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Unformatted text preview: ition to performing the functions of a regular
modem. A computer equipped with a FAX modem can send/receive text and
images as a FAX to/from a remote FAX machine or another computer equipped
with a FAX modem. Like other kinds of modems, FAX modems can be external
Error correction. Data transmission errors may occur while data moves on
telephone lines from one modem to another. To take care of this problem, many
of the more sophisticated modems use methods of recovering from data
transmission errors. The two standard methods (protocols) used in the design of
modems for error correction feature are MNP4 (Microcom Network Protocl
Version 4) developed by Microcom, Inc. and V.42 standard developed by CCITT.
MNP4 error-correction protocol is suitable for low-speed modems (such as those
operating at 2400 bps), whereas V.42 standard is suitable for higher-speed
modems (those operating at 9600 bps and 14400 bps).
5. Data Compression. Data compression techniques (discussed in Chapter 19)
help in reducing the volume of the data. The reduction in data volume depends
mostly on the kind of data that is being compressed and the compression algorithm
used, but can be as high as 10:1 ratio. By reducing the volume of the data to be
transmitted, modems with built-in data-compression techniques can achieve higher
throughput (can transfer more data in the same amount of time) than conventional
modems of the same speed.
Keeping interoperability in mind, the two standard data compression techniques
used in the design of modems are Microcom's MNP Class 5 specification and
CCITT's V.42bis specification. MNP5 specification is commonly used for lowspeed modems, whereas V.42bis specification is commonly used for high-speed
modems. The effective throughputs of the base data rates for the two standard
specifications are given in Figure 17.10. Note that whereas MNP5 can almost
double the transmission rate of a modem, V.42bis can almost quadruple it. Of
course, for data compression to work effectively, two communicating modems
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This document was uploaded on 04/07/2014.
- Spring '14