3Com Courier Reference page 106

13-10 C 13: H
HAPTER ANDSHAKING
, ,
ERROR CONTROL DATA COMPRESSION
Installations with specialised software may want to enable a fixed
serial port rate for ARQ calls and a variable serial port rate for
non-ARQ calls. See the &B2 command in Appendix B, Alphabetic
Command Summary.
The call is under data compression.
The data is made up of text files rather than binary files such as .EXE
or .ZIP files
MNP5 compression is disabled for files that are already compressed,
and 8-bit binary files that appear to modems to be already
compressed. Disable MNP5 compressing by sending the Courier
V.Everything modem AT&K3.
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 the best throughput on error-controlled connections only and with
hardware flow control we recommend the most current version of
ZMODEM file transfer protocol.
For example, certain public domain file transfer protocols have the
following effects:
Public Domain
Kermit
XMODEM
YMODEM
The protocols listed above further reduce throughput when an error
control connection is established. The accuracy of the data is checke
both by the file transfer protocol and the Courier V.Everything
modem. To avoid redundancy, use the above protocols only for
non-ARQ connections, and only at speeds of 2400 bps and below.
,
AND THROUGHPUT
Effects
Newer versions support packets up to 9K and a sliding
window design to eliminate turnaround delay. With earlier
versions, however, throughput may be severely reduced
due to short block lengths (possibly under 128 bytes) and
acknowledgment turnaround time.
Throughput may be reduced if your version uses short
block lengths (128 bytes). Some versions use larger blocks
(1K blocks). Throughput is also reduced by overhead (error
control protocol information).
There is an improvement over XMODEM, due to larger
block lengths (1K bytes), but throughput is still reduced by
the protocol's error control overhead.