Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP Path: utzoo!mnetor!seismo!kitty!larry From: larry@kitty.UUCP (Larry Lippman) Newsgroups: comp.dcom.modems Subject: Re: Pending FCC ruling threat to modem users Message-ID: <1496@kitty.UUCP> Date: Thu, 25-Dec-86 00:25:08 EST Article-I.D.: kitty.1496 Posted: Thu Dec 25 00:25:08 1986 Date-Received: Thu, 25-Dec-86 02:48:34 EST References: <1572@brl-adm.ARPA> <3454@curly.ucla-cs.UCLA.EDU> <403@pixar.UUCP> Organization: Recognition Research Corp., Clarence, NY Lines: 120 Keywords: modems, bandwidth, carrier, sex (no, not really) Summary: Arrgh! Misinformation strikes again... In article <403@pixar.UUCP>, aaa@pixar.UUCP (Tony Apodaca) writes: > In article <3454@curly.ucla-cs.UCLA.EDU> stiber@zeus (Michael D Stiber) writes: > >>First: MODEM calls DO NOT cost the phone company the same amount as > >>other calls. For all practical intents and purposes, this statement is INCORRECT. Read on for more details. > >2) Modems use the same lines as voice. The assertion that they tolerate > >noise less well is irrelevant, since they do not get special priveleges. This statement is correct. > I'm sure that this discussion is going hot and heavy in net.dcom.etc but > I'll answer here anyway. Flame off, Mike. The point is true even if the > rationale is messed up. Modem calls DO cost the phone company more, for > several reasons: > 1) They are continuous. The dual carrier never stops. Therefore, > the phone company must supply bandwidth to the call continuously even if > there is no "valid" data. They cannot time-multiplex their signals. > There are small breathing and thinking pauses in all voice conversation, > and 99% of voice is half-duplex, even a teenage girl's conversations. If you are alluding to most telephone calls being statistically multiplexed (for lack of a better simple term) by a process known as TASI (Time-Assigned Speech Interpolation), you are wrong. TASI was first developed by Bell Labs during the 1950's to improve the effective circuit capacity of transoceanic cables where the number of channels was limited, and the per channel cost was very high. TASI does make use of the fact that an intertoll circuit is 4-wire, with separate transmit and receive channels; during normal human conversation each of the two channels is used only about 45% of the time. Therefore, TASI apparatus can almost double the effective channel capacity by assigning a given intertoll trunk to a transmit or receive channel only when speech energy is detected. HOWEVER, TASI apparatus is complex, expensive, and has some objectional human factors (like clipping of syllables). Operating telephone companies have used TASI only for transoceanic circuits, and for some early satellite circuits where TASI was cost-effective. As far as I know, TASI has never been used within the continental U. S. by an operating telephone company. In recent years, there has been a resurgence in interest for TASI apparatus, and a few vendors have offered TASI apparatus for use on private line communication facilities - however, this does NOT apply to use by operating telephone companies for the DDD switched network. So the point is: operating telephone companies do NOT vary the "bandwidth" of telephone call by TASI or other means (with the exception of transoceanic cable use). > 2) They are high bandwidth. The phone line was designed with human > voices in mind, and they are pretty low bandwidth, as everyone knows. Also, > everyone knows that their modems strive to get the most out of it, so they > use it all up (if they didn't you'd buy a new one!). However, the phone > company "counts on" the signals being voice-like, so they can cram as many > signals into one wire as possible, and a modem transmission screws up their > frequency-division multiplexing. This is sick. For all intents and purposes, the bandwith of any dialed telephone call is _limited_ to being well under 4 KHz for several reasons: 1. Many central office subscriber loops are loaded to minimize the effects of capacitive loop attenuation. The result of placing loading coils in the subscriber loop is a distributed L-C low-pass filter which drastically limits bandwith to less than 4 KHz. 2. Frequency-division multiplex apparatus (like N-carrier) effectively limits channel bandwith to less than 4 KHz by virtue of bandpass filters. 3. Time-division multiplex apparatus (like T1-carrier) effectively limits channel bandwith to less than 4 KHz since the T1 sample rate is 8 KHz. 4. Digital central offices effectively limit channel bandwith because they too sample at typically 8 KHz. So the point is: there just ain't no way to achieve more than 4 KHz in bandwidth on the switched DDD network (you're lucky to get 300 to 3,000 Hz). There is no bandwith allocation scheme being used by an operating telephone company that is somehow defeated by a modem. Furthermore, the bandwith required to transmit intelligible speech is the same as is used by any dial-up modem. > 3) The carrier on some modems just happens to overlap a critical > region of part of the phone company's equipment's frequency allocation. > The circuits known as "echo suppression" use it to monitor themselves, and > kick in higher bandwidth and better circuits if there is too much echo on the > line. If they didn't have it, you'd bitterly complain about the quality of > your long-distance connections. Voice doesn't have much of these freqs, > but the carrier flips it out, causing it to allocate too much signal to > your call. Echo supressors do NOT affect channel bandwith. On a voice call an echo suppressor detects speech energy (say on an E-W direction) and inserts a large amount of attenuation (as much as 40 dB) in the opposite direction (say, W-E in the above example). This is why one person may not be able to interrupt another until they stop talking on a DDD circuit with an echo suppressor. Echo suppressors are used sparingly in the DDD network, and generally only on regional intertoll trunks. Only ONE echo suppressor can ever be in a given DDD connection. Echo suppressors are intentionally designed to be disabled by a SF tone between 2,000 Hz and 2,200 Hz which lasts for at least 400 ms; it is a necessary requirement that echo suppresssors be disabled for any data call. The primary purpose of the "answer tone" furnished by a modem is to disable any echo supressor for the duration of the call. Your description of echo suppressor operation is totally wrong. > I don't work for the phone company ... Perhaps you should - then you'd learn something about telephony. I don't enjoy being harsh on you - but your article contained some serious misinformation than has to be corrected. I try to consider Usenet as an educational forum, and it always troubles me to see articles such as yours. <> Larry Lippman @ Recognition Research Corp., Clarence, New York <> UUCP: {allegra|bbncca|decvax|nike|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?"