vns@mhres.UUCP (03/23/87)
A friend of mine is using uucp as follows and has some questions, has
somebody out there experience with this?
..As you can see, we finally have our uucp link working all the way down
..to our site. We installed an IBM PC/AT in Bogota and have Unix V, rel.
..2 running on it (from Microport). The connection between Bogota and
..Dallas is just with modems and international long-distance calls. But
..the connection between Bogota and us is probably unique. Since
..our only communication from us is via radio, for that connection,
..we are using packet radio controllers over our voice UHF repeater link,
..and it has been working flawlessly for 4 or 5 months. However, it took
..quite a lot of experimenting with the packet controllers to get the
..right combination of parameters.
..
Two questions on uucico and modems:
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IMPROVEMENTS TO UUCICO THROUGHPUT
1. There is about 3 seconds dead time between each file sent. This is
significant in the common situation where mail files consist of one
very short control file in addition to the message itself. If all
spool files could be transmitted as one long file, there would be a
substantial improvement in throughput. This problem also might instead
be addressed with an improved protocol as in 3. below.
2. There has been posted on the network a very good compress program.
Has anyone incorporated it into uucico so that data is automatically
compressed and decompressed.
3. With the use of satellites, international calls have an approximately
.4 second round-trip delay time. Fortunately, uucico appears to send two
packets before waiting for an acknowledgement. At 1200 baud, the two
packets take sufficiently long that there is no pause waiting for
acknoledgements. However, at 2400 baud, we see a pause such that the
maximum effective data rate is about 1500 baud. (Actually, our modems
are capable of an effective 2900 baud throughput because in the error-
correcting mode, the start/stop bits are not transmitted).
4. Ordinary modems are capable of simultaneous transmission in both
directions. Has anyone developed a uucico protocol to do this?
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MODEM PROBLEMS
We have the following problem with our modems. Perhaps you have some
ideas, suggestions, clues or experience with this kind of problem:
We have in operation a uucp link between Bogota, Colombia and Dallas,
Texas, U.S.A. At both ends, we are using Microcom AX/2400 modems with
an error correcting protocol and, with the proper international
parameters set, they work very well when the call originates from
Bogota. If we could originate calls from Dallas, we could save at least
half of the long distance charges, but, we have not been able to get
this to work with our current modems, or with our previous cheap Bell
212A modems.
What happens is that the modem answers in Bogota, but the carriers do
not lock, and so the modems hang up after timing out. We have tried
1200 and 2400 baud, as well as error-correcting and non-error-correcting
modes and different parameter settings on the modems. They all fail.
I suspect that the problem is related to echo suppression. In a normal
voice call, I have observed that the phone company equipment suppresses
echo by simply shutting off the incoming signal while I am talking,
and for the duration of the echo thereafter. Since a modem is
"speaking" all the time, with echo suppression on, the incoming signal
is continuously shut off, hence no carrier connection can be made. To
solve this problem, apparently the CCITT international modem standards
call for a "guard tone" which is supposed to turn off the echo
suppression circuitry of the phone company. It appears that in
answering the call in Dallas, the guard tone is properly recognized, but
answering in Bogota, the guard tone is not recognized. This is borne
out by the fact that once we installed the modem with the guard tone in
Dallas, a cheap Bell 212A non-CCITT compatible modem in Bogota worked
quite well when the calls originated from Bogota.
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--
Real-Name: Gertjan Vinkesteyn, back in Holland
UUCP and other network )\/( in America: ..!seismo!mcvax!mhres!vns
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