USRobotics V.34 user manual Throughput Guidelines

COURIER HIGH SPEED MODEMS

Throughput Guidelines

The following guidelines should help to make the most of the modem's advanced performance features. In many instances, experimentation and experience will indicate what works best for your applications.

1.Optimal throughput is attained under the following conditions:

•The communications software allows fixing the serial port rate higher than the connection rate, by setting the software to 115.2K, 57.6K, or 38.4K bps and setting the modem to &B1.

If the software automatically switches serial port rates to follow the connection rate, the modem's serial port rate must be also set to follow the connection rate for each call, &B0, and throughput will be limited.

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 Chapter 4.

•The call is under data compression.

•The data is comprised of text files rather than binary files such as .EXE or .COM files. See the table at the end of this appendix.

2.MNP5 compression is disabled for files that are already compressed, and 8-bit binary files that appear to the modem to be already compressed. MNP5 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	Newer versions support packets up to 9K and
	a sliding window design to eliminate turn-
	around delay. With earlier versions, however,
	throughput may be severely reduced due to
	short block lengths (possibly under 128 bytes)
	and acknowledgment turnaround time.

Link Negotiation and Error Control A-7