USRobotics V.34 user manual Bits Entries

COURIER HIGH SPEED MODEMS

Possible sizes are as follows:

Bits	Entries
9	512

101024

112048

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.

A-6 Link Negotiation and Error Control