mulligan@coral.bucknell.edu (03/19/91)
The quest continues. . .
First of all, I'd like to send a *BIG* thank-you to everyone who sent me
a response to my questions about modem line noise and call waiting. Many
people told me to just switch off the call waiting before I dial. However,
I'm on Bucknell University's private phone system, and there is no such code.
I called the phone center and the lady there told me that shw couldn't help,
"but she'd ask the technicians and get back to me if she found out anything."
So, while I'm breathlessly waiting for a reply (yeah, right) I'm back to square
one.
On to the line noise. The noise affects only ingoing or outgoing data, not
both at once, since my modem is running half-duplex. I can't change modems,
since I'm not on a PC, but a (very) dumb terminal - the modem must have the
communications software built-in. I did get a schematic for a filter, which
I'll try after spring break. I'm starting to think that it is something that
I'll}i just have to live with.
^
| this is an example of the junk I get!
Anyway, thanks again to all who helped out! Anyone have anything else to try,
or any VT100 info (I've got an owner's manual, that's it) let me know - I love
to get mail! 8^)
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Ed Mulligan Support your local E.E. student,
mulligan@coral.bucknell.edu give to the Ed Mulligan college fund!
km3t@jjmhome.UUCP (Dave Pascoe) (03/19/91)
In article <1991Mar18.152432.3078@coral.bucknell.edu>, mulligan@coral.bucknell.edu writes: > The quest continues. . . > > On to the line noise. The noise affects only ingoing or outgoing data, not > both at once, since my modem is running half-duplex. I can't change modems, > since I'm not on a PC, but a (very) dumb terminal - the modem must have the > communications software built-in. I did get a schematic for a filter, which > I'll try after spring break. I'm starting to think that it is something that > I'll}i just have to live with. I've had the same basic problem with line noise. In my case, noise comes from only one direction as well. And this makes sense when you think about it. It should happen that way, unless the noise is generated locally, which is very seldom the case. I would suggest that you talk to the local telephone company, not your campus telephone people. Chances are that the problem lies in one of the telephone company circuits. I've seen this happen in several cases of computer line noise. I would dial from my exchange to one particular exchange over and over and line noise was present. I could dial other exchanges outside the central office where the noise was and had no problems. It turns out that the noise was found by an astute technician at the remote central office. The likelihood of finding a technician who can find stuff like this is small, but it is possible. The key is to give the phone company as many clues about the problem as possible. The problem, in this case, turned out to be noise on one of the digital trunk lines between central offices. Digital lines, by definition, don't have any noise per se, but the converter equipment at each end of the line can generate noise. That's apparently where the problem was. So, call the phone company! But be prepared to wade through a ton of customer service people who won't have the slightest idea what you're talking about. Just try to be nice.... -- Dave Pascoe | Internet: km3t@jjmhome.m2c.org KM3T | UUCP: km3t@jjmhome.UUCP
elliott@optilink.UUCP (Paul Elliott x225) (03/22/91)
In article <9878@jjmhome.UUCP>, km3t@jjmhome.UUCP (Dave Pascoe) writes: [ noise on phone lines mangles modem data... ] > The problem, in this case, turned > out to be noise on one of the digital trunk lines between central offices. > Digital lines, by definition, don't have any noise per se, but the converter > equipment at each end of the line can generate noise. > That's apparently where the problem was. (I haven't really been paying attention, and don't know what's already been covered, but...) The digital trunk noise referred to above is probably due to a problem called "frame slip". While it is possible that the noise was injected into the analog portion of the interface, a common problem occurs where the digital bit rates at both ends of a digital trunk are not exactly synchronized. This equipment uses buffers to "soak up" short- term frequency variations in the digital signal (called wander and jitter). If there is a long-term frequency difference, the buffers overflow or underflow, causing a time/phase shift in any signal being transported. This is called a frame slip. Buffers are commonly sized to hold one or two "frames" of data, a frame being 125 microseconds (8 Khz frame rate). When the buffers wrap (over/underflow), this can be sometimes be heard as a small "click" on a voice call. Most modems use some form of PSK (Phase Shift Keying), and the frame slip imposes a spurious phase transition on the modem signal, commonly generating the '{' character in the case of the Bell 212 1220 baud modem (or is it '}' ?). The slip rate is usually less than on per second. This frame slip problem was often ignored by the phone company, especially for voice traffic. It is common practice now to synchronize the digital trunks, but the synchronizatin heirarchy can be improperly configured. It can take some persistance to get the phone company to admit/recognize the problem and fix it. I have no experience in this aspect of it, I just design the things. -- Paul M. Elliott Optilink Corporation (707) 795-9444 {uunet, pyramid, pixar, tekbspa}!optilink!elliott "If I had known it was harmless I would have killed it myself." - P.K. Dick
gt5302b@prism.gatech.EDU (R. Steve Walker) (03/27/91)
There are companies that sell modem noise filters that "claim" to fix 99% of noise problems. One company that sells them is BallCo (404) 979-5900. They have low prices and a money back guarantee. Good luck! Richard S. Walker Georgia Tech Research Institute GA Tech Box 35302 SWALKER@gtri01.gatech.edu (vm) Atlanta, GA 30332 swalker@vms62a.gatech.edu (vms) [404] 874-1886[W] gt5302b@prism.gatech.edu (unix) [404] 607-0958[H] 71021.1544@compuserve.com (cis)