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COURIER HIGH SPEED MODEMS
Link Negotiation and Error Control A-5
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 ITU-T 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, U.S. Robotics 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, and in the CONNECT message if
the modem is set to &A3.
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.)