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CHAPTER 4: Operation
The result? Your personal computer
can talk to the Tote-A-Fax 542
modem at high speeds (that is, your
PC can feed data to the modem in
large quantities) and yet your
modems can still talk to each other
at 2400 bps. And the data travels
from one computer to the other
computer without errors.
4.7.2 CCITT V.42, V.42
BIS
The same scenario applies when the
V.42 or V.42 bis standards are
applied to the data exchange
between DTE and modem. The
V.42 specifications formally
standardize and enhance the error
correction functions that make up
MNP Levels 2–4 (MNP protocols
have been, until recently, the
de facto
industry standards).
CCITT V.42 is now an internation-
ally recognized standard for error
correction. Similarly, CCITT V.42
bis is the new accepted standard for
data compression techniques; V.42
bis roughly corresponds to MNP
Level 5. One big difference is in the
amount of data compressed: V.42
bis generates about a 4:1 ratio of
data compression, while MNP Level
5 generates a 2:1 ratio of data
compression.
NOTE: The extent of compression is always
dependent on the type of file that is
being transferred.
Finally, no matter which data
compression and error correction
protocols you apply to your data
transmissions, a vital factor for the
overall success of your data trans-
missions will be due to the
quality
of the connection made over dialup
telephone lines between your modem
and the remote modem
.
4.7 Data Compression and Error
Detection for the Modem
This section explains the concepts
behind a set of data protocols (MNP
Levels 1–5) and a set of internation-
al standards (V.42/V.42 bis), both of
which involve the application of data
correction and compression
techniques.
4.7.1 MNP L
EVELS
1—5
The Microcom Networking Protocol
(MNP) doesn’t apply to data
transmission
between
modems; rather,
they apply to data exchanges
between
a DTE and modem
.
What does this mean to the user of a
modem that supports these proto-
cols? You can send data from your
DTE (PC, data terminal, etc.) to the
modem at speeds higher than 2400
bps. The modem then implements
the MNP 1–5 protocols, which
compress the data using an encod-
ing scheme, and checks the data for
errors, corrects the errors, and sends
the data over the phone line.
The transmission is still, for
example, 2400 bps. But the data has
been encoded and compressed
(eliminating “wasteful” data), so the
amount of data sent represents more
than the 2400 bps.
The remote modem that receives the
signal does the following: it
“rebuilds” or (decompresses) the
data, checks it for errors, corrects
the errors, and funnels the data to
the remote DTE (computer).
NOTE: Data correction/compression
techniques are applied to
asynchronous data only.
