Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!wuarchive!rex!ames!bionet!hayes.ims.alaska.edu!floyd From: floyd@ims.alaska.edu (Floyd Davidson) Newsgroups: comp.dcom.modems Subject: Re: PC mag review comments Message-ID: <1990Dec4.125743.15230@ims.alaska.edu> Date: 4 Dec 90 12:57:43 GMT References: <1990Dec03.011952.15024@mthvax.cs.miami.edu> <28585@usc> Organization: University of Alaska, Institute of Marine Science Lines: 56 In article esmith@goofy.apple.com (Eric Smith) writes: >In article <28585@usc> kjh@pollux.usc.edu (Kenneth J. Hendrickson) writes: > > >> What is the receiver sensitivity? > >Good question. How is this usually specified for modems? > Warning: Boring techie discussion of limited value follows: It may not be specified. I'll quote some figures off the top of my head, which is pointed, so the figures may be twisted. If I remember right most modems have an output of -10 Dbm and most of them claim to function with an input signal down to something like -45 Dbm (300 and 1200 bps modems, I don't know about others). The standard telephone line is expected to have a test tone level of 0 Dbm input and an output of -9 Dbm (sometimes this is set to -6 Dbm). The modem is using a data level of -10 Dbm0. (Ten db down from test tone level. It is interesting to note that leased lines use -13 Dbm0.) The received signal going into the modem would be -19 Dbm (if in fact you have a -9 Dbm receive level for a test tone, which is not likely -- -6 to -12 Dbm is likely). Ok so the modem expects -19, may get as much as -16, and is supposed to work at -45. That is a 29 Db range and most likely if you actually see a level vary over that range the modem will not work for other reasons (phase hits etc.). That is why you are not likely to see anyone bragging about modem sensitivity measurements. It doesn't mean much about how well the modem will handle bad lines. Much more interesting is whether the modem can handle a momentary change of say 6 Db or 3 Db up or down in the signal level. I have done trouble shooting on leased lines where the problem was the levels were 4 Db off and the modem could not handle it at all. That is really poor performance and causes no end of headaches if the end user, or some intermediate carrier, is in the habit of adjusting levels every month or two just to be sure. Once again, I'm not sure the figures I gave above for the modem output or the minimum receive signal are correct. It doesn't really make any difference. Floyd -- Floyd L. Davidson floyd@hayes.ims.alaska.edu Salcha, AK 99714 paycheck connection to Alascom, Inc. When I speak for them, one of us will be *out* of business in a hurry.