zougas@me.utoronto.ca ("Athanasios(Tom) Zougas") (09/02/89)
I have lots of what looks like line noise when running 2400 baud logging into a VAXen running VMS version 4.6. The noise comes out as funny, extended, international characters as well as control characters just popping up. When running 1200 baud, there is no problem. Also, 2400 baud connected to anything else is fine. Which leads me to ask if this just line noise between the two modems, or could it be anything to do with the VAXen? Any suggestions? Thank you, Tom. -- I can be reached at... CDNNET: zougas@me.utoronto.ca | ARPA: zougas%me.toronto.edu@relay.cs.net CSNET: zougas@me.toronto.edu | BITNET: zougas@me.UTORONTO.BITNET dumb: ...!utai!me!zougas | UUCP: zougas@me.uucp
wtm@neoucom.UUCP (Bill Mayhew) (09/06/89)
Why are 2400 bps connections noisier than 1200 bps on the same line? Answer: both 1200 and 2400 transmission rates use the same 300 bps carrier but differ in the number of phase-angle modulation states that the carrier can have. For 1200, there are 4 states (300 * 4 = 1200), and for 2400 there are 8 phase states (300 * 8 = 2400). The If you draw an S-plane map, the configuration is something like: ^ ^ | | | * | * * | * * | * | | ----+----> ----+----> | | * | * * | * | * | * | | Some of the fancier modems such as the AT&T 9600 baud V.32 modem have so-called constellation generators that can be attached to an X-Y mode oscilloscope to generate patterns similar to the above. What you see is actually a jumbly ring in the vicinity of the ideal locations as the bits are recieved. Trouble happens when the jumbly rings overlap; this indicates that there is phase jitter somewhere in the communications network. Jitter can come from poorly equalized lines that have group delay (some frequencies propate faster than others), from digital links that insert administrative packes at irregular intervals, or from a defective defective modem component. The reason that 1200 works wehn 2400 doesn't is that the distance between the centers of the phase states is greater for 1200 than 2400, thus it is easier for the demodulator to make a decision to which phase state a particular buad belongs. I have seen a number of instances where a noise free line at 1200 bps is completely unusable at 2400. Note that in such a case, MNP error correction is of no help, as MNP is really only effective is the noise is sporadic. The axiom of, 'you get what you pay for', applies at least partially to modems. More expensive units often employ more adpative equalizers and tunable filters that can help compensate for group delay and echoes. Often the special features are integrated into a chip and may not be readily apparent upon visual inspection. Often the glossy sales brochures don't provide much real information about a modem's signal processing capabilities. Field testing seems to be the best method for guaging modems' performance. Bill