COURIER HIGH SPEED MODEMS
1.
Optimal
throughput
is attained
under
the
following
conditions:
• The communications software allows fixing
the
DTE rate
higher
than
the link rate, e.g., setting the software to
38.4K
or
19.2K
bps
and
setting
the
modem
to &Bl.
If
the
software automatically switches bit rates to follow
the link rate, the
modem's
DTE rate
must
be
also set to
follow
the
link rate for each call,
&BO,
and
throughput
will
be
limited.
Installations
with
specialized software
may
want
to
enable a fixed DTE rate for ARQ calls
and
a variable
DTE rate for non-ARQ calls. See
the
&B2
command
in
Chapter
4.
• The call is
under
data
compression.
• The
data
is
comprised
of text files
rather
than
binary
files
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such
as
.EXE
or
.COM files. See
the
table
at
the
end
of -
this appendix.
2.
MNPS compression is disabled for files
that
are
already
compressed
and
8-bit
binary
files,
that
appear
to the
modem
to
be
already
compressed. MNPS is disabled
by
setting
the
modem
to &K3.
3.
The file transfer is
not
slowed
down
by
a file-transfer
protocol.
Many
non-text files require a file transfer protocol,
but
the
results vary. For example, certain public
domain
file
transfer protocols
have
the
following effects:
Kermit
Xmodem
Ymodem
Throughput
is severely
reduced
due
to
Kermit's
short
block lengths
(under
128 bytes)
and
acknowledgment
turn-around
time.
Throughput
may
be
reduced
if
your
version
uses
short
block lengths (128 bytes). Some
versions
user
larger blocks
(lK
blocks).
Throughput
is also
reduced
by
overhead
(error
control protocol information).
There is
an
improvement
over
Xmodem,
due
to
larger block lengths
(lK
bytes),
but
throughput
is still
reduced
by
the
protocol's
error
control
overhead.
A-8 Link Negotiation and Error Control
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