Compression

Another way to increase the number of packets that can be transmitted is to reduce the size of frar enabling compression. Because compressed frames are smaller in size, more compressed frames ca sent across the media, improving transmission times. Compression is implemented either in hardw in software, depe nd mg on the Cisco IOS Software version installed, the type of interface and encap in use, and the hardware platform that it is installed onto. This chapter covers only software compr techniques, in paaticular the STAC and Predictor compression algorithms.

Before enabling compression on any router, it is very important to check the processor and memor utilization. If a router's memory utilization exceeds 40 percent, compression will not be a helpful s< It is also important to note that STAC and Predictor both support different encapsulation protocols have different memory and CPU requirements. Table 4-10 outlines these issues.

Table 4-10. Compression Issues

Compression Method

Protocol

System Requirements

STAC

HDLC, PPP, LAPB, X.25

Higher CPU requirements

Predictor

PPP, LAPB

Higher memory Requirements

The amount of traffic being transmitted, the type of packets being sent, and the amount of availabl bandwidth also affect the i mpact that compression h as o n a to uter. If you are considering impleme compression on an interface that is mainly used to download data that has already been compresse example, enab ling;] rompression w ill not be s>eneficip' because daea cannot be compressed twice. If interface has a large amount of bandwidth, and large amounts of data are being transmitted, the niictionaries in memory are >ikel y to be ve^ l arge. To check mem ny utilization, use the show mer oommary com ma nd aed compare the: total memory to the free memory. If you do n ot have much memoryr nour router wsl probably not te able to handle compres^on. To verify CbU utilization, use nhow process cpu co m mand; note the average processor utiliza tion over a period of time. If it Consistent |y reaches 40 perce nt, co mpression is p robably pot a r>erformarce so lut roe Examp l e 4-2 the processor utilizat i on ftx a rou ter before and after STAC compressio n was en abled.

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