minow@decvax.UUCP (Martin Minow) (05/15/84)
The UUUU pattern is an alternating 1010 bit pattern -- which might help you track down the problem. Another problem which I have seen on some cross-country calls is a once-per-second garbage character, regular as clockwork. The best explanation I have seen for this (which might not be correct, of course) is that if the transmission is via a digitized channel, and the sender and receiver clocks get out of synchronization, the transmission-line hardware "steals" an 8-bit datum for resyncronization. This just happens to force a 180-degree phase reversal into the signal, which manifests itself as a garbage character on an otherwise idle line. Sorry if this is inoherentt -- perhaps a more technically sophisiticated reader could explain better. Martin Minow decvax!minow
piet@mcvax.UUCP (Piet Beertema) (05/17/84)
<> >....a once-per-second garbage character, regular as clockwork. >....the transmission-line hardware "steals" an 8-bit datum >for resyncronization. I don't think that's the explanation. At least on international (and maybe too on cross-country) lines a very low pitched note is used for pulse (tick) counting for connect duration timing. The interval between ticks depends on the distance over which the call is made. But an ordinary telephone set can't reproduce such low notes. The only way to hear it is by "tapping" (via a coil) the phone line and reproducing the sound via a Hifi amp. You may ever have noticed it when listening to some direct phone talk on TV (at least in most European countries, where the TV sound is transmitted via FM) or to the (FM!) radio.
jis@hocsd.UUCP (05/24/84)
MESSAGE-ID: <454247417.1194@hocsd> REFERENCE: <4644@amd70.UUCP> <485@decvax.UUCP> <5809@mcvax.UUCP>, <533@dual.UUCP> Even ATT guarantees a synchronized digital trunk only if you are paying for a data grade line. If you are using a modem on a voice grade line you are very likely to get a bunch of unsynchronized digital trunks on a call over a relatively long distance. Jishnu.
paul@dual.UUCP (Baker) (05/29/84)
>>....a once-per-second garbage character, regular as clockwork. >>....the transmission-line hardware "steals" an 8-bit datum >>for resyncronization. >I don't think that's the explanation. At least on international (and >maybe too on cross-country) lines a very low pitched note is used for >pulse (tick) counting for connect duration timing. The interval between >ticks depends on the distance over which the call is made. I have to disagree with this proposition. Although almost all European calls are (very sensibly) charged by "ticks", almost no U.S. calls are charged in this way. Typically one is charged x dollars per minute. For the U.S., where a fair percentage of connections are digital the first explanation is pretty sensible. The Bell System, now ATT, is supposed to be all locked to a master clock generator in some unheard of town in the centre of the country. Alternative carriers such as MCI and SPRINT usually can't be bothered to supply passable voice quality, so niceities like synchronizing all their clocks would never occur to them. In Europe, so that one can have an instant measure of billing, it is common to put a "common mode" signal on the line for every "tick" of charge recorded at the exchange. If there is some imbalance in the connection between the exchange and the telephone, these pulses can be heard as a faint buzzing noise. Paul Wilcox-Baker