Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!swrinde!zaphod.mps.ohio-state.edu!casbah.acns.nwu.edu!hayes.ims.alaska.edu!floyd From: floyd@ims.alaska.edu (Floyd Davidson) Newsgroups: comp.dcom.modems Subject: Re: Capacity of a Channel (Was: Inexpensive 9600 baud modems) Keywords: Shannon, 9600, V.32, V.42, bis Message-ID: <1990Dec12.131736.10478@ims.alaska.edu> Date: 12 Dec 90 13:17:36 GMT References: <1990Dec10.162413.13959@sparrms.ists.ca> <1990Dec11.185205.13752@wsrcc.com> <3599@jaytee.East.Sun.COM> Organization: University of Alaska, Institute of Marine Science Lines: 87 In article <3599@jaytee.East.Sun.COM> gsteckel@east.sun.com (Geoff Steckel - Sun BOS Hardware) writes: >In article <1990Dec11.185205.13752@wsrcc.com> wolfgang@wsrcc.com (Wolfgang S. Rupprecht) writes: >> >>Also, remember that the 3.4 kHz is the MINIMUM bandwidth the phone >>line will present to the modem. The sky is the limit. Don't folks >>regularly send 10 Mhz ethernet connections over (admittedly shorter >>sections of) these wires? I believe 64k baud ISDN links actually go >>from the CO to your jack over these same wires. The trick here is >>that it is fairly easy to get good freq responses out of short >>sections of twisted pair. > >Not quite true. I refer people to the experts on comp.dcom.telecom, >but the 3.4 KHz is the limit for which the telco people minimize dispersion >and maintain a reasonable frequency response through amplifiers, >multiplexors, and all the other interesting equipment between your modem and >the other one. With the same twisted pair, but different compensating >networks, amplifiers, etc, at intermediate points in the circuit, you >can get pretty much any response you want. The telco can also make your >circuit pretty quiet. > >A twisted pair, per se, can be used into the megahertz if you know a >LOT about its properties. ISDN works (at 176 KB/s) because the engineers >worked very hard to overcome two problems: dispersion and attenuation. >The subtler part is dispersion: differential propagation delay versus >frequency. Messes the heck out of pulses, and really destroys complex >modulation schemes. Within limits, however, the dispersion and frequency >dependent attenuation are predictable for a copper loop to the Central >Office, and this information is used to `pre-distort' the ISDN loop >waveforms and to recover the data at the far end. > >T1 (1.544 MB/s) also uses twisted pair - with sophisticated phase, >amplitude, and frequency compensation on the transmitted and received >signal. > >Once at the central office, ISDN uses all-digital transmission to the >terminating central office, which then uses the twisted pair loop to >the user. > >These techniques of precompensation and recovery are used by some `analog' >modems as well, but the possibility of FDM or TDM links (with attendant >brick wall anti-aliasing filters) in the transmission path limits their >usefulness. 14400 B/s with V32bis represents a triumph of engineering >over ugly limitations. What Geoff is saying is precisely correct, but I'd like to add a couple things that may make it easier to see what he means. T1 and ISDN are *digital*, just like your RS232 line. Your modem on the other hand encodes that digital signal into an analog transmission system. Two very different things and do not compare directly. When your 3.4 Khz voice line is digitalized and put on a T1 stream it takes up 56kbps. (It is done at a 64Kbps rate, but 8Kbps are used for other than voice data.) The absolute max bandwidth for an analog transmission over a telephone system is designed to be 4Khz, but in fact you can forget about using the upper end or the lower end of that spectrum. Most lines will have much less that 3.4Khz bandwidth. We only test the freq response from 400Hz to 2800 Hz! If you have ISDN or a T1 stream then each bit will be transmitted from one end of your connection to the other, using whatever bandwidth you are paying for. If you have an analog drop (a normal telephone voice line) you can put whatever you wish on it, but what comes out the other end is still going to be limited at best by a A-D conversion to 56Kbps. That would be assuming you are switching through a digital switch. On a local loop using an analog switch you might get better. But *any* type of carrier used to trunk your call to another switch will limit the bandwidth (4Khz for analog systems, 56Kbps for digital systems). There is an answer to "How do we get more bandwidth?" though. That is exactly what ISDN is designed/supposed/going to do. > geoff steckel (gwes@wjh12.harvard.EDU) > (...!husc6!wjh12!omnivore!gws) >Disclaimer: I am not affiliated with Sun Microsystems, despite the From: line. >This posting is entirely the author's responsibility. Despite my address, I have no connection with the Institute of Marine Science at the U. of Alaska Fairbanks. Despite my .signature, I do test toll trunks for a living. 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.