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COURIER HIGH SPEED MODEMS
A-6 Link Negotiation and Error Control
Possible sizes are as follows:
Bits Entries
9 512
10 1024
11 2048
U.S. Robotics modems use 11-bit, or 2048-entry
dictionary, but drop down if the remote modem uses a
9- or 10- bit dictionary. The size of the dictionary for a
call is reported in the ATI6 display.
• Maximum string length of each entry. As the dictionary
fills, the modem deletes the oldest unused strings.
V.42 bis compression is more efficient than MNP5 compression
in part because it dynamically deletes entries that are no longer
used. In addition, it works better with files that are already
compressed. These include .ZIP files downloaded from many
Bulletin Boards and 8-bit binary files, which seem to the modem
to be compressed.
MNP5 compression should not be used with such files because
it adds data to them, which lessens throughput. (The additional
data is stripped when the file is decompressed by the remote
modem.) When transferring such files, it's best to set the
modem to &K3: this allows V.42 bis compression to work
dynamically with the compressed data, but disables MNP5.
Flow Control
Flow control of data from the computer is required under error
control for two reasons:
1. The transmitting modem buffers a copy of each frame it
transmits to the remote end until it is acknowledged by the
receiving modem.
2. If errors are encountered, retransmission activity can cause
a steady stream of data from the computer to overflow the
buffer.