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Advanced Modem Concepts
Data Compression
Data compression minimizes the amount of data sent over the
telephone line by removing redundant information, thereby
compressing the data it receives from the computer. To do this,
the modem uses the V.42bis or MNP 5 data compression
protocols. Once a file is compressed, it can be sent faster
across the telephone line since there is less data to send. Once
the file is received by the other modem, the file is decompressed
to its original form before being sent to its attached computer.
The amount of data compression achieved is dependent on the
type of file being transferred. The more repetitive the data, the
more compressible it is.
Data compression is useful when transferring uncompressed
files. However, if the MNP protocol is used, and the file has
been compressed by PC software, such as PKUNZIP
the transfer, then data compression should be turned Off. Use
the %C command to turn data compression ON or Off.
The format for the command which controls data compression
is:
TYPE:
Flow Control
Flow control is a method of stopping and starting data flow
between two devices. This prevents one device from accepting
so much data from another that its buffer (a temporary storage
area) fills and then overflows resulting in data loss or errors.
Flow control can occur between a modem and computer or
between two modems or between two computers. The following
sections use Figures D-1 and D-2 to help illustrate various
methods of flow control.
AT%C n
where n is 0 for disable and 1 for enable.
Issue 1 November 1994
r
, prior to
D-3
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