gt5302b@prism.gatech.EDU (R. Steve Walker) (04/06/90)
The noise is probably caused by the telephone switching equipment used by your local phone co. I'm not sure why the problem is one-way, but I would guess the noise only affects the originator of the call. There is a solution to your problem. BallCo, Inc makes a modem noise filter and telephone surge supressor for your modem. I've had one for a year and it works great. Link-Up Magazine is supposedly reviewing the product in their next issue. Mine cost $29.95 and BallCo's number is (404) 979-5900. 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] 894-7161[W] gt5302b@prism.gatech.edu (unix) [404] 874-1886[H] 71021.1544@compuserve.com (cis)
cpcahil@virtech.uucp (Conor P. Cahill) (04/07/90)
In article <1990Apr5.011827.6918@mccc.uucp> pjh@mccc.uucp (Pete Holsberg) writes: >OK - I can get rid of the screen garbage. But suppose the garbage >occurs when I am composing a message. Why don't any of the garbage >characters get into that message file? Because the "noise" is occuring in only one direction. When you get noise generated garbage it can occur in one direction, the other, or both. In your case it seems that you are only getting noise in the host to terminal direction. -- Conor P. Cahill (703)430-9247 Virtual Technologies, Inc., uunet!virtech!cpcahil 46030 Manekin Plaza, Suite 160 Sterling, VA 22170
jeff@tc.fluke.COM (Jeff Stearns) (04/13/90)
In article <7718@hydra.gatech.EDU> gt5302b@prism.gatech.EDU (R. Steve Walker) writes: >The noise is probably caused by the telephone switching equipment used >by your local phone co. I'm not sure why the problem is one-way, but >I would guess the noise only affects the originator of the call. > >There is a solution to your problem. BallCo, Inc makes a modem noise >filter and telephone surge supressor for your modem. I've had one for >a year and it works great. Link-Up Magazine is supposedly reviewing >the product in their next issue. Mine cost $29.95 and BallCo's number >is (404) 979-5900. > >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] 894-7161[W] gt5302b@prism.gatech.edu (unix) >[404] 874-1886[H] 71021.1544@compuserve.com (cis) There are many phenomena than can disrupt modem communication. The more interesting ones aren't going to be solved with surge suppressors. For example, phone calls are typically digitized and sent at approximately 64kb per second. A group of calls all headed for the same central office are bundled into a 1.5Mb bit stream and sent over a single set of wires. This allows many calls to be carried on fewer wires than if they all went separately. The digital encoding and decoding scheme was craftily designed by clever engineers to allow copious bit errors on the digital stream without causing significant degradation of the audio quality at the earpiece. The human ear is surprisingly forgiving, and the engineers took full advantage of its characteristics. When the digital data is transmitted from one central office to another, the transmitter and receiver may be out-of-synchronization in such a way that the digital bit stream is corrupted randomly or at periodic intervals, say once per second. Your ears will not hear this. A modern modem, however, is sensitive to both amplitude and phase, and it cannot tolerate signal corruption that your ears cannot even hear. Depending on the signalling method used by your modems, the corruption will show up in various ways. On a typical 2400 baud modem, it's common for the modem to misinterpret this type of glitch as a ~ or {. Some PBX's compress the digital signal so that even more conversations can be carried in one group. This compression can be very hostile to modems, which depend on signal characteristics that get altered by the compression. Phone calls are also subject to a certain amount of echo. This arises from the fact that we're trying to carry a bidirectional call on a single circuit. Modems are built with circuitry to compensate for the fact that they will hear themselves transmitting. This correction cannot be perfect. Depending on the qualities of the local line and the particular connection, it can make it very difficult for a modem to hear the far end without getting interference from its own signal. There are many other forces that conspire against modems. I'm wary of blanket statements that pronounce simple over-the-counter cures. If solutions were this easy, modem manufacturers would have stuck the magic circuits into their modems long ago. -- Jeff Stearns John Fluke Mfg. Co, Inc. (206) 356-5064 jeff@tc.fluke.COM {uw-beaver,microsoft,sun}!fluke!jeff