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Micro House PC Hardware Library Volume II: Network Interface Cards And Modems Micro House PC Hardware Library Volume II: Network Interface Cards And Modems
by Micro House International, Inc. and Scott Mueller
Que, Macmillan Computer Publishing
ISBN: 078971664x   Pub Date: 06/17/98
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Error-Correction Protocols

Error correction refers to the capability of some modems to identify errors during a transmission, and to automatically resend data that appears to have been damaged in transit. For error correction to work, both modems must adhere to the same correction standard. Fortunately, most modem manufacturers adhere to the same error-correction standards.

MNP 1-4
This is a proprietary standard which was developed by Microcom which provides basic error correction. The Microcom networking Protocol (MNP) is covered in more detail in the “Proprietary Standards” section.
V.42
V.42 is an error-correction protocol, with fallback to MNP 4., and version 4 is an error-correction protocol as well. Because the V.42 standard includes MNP compatibility through Class 4, all MNP 4-compatible modems can establish error-controlled connections with V.42 modems.
This standard uses a protocol called LAPM (Link Access Procedure for Modems). LAPM, like MNP, copes with phone-line impairments by automatically retransmitting data corrupted during transmission, assuring that only error-free data passes between the modems. V.42 is considered to be better than MNP 4 because it offers about a 20 percent higher transfer rate due to its more intelligent algorithms.

Data-Compression Standards

Data compression refers to a built-in capability in some modems to compress the data they’re sending, thus saving time and money for long-distance modem users. Depending on the type of files that are sent, data can be compressed to one-fourth its original size, effectively quadrupling the speed of the modem. For example, a 14,400 modem with compression can yield transmission rates of up to 57,600 bps, and a 28,800 can yield up to 115,200 bps.

MNP 5
Microcom continued the development of its MNP protocols to include a compression protocol named MNP 5. This protocol is discussed more fully in the section “Proprietary Protocols.”
V.42bis
V.42bis is a CCITT data-compression standard similar to MNP Class 5, but providing about 35 percent better compression. V.42bis is not actually compatible with MNP Class 5, but nearly all V.42bis modems include the MNP 5 data-compression capability as well.
This protocol can sometimes quadruple throughput, depending on the compression technique used. This fact has led to some mildly false advertising; for example, a 2400 bps V.42bis modem might advertise “9600 bps throughput” by including V.42bis as well, but this would be possible in only extremely optimistic cases, such as in sending text files that are very loosely packed. In the same manner, many 9600 bps V.42bis makers now advertise “up to 38.4K bps throughput” by virtue of the compression. Just make sure that you see the truth behind such claims.
V.42bis is superior to MNP 5 because it analyzes the data first, and then determines whether compression would be useful. V.42bis only compresses data that needs compression. Files found on bulletin board systems often are compressed already (using PKZIP or a similar program). Further attempts at compressing already compressed data can increase the size of the file and slow things down. MNP 5 always attempts to compress the data, which slows down throughput on previously compressed files. V.42bis, however, compresses only data that will benefit from the compression.
To negotiate a standard connection using V.42bis, V.42 also must be present. Therefore, a modem with V.42bis data compression is assumed to include V.42 error correction. These two protocols, when combined, result in an error-free connection that has the maximum data compression possible.

Proprietary Standards

In addition to the industry-standard protocols for modulation, error correction, and data compression that generally are set forth or approved by the ITU-T, several protocols in these areas were invented by various companies and included in their products without any official endorsement by any standards body. Some of these protocols have been quite popular at times and became pseudo-standards of their own.

The most successful proprietary protocols are the MNP (Microcom Networking Protocols) that were developed by Microcom. These error-correction and data-compression protocols are supported widely by other modem manufacturers as well.

Another company that has been successful in establishing proprietary protocols as limited standards is U.S. Robotics, with its HST (high speed technology) modulation protocols. Because of an aggressive marketing campaign with BBS operators, it captured a large portion of the market with its products in the 1980s.


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