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Error-detection and error-correction methods were developed to ensure data integrity at any speed. Some widely used methods include Microcom Networking Protocol
(MNP) and Link Access Procedure for Modems (LAPM).
MNP actually has ten different levels.
They are as follows:
| MNP1 |
Asynchronous, half-duplex transfer |
| MNP2 |
Error correction, asynchronous,
full duplex |
| MNP3 |
Error correction, synchronous |
| MNP4 |
Error correction, better
throughput than MNP2 and 3; this modification rides on top of
MNP2 or 3 to improve throughput |
| MNP5 |
Simple data compression, about 2:1 |
| MNP6 |
Statistical duplexing and
Universal Link Negotiation; with V.29, modems can emulate
full-duplex operation; also supports fallforward operation
between two MNP modems |
| MNP7 |
Data compression, about 3:1 |
| MNP8 |
MNP7 for pseudo-duplex modems |
| MNP9 |
Data compression, about 3:1;
includes V.32 technology |
| MNP10 |
Dynamic fall-back and fall-forward
adjusts modulation speed with link quality; intended for use
with cellular technology |
Compression algorithms typically
require error-correction algorithms, so V.42bis and MNP 5 compression
usually run over LAPM or MNP 4 correction. 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 is rarely
achieved.
How well the modem compression works
depends on the kind of files being transferred. In general, you will
be able to achieve twice the speed for transferring a standard text
file (like the one you are reading right now). Decreasing by 50
percent means that you can double the throughput on the line so that a
9600-bps modem can effectively transmit 19200 bps. However, V.42bis
and MNP5 modems cannot compress a file that is already compressed by
software. In the case of MNP5, it will even try to compress a
precompressed file and actually expand it, thus slowing down the file
transfer! If you have ever downloaded files from a BBS or online
service, you know that almost all files are in a compressed format.
The modern data-compression technique is analogous to the video-compression or disk-packing algorithms used in computers. The compression efficiency is highly dependent on data content. Some data (such as ASCII files) compress readily, whereas others can be compressed only a little.
Although some application software
supports data compression, it is usually better to let the modem
compress transmitted data. Data-compression algorithms that run in
modem hardware are faster than those performed by host software. If
two modems have agreed on V.42bis compression, you need to turn off
the compression capability of the application. This means transferring
data at a higher speed on the interface between the DTE and the DCE.
Confusion often arises between the DCE-to-DCE modulation speed and DTE-to-DCE speed.
DCE-to-DCE represents how fast the modems communicate with each other
across the telephone network. DTE-to-DCE represents how fast your
computer communicates with the attached modem.
To gain full benefits from compression in an ideal situation, the DTE (for example, a PC) must send to the DCE (for example, a modem) at speeds matching the potential compression ratio (as shown in Figure
).
However, the EIA/TIA-232 serial interface (COM port) that is widely
found on PCs and some Macintosh computers might operate considerably
slower than the full potential of V.34. The problem is that some PCs
and Macs use the EIA/TIA-232 serial interface with a combination of
UARTs and character-oriented communications software packages, which
are not reliable at higher data rates. In a PC, DTE should be set to
clock the modem at its fastest rate to take advantage of compression.
An improperly configured modem might
automatically adjust DTE-DCE speeds to match the established DCE-DCE
speeds. This is often called speed mismatch. To avoid speed mismatch,
you must lock the DTE-DCE speed so it remains constant, as originally
configured. This speed-locking mechanism is called speed conversion
(also known as port-rate adjustment or buffered mode).
Figure shows the maximum theoretical speeds that are possible for selected modem-modulation standards. Also shown are the possible speeds if V.42bis compression is used with the same standards.
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