US Robotics COURIER User Manual page 109

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
-
-
-
-
-I
-
-
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
-
-
-
-
-
-
-
-
-