Path: utzoo!utgpu!news-server.csri.toronto.edu!rpi!usc!elroy.jpl.nasa.gov!decwrl!hayes.ims.alaska.edu!floyd From: floyd@ims.alaska.edu (Floyd Davidson) Newsgroups: comp.dcom.modems Subject: Re: "Loud Carrier"?!? (was Re: Telebit vs. Intel) Message-ID: <1991May28.124617.29257@ims.alaska.edu> Date: 28 May 91 12:46:17 GMT References: <1991May26.044327.5598@clark.edu> <1991May26.231210.14794@netcom.COM> Organization: University of Alaska Institute of Marine Science Lines: 56 In article <1991May26.231210.14794@netcom.COM> gandrews@netcom.COM (Greg Andrews) writes: >In article <1991May26.044327.5598@clark.edu> steve@pro-hindugods.cts.com (Steve Fenwick) writes: >>What _is the advantage of a louder carrier? > ...there is no such thing for dialup lines. ... > >Over leased lines, the advantage of boosting the output level is the same >as when you shout on a noisy phone call. Your louder talking helps the >other person hear your voice over the noise. If the phone line makes the >modem signals too faint, they will be lost in noise when they get to the >other end of the line. Boosting the output can help get the modem signals >louder than the noise so the other modem can hear them properly. The output level on dial-up modems is almost always fixed at about -10 Dbm (within a couple db). That is because the normal line loss for a dial-up line is estimated at about 3 dB, and the desired level for a modem is -13 dBm at the central office switch (at a point where a test tone would be 0.0 dBm). Since most dial-up modems have a very good dynamic range, there is no practical effect if the level is plus or minus 3-4 dB from the -13 dB design figure (as could happen if you live very close or very far from the telco). Leased line modems can usually be adjusted for just about any output level from -21 to +5 dBm, but the level that is actually used is carved in stone by the circuit engineer who specifies the various equipment that is on the circuit at different places. The level out of the modem depends on what equipment the customer has and what kind of an interface the phone company has. But the result is always the same: At any point along the circuit where a full level 1 KHz tone would be 0.0 dBm, the modem level is going to be -13 dBm. The actual levels vary, of course, but the 13 dB difference is set (there are some very few exceptions). If the modem output is less than the design level, the signal to noise ratio is worse, and the result is as Greg described. If the modem output is too much, it can cause distortion as it passes through equipment at the telco (analog to digital converters, analog or digital carrier systems, etc.). Almost everything the telco puts in a circuit is designed to either clip or compress anything that is too "loud". The resulting distortion will severely interfere with the data transmitted. And any testboard technician who measures a modem tone that is more than 3 dB "hot" is going to plug the circuit off until the level is corrected. Thus a "louder" modem, even a leased line modem, really doesn't exist. One modem may have a couple dB or so higher level than another, but that small a difference doesn't affect the data. Floyd -- Floyd L. Davidson | Alascom, Inc. pays me, |UA Fairbanks Institute of Marine floyd@ims.alaska.edu| but not for opinions. |Science suffers me as a guest.