Our company specializes in satellite communication, and I was pleased to see that your July Lab Report on modems included satellite channels. You were right about the fastest modem on impaired lines, the Motorola Codex 3265 V.Fast. However, to exploit its capabilities, you need special software.
We have found ZMODEM to be better than XMODEM on satellite channels, and to overcome the inherent limitations of ZMODEM, we have developed our own protocol. One of the characteristics of satellite channels is that the signal-to-noise ratio can be very good for 20 seconds and then drop sharply for 500 milliseconds and then bounce back. Normal software/modem combinations waste time when they react by falling back to a slower speed and then picking up at a faster speed.
Our software treats signal degradation as a ``blank.'' It waits for the signal degradation to go away an
d then resumes at the initial speed. Our system also assumes that ACK (acknowledge) packets will take a while to reach their destination; it will continue to pump data and recover missing frames later. Modems must also be capable of supporting output-level adjustment. We achieve throughputs of 2000 characters per second; a V.32bis modem and XMODEM will not exceed 700 cps. With a fleet of five vessels, a shrewd shipowner can save $50,000 per year.
Frank Guinard
Arka Ltd.
London, U.K.
Flexible C++
Matthew Wilson
My approach to software engineering is far more pragmatic than it
is
theoretical--and no language better exemplifies this than C++.
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Satellite Savvy
