CPU cycles versus memory

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Other considerations when selecting a compression algorithm to optimize your WAN

utilization include:

■ Modem compression: In dialup environments, compression can occur in the modem. Two common modem compression standards are Microcom Networking Protocol-5 (MNP-5) and the International Telecommunication Union Telecommunication Standardization Sector (ITU-T) V.42bis. MNP-5 and V.42bis offer up to two times and four times compression, respectively. The two specifications are not compatible. The modems at both ends of the connection negotiate the standard to use. If compression occurs at the modem, do not configure the router to run compression.

■ Encrypted data: Compression is a Layer 2 function and encryption occurs at Layer 3. When a data stream is encrypted by the client application, it is then passed onto the router for routing or compression services or both. When the compression engine receives the encrypted data stream, which by definition has no repetitive patterns, the data expands and will not compress. LZ will then compare the before and after images to determine which is the smallest and send the uncompressed data as it was originally received if expansion occurred. If data is encrypted, do not compress the encrypted data using a Layer 2 compression algorithm.

■ CPU cycles versus memory: The amount of memory that a router must have and that the network manager must plan on varies. The amount of memory that is required varies according to the protocol being compressed, the compression algorithm, and the number of concurrent circuits on the router. Memory requirements will be higher for Predictor than for STAC, and payload will use more memory than link compression. Likewise, link compression uses more CPU cycles.

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