, for a total of 128 KB. Hardware compression entails similar

requirements.

If you select different values for history size for the two sides of the link, the

smaller of the two sizes becomes the history size for the link.

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Avaya Configuring Data Compression Services Manuel d'utilisateur Page 25