3Com 56K manual Public Domain Effects

12-10CHAPTER 12: HANDSHAKING, ERROR CONTROL, DATA COMPRESSION, AND THROUGHPUT

Installations with specialized 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 Business 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	Effects
Kermit	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.
XMODEM	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).
YMODEM	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.

The protocols listed above further reduce throughput when an error control connection is established. The accuracy of the data is checked both by the file transfer protocol and the Business Modem. To avoid redundancy, use the above protocols only for non-ARQ connections, and only at speeds of 2400 bps and below.