Error Control and Data Compression Standards


Error Detection/Correction

• Microcom Networking Protocol (MNP)

- MNP 2-4 in public domain

- MNP 10 for cellular

© 2004 Cisco Systems, Inc. All rights rese^ed. BCRAN v2.1—2-11

Error detection and correction methods have been developed to ensure data integrity at any speed. Some widely used methods include MNP and LAPM.

Compression algorithms typically require error-correction algorithms. So compression under V.42bis and MNP-5 is usually run over LAPM or MNP-4. V.42 and V.42bis are not limited to V.32 and V.34 modems. They can also be implemented in lower-speed equipment. The 4:1 compression ratio provided by V.42bis is theoretical and rarely achieved.

V.44 is the newest compression standard that is designed to be used by V.90. V.44 offers up to a 6:1 compression ratio, compared to the 4:1 maximum compression from V.42bis. This 20-to-60 percent increase in throughput is due to a new compression algorithm that is optimized for typical web content.

The modern data compression technique is analogous to the video-compression or disk-packing algorithms that are used in computers. The compression efficiency is highly dependent on data content. Some data (such as ASCII files) compresses readily; other data compresses very little.

Some application software supports data compression. However, it is usually better to let the modem compress transmitted data. Data compression algorithms that operate in modem hardware are faster than those performed by host software. If two modems have agreed on V.42bis compression, you must disable the compression capability of the application. This modem-provided compression means transferring data at a higher speed on the interface between the DTE and the DCE.

Data Compression

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