Mnp Handshaking; Data Compression - US Robotics COURIER User Manual

COURIER HIGH SPEED MODEMS
CC/TT V.42 Handshaking
This international standard includes a two-stage handshaking
process:
• A Detection phase that is based on an exchange of
predefined characters.
• LAPM (Link Access Procedures for Modems) Negoti-
ation. In this phase, the modems identify their
capabilities concerning maximum data block size and
the number of outstanding data blocks allowed before
an acknowledgment is required.
MNP Handshaking
This protocol is supported by the CCITT V.42 Recommendation.
It was originally developed by Microcom, Inc. and is now in the
public domain.
MNP is based on special protocol frames. If the remote modem
doesn't recognize an MNP Link Request, error control isn't
possible. (In HST asymmetrical mode, USRobotics modems use a
proprietary scheme similar to MNP.)
Data Compression
If the modems successfully establish a V.42 connection, they also
negotiate for V.42 bis data compression. If they successfully
establish an MNP connection, they negotiate for MNP5 data
compression. The type of compression for a call, if any, is
reported in the ATI6 display.
Modems using V.42 bis compression negotiate the following
options and report them in the ATI6 display.
• Dictionary size, that is, the amount of memory available
for compression table entries. (Entries are codes devised
for redundant data. The data is packed into shorter data
units, called code words, and unpacked by the receiving
modem.)
Possible sizes are as follows. USRobotics modems use
11-bit, or 2048-entry dictionaries, but drop down if the
A-6 Link Negotiation and Error Control
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