COURIER HIGH SPEED MODEMS
they
shift
up
again
to 12K
bps,
14.4K
bps,
and
again
to 16.SK
bps;
or
they
fall
back
to 7200
bps
and
again
to
4SOO
bps,
if
the
remote
modem
is
operating
at
one
of those
lower
speeds.
Initially, the
high-speed
channel
is allocated to the an-
swering
modem,
the 450-bps
channel
to the calling
modem.
This allocation reverses if
the
calling
modem
has
more
data
to
transmit
than
the
answering
modem.
Specialized High-Speed Connect
It's possible for
two
modems
in HST
mode
to connect directly
at
9600, 12K, 14.4K
or
16.SK
bps,
without
2400-bps
handshaking.
However,
this is
only
recommended
for faster connections
in
specialized installations
where
the
computer
and
software
manage
error
control.
The
reason
is
that
there
can
be
no
error
control because HST
modems
negotiate
error
control
during
2400
bps
handshaking.
Both
modems
must
be
set as follows.
High
speed
handshaking
requires a fixed link rate:
the
modem
only
connects if the
remote
modem
is
operating
at
the
same
link
rate-9600
(&N6), 12K
(&N7), 14.4K
bps
(&NS),
or
16.SK
bps
(&N9).
Serial
port
rate: 57.6K/3S.4K/19.2K/9600
bps
(57.6K, 3S.4K,
or
19.2K
required
for 16.SK
or
14.4K connections)
Modem
settings:
Bl,
&Bl,
&Hl
I
&H2/
&H3,
&MO,
&N6/
&N7 I
&NS/
&N9
HST Mode, Error Control Disabled
If
error
control is
disabled-that
is,
the
Courier
HST is set
to
&MO
or
the
remote
modem
isn't set for
error
control
and
the
Courier
HST reverts to
&MO-the
Courier
operates
in
one
of
the
following ways.
1.
If
set to
&NO,
it
only
connects
at
2400
bps
or
lower. This
ensures
that
there
will
not
be
data
transfer
at
high
speeds
unless
the call is
protected
by
error
control.
Courier
HST
modems
negotiate
error
control
during
2400
bps
handshaking.
If
they
cannot
connect
under
error
control,
they
stay
at
2400 bps. Or,
if
the
remote
modem
is
operating
at
a
lower
rate, the
Courier
HST switches to
the
lower
rate.
A-2 Link Negotiation and Error Control
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