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Configuring Data Compression Services

3-4

117352-D Rev. 00

Table 3-1 lists the amount of memory required for 8 KB and 32 KB history sizes.

If you select different values for history size for the two sides of the link, 8 KB

becomes the history size for both sides of the link.

Hardware compression has similar requirements (see “History Size with

Hardware Compression” on page 3-4).

8 KB History Size

In general, 8 KB histories are appropriate for frame relay and X.25, when you

have large numbers of VCs and need to conserve memory for each of them.

32 KB History Size

For PPP, with only one circuit per line, you may want to use a 32 KB history. You

may also want to allocate a 32 KB history size for links with speeds higher than 64

Kb/s to improve throughput. The compressor can find a data pattern match up to

three times faster using a 32 KB history than an 8 KB history.

History Size with Hardware Compression

Hardware compression daughterboards and net modules are available in 32, 128,

and 256 contexts. The numbers 32, 128, and 256 assume a history size of 8 KB

per context, although you can set the history size to either 8 KB or 32 KB. For

example, if you set the history size to 32 KB, you use four 8 KB contexts, and you

have that many fewer contexts available to run hardware compression on your

network.

Table 3-1. Memory Allocation for Software Compression History

Memory Requirements 8 KB History Size 32 KB History Size

Compression history 8 KB 32 KB

Lookup table 16 KB 64 KB

Decompression history 8 KB 32 KB

Total memory required 32 KB 128 KB

Note:

Although software compression does not place strict limits on the

number of contexts you can configure, memory requirements for history size

are the same for both software and hardware compression.

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Avaya Configuring Data Compression Services Manuel d'utilisateur Page 50