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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