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Chapter 10: Principles of Data Communication
Files that have already been compressed using PC-based compression software
such as ZIP or ARC
®
are not particularly suited to further compression, and
will only receive a small percentage increase in throughput time when
transmitted using the V.42bis standard.
To achieve the maximum gain in throughput offered by data compression,
the data-transfer speed between your computer and the ISDN TA/V (data
rate of the computer’s serial port) should be faster than the transfer speed of
the ISDN line traffic. It is recommended that the DTE connection is set to
run as fast as possible, since the data has to be decompressed (back to its
original size) before it is sent on to the computer.
10.1.4 F
LOW
C
ONTROL
The serial data rate (speed of transfer between your computer and the ISDN
TA/V) and the line rate (speed of transfer down the ISDN line) normally
differ in data communications, as outlined in
Section 10.1.3
. Because of this,
a technique of data buffering to prevent data loss during transmission is
required.
Flow control is the method by which your ISDN TA/V and computer tell each
other when they have the capacity to receive more data, and when they have
not—in which case the device which is attempting to pass on the data holds it
within a memory buffer until the receiving device is ready.
Two types of flow control are supported by your ISDN TA/V. The default
setting (hardware flow control) is appropriate when the ISDN TA/V,
computer, and the cable between the ISDN TA/V and your computer support
the RTS and CTS signals. Your ISDN TA/V can also be configured to support
software flow control, where the ISDN TA/V and computer use special XON
and XOFF characters to communicate. Refer to
Section 17.3
for details of the
AT commands which determine the type of flow control used by the ISDN
TA/V.
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