Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP Path: utzoo!utgpu!water!watmath!orchid!clyde!rutgers!mit-eddie!genrad!decvax!ucbvax!kitty.UUCP!larry From: larry@kitty.UUCP.UUCP Newsgroups: comp.dcom.telecom Subject: Submission for mod.telecom (Modems and data jacks) Message-ID: <8704111730.AA04601@seismo.CSS.GOV> Date: Sat, 11-Apr-87 12:30:30 EST Article-I.D.: seismo.8704111730.AA04601 Posted: Sat Apr 11 12:30:30 1987 Date-Received: Sat, 18-Apr-87 00:28:24 EST Sender: daemon@ucbvax.BERKELEY.EDU Distribution: world Organization: The ARPA Internet Lines: 72 Approved: telecom@xx.lcs.mit.edu In a recent article howard@cos.UUCP (Howard Berkowitz) writes: > One minor point (minor until it gets you): dial modems, > especially the higher speed ones, may not work properly > because the network interface device has resistors set > for voice use, not data. It is worth letting your telco > know you are running higher speed data, because your > network interface should then be configured to give you > better signal to noise ratios, not important for voice. > > If they're going to charge more for data access, do this > only after you have unacceptable problems. Your problems, > however, often are in the jack, not the local loop. The above article raises a good point, but it is not quite accurate. First, let's talk about data jacks. There are two types of data jacks with resistors: 1. The Fixed Loop Loss (FLL) jack, such as the RJ41S (RJ42S and RJ43S for A-lead control where data lines terminate in key equipment). This jack provides direct tip and ring access through pins 5 and 4, respectively. It provides an attenuated tip and ring access through pins 2 and 1, respectively. It is intended that voice signals utilize the direct line connection, while data signals utilize the attenuated line connection. The value of the attenuator pad will be set by the telephone company according to loop loss so that the effective loss (total of loop loss and pad loss) between the jack and the central office will be between 8 and 9 dB. The net result is that a 0 dBm data signal sent by the modem will reach the central office at -8 to -9 dBm. So the point is: the line connection through the resistor pad is for DATA use, and NOT for voice use. 2. The Programmable Loss jack, such as the RJ45S (RJ46S and RJ47S for A-lead control where data lines terminal in key equipment). This jack provides direct tip and ring access through pins 5 and 4, respectively. A resistor installed by the telephone company across pins 7 and 8 (leads PR and PC) tells the modem what output level to send. An infinite resistance tells the modem to send at -9 dBm; 19,800 ohms sends at -8 dBm; 9,200 ohms sends at -7 dbm; 5,490 ohms sends at -6 dBm; ...; and a direct short sends at 0 dBm. This resistor is sensed by internal modem circuitry, and has nothing to do with direct telephone line tip and ring. The telephone company will set the programming resistor such that the transmitted data signal will enter the central office at between -8 and -9 dBm (i.e., modem transmit level + loop loss = -9 dBm). Telling the telephone company that you are running "higher speed" data will most likely accomplish nothing. The telephone company could care less about your data rate or noise concerns (at least for the price of a POTS line); all the telephone company cares about is that your transmitted data signal reaches the central office at between -8 and -9 dBm. Period. Speaking candidly, if you feel compelled to send data at a greater transmit level, you can defeat any FLL or Programmable data jack and send at 0 dBm - regardless of actual central office loop loss. While I don't want to get off on a tangent here, the fact is that transmitting at a higher level is NOT necessarily going to result in a lower error rate. I can vouch for this from extensive personal experience. Most of the newer modems have receive threshholds of < -40 dBm; this is really quite a bit of sensitivity. Transmitting at a higher-than-necessary level can "strain" the ability of the band-pass filters in the modem to reject the locally-generated transmit signal, and leakage of the transmit signal into the modem receiver can often exacerbate a data line error situation. So the moral is: LOUDER is not always BETTER. :-) <> Larry Lippman @ Recognition Research Corp., Clarence, New York <> UUCP: {allegra|ames|boulder|decvax|rocksanne|watmath}!sunybcs!kitty!larry <> VOICE: 716/688-1231 {hplabs|ihnp4|mtune|seismo|utzoo}!/ <> FAX: 716/741-9635 {G1,G2,G3 modes} "Have you hugged your cat today?"