COURIER HIGH SPEED MODEMS
3.
You're
transmitting
text files.
Throughput
is
higher
for text
files
than
for
other
types
of files,
such
as .EXE
or
.COM
binary
files. For a
comparative
table, see
the
end
of
Appendix
A.
4.
File
transfer
is
not
slowed
down
by
a file-transfer protocol.
Many
non-text files
require
a file
transfer
protocol,
but
throughput
result
vary.
Certain
public
domain
file
transfer
protocols, for
example,
have
the following effects:
Kermit
Throughput
is severely
reduced
due
to Kermit's
short
block
lengths
(under
128 bytes)
and
acknowl-
-
-
-
-
-
edgment
tum-around
time. -
Xmodem
Throughput
may
be
reduced
if
your
version
uses
short
block lengths, for
example
128 bytes.
Some
versions
use
blocks of 1K bytes,
which
is
much
better,
although
overhead
(error control
protocol
information) still affects overall
throughput.
Ymodem
This protocol is
better
than
some
because
of
larger
block
lengths
(1K bytes),
although
overhead
(error
control protocol information) still affects
overall
throughput.
The
above
protocols
further
reduce
throughput
during
error
control (ARQ) connections. The accuracy of
the
data
is
checked twice,
by
the
file
transfer
protocol
and
the
modem.
To
avoid
redundancy,
use
the
above
protocols
only
for
non-
ARQ connections,
and
only
at
speeds
of 2400
bps
and
lower.
For
the
best
throughput,
but
on
ARQ connections
only
and
with
hardware
flow control,
we
recommend
the
most
current
version
of Zmodem.
Overhead
is
minimal
with
this
protocol,
with
throughput
almost
equal
to
that
obtained
with
no
file-transfer protocol. Leave
the
modem
at
its &M4
and
&Kl settings for
both
error
control
and
data
com-
pression.
Ymodem-G is
another
good
choice,
but
only
on
error-controlled calls,
because
Ymodem-G
aborts
the
connection if it detects
an
error.
WARNING:
If
you
are
using
an
X,
Y
or
Zmodem-type
protocol,
do
not
use
the
modem's
software
flow control. See
the
more
detailed
warnings
in
this
chapter
under
Transmit
Data
Flow
Control
(&Hn)
and
Received
Data
Software
Flow
Control
(&In).
4-4 Interface Controls
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