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tries to “negotiate” error correction and data compression protocols with the other modem. Your
modem tries protocols in the following order: first, error correction; then data compression, and
finally, neither error correction or data compression. The protocols used are the most powerful
protocols that both modems support.
Because your modem supports so many rates and protocols, the negotiation process can take several
seconds to complete. If it seems to take an extremely long time, you may be having line noise
problems. Refer to “Problems with High Speed Connections” in Chapter 10, Solving Problems.
Once connected, your modem sends your computer a Result Code that tells it the connection rate.
Depending on how your software handles the Result Code, you see either the Result Code on your
screen, or you see your software’s interpretation of it. Also depending upon how the modem is set,
the Result Code will show the connection rate between the two modems (DCE), or the rate at which
the computer is communicating with the modem (DTE).
E
RROR
C
ORRECTION
AND
D
ATA
C
OMPRESSION
Your modem features error correction and data compression protocols that allow you to transmit
data fast and error-free.
Error Correction
For error correction, the modem breaks your transmission down into blocks of data (frames) and
calculates a checksum (the sum of all data bytes in the block). The block and its checksum are
transmitted. The receiving modem calculates a checksum on the received data block and compares
it with the received checksum. If these numbers are different, the data block was not transmitted
properly, and the receiving modem instructs the transmitting modem to resend the data.
Error correction is necessary for high speed communications. However, error correction will not
perform miracles on extremely noisy lines. If you have lots of errors, you may need to have your
telephone circuits checked or change long distance carriers.
Data Compression
To accomplish data compression, the sending modem analyzes the outgoing data for often repeated
patterns (such as the letters “A” and “E” in English text). It then assigns a shorter data code that
transmits faster than the original data. The more often the same data code occurs in the
transmission, the better compression the modem can achieve.
Certain file types such as text files and graphics screens are good candidates for optimal
compression. Files that have already been compressed with a program like Stuffit, PKZIP, or ARC
may see little or no performance improvement, because the files are already compressed.
Error Correction and Data Compression Standards
The error correction and data compression standards your modem supports include MNP 2-4 and
V.42 for error correction, and MNP 5 and V.42bis for data compression. MNP 5 offers a compression
rate of up to 2:1. MNP 5 connections also use MNP error correction.
