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posted by martyb on Monday November 20, @11:02PM   Printer-friendly
from the good-question dept.

If you've ever had dialup internet service, or still do, or just know someone that does, you have probably heard terms like "56k modem". "56k" has become almost synonymous with dialup Internet access. But it's such an arbitrary number. It's not divisible by ten, it's not a power of two... so why was it chosen as the fastest dialup speed? For the answer, we will have to travel back in time quite a while.

Our visitors from Google should be warned that this is not a "stripped down" explanation; it is intended for relatively technical readers. But if you really want to know where this magic number comes from, you need to understand some of the technical background. As we shall see, "56k" was not just pulled out of a hat.

[...] Anyone that has ever used a dialup modem knows full well that they don't actually get to connect at that speed, though. And that their connection speed varies each time they dial in. There are two factors at work here.

The first is the FCC. If you are in the United States, the FCC places a restriction on the power output of devices connected to the phone network. The result is that you will never be able to connect at a speed faster than 53.3 kbit/s.

The second is the overall complexity of the phone network. 56 kbit/s (or 53.3 kbit/s) requires very good operating conditions, as it is really operating beyond the paramaters of what the phone network is required to be capable of. Operating at these speeds requires that there only be one ADC between the user and their ISP (which is not guaranteed to be true, but typically is), and that the copper wiring in the user's "local loop" have very good electrical properties. Part of the dialup process that is used to initiate a connection is an evaluation of the overall quality of the connection; if it is determined to be lacking, the modem will automatically drop down to a lower data rate.

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  • (Score: 3, Interesting) by drussell on Tuesday November 21, @03:52AM (3 children)

    by drussell (2678) on Tuesday November 21, @03:52AM (#1333686) Journal

    You obviously have a different knowledge base, but... the 33.6k USR (a real USR) had a update to v.92 and those would indeed do 56k, or very close to it.

    You were obviously dialing into a digital system from your analog POTS line... So yes, you could do up to 53,333k from their (digital) end downstream to your analog end. Upstream from you was probably analog (31200 max, recommendation V.90) or possibly up to a theoretical 48000 if you were in recommendation V.92 mode on the upstream, so your modem was sending raw PCM back, essentially trying to act like a digital ISDN connection bandwidth wise, but that only works when you're direct to the CO and physically close, the 64k channel at the CO end still has to "see" what your modem is sending, accurately.

    If you tried dialing up another line in your house or your neighbor or whatever from your analog telephone line, to their analog telephone line instead of an ISDN or other digital line at some CO head-end equipment or whatever, you would only get 33600.

    I still have a bunch of those modems around here somewhere, but no longer have the phone line. So I guess their current speed is 0k.

    Go ahead and try it. Grab two V.90/V.92 modems connected to your (whatever local terminal devices') serial ports, connect them together with a phone cord, type ATA on one modem, and ATD on the other and see what it says it connects at. You WILL get a CONNECT 33600/ARQ. You cannot get digital speeds between two analog Couriers.

    In theory, someone with a disassembly of a recent enough firmware could perhaps somehow hack the firmware to allow a back-to-back local connection in V.PCM-Upstream mode or something to get maybe 48000/48000, but if you're going to really hack the firmware for a back-to-back modem connection, why not just trick the software in the TI DSP chipset into doing some way higher frequencies and speeds... That could be a new electro-sport, I suppose... Pimp the COURIER! You could probably easily get whatever data rate the UART in the modem is capable of, maybe 230,400? I know the stock firmware on mine are all limited to 115,200 but I think the chips can probably do at least 230,400 and one could possibly bodge in one of those ones that does up to 1-1.5 MHz bitrates, though you'd have to replace the stock TI DSP with a faster part, and possibly overclock the '186.... Or 80188 or whatever it is in there. 188 I think. Again, I digress... :)

    Lived on dialup from 1993 to 2008

    My sympathies.

    I guess I'm lucky, I had at least 1.5 mbps to the office by 94 and at home by 98 without extraneous effort or expense.

    ... or so, and aside from what the modem reported I regularly ran speed tests, and when I was on a good near-town phone line that v.92 always got very close to 56k. When I moved out to rural telephone shitland, then the same modem only got 26k.

    Right. You were able to get connected to a digitally-connected remote end in digital mode when you were near town. Once you got too far away, you could only get a regular analog-both-ways connection. When the remote end is able to generate a full 64k signal at the CO end with nothing between there and your modem except a (relatively) short piece of wire, it can generate signals that simply cannot be generated on an analog-analog-only connection within the system as it has to be digitized by an ADC. The DAC on the output can precisely generate things all the way up the available 64k-bitwise spectrum.

    Your slow speed had nothing to do with your modem itself, it's limitations of the physical connection infrastructure of your telco provider. You must be one-hop digital to the CO to expect "56k class" connections.

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  • (Score: 2) by Reziac on Tuesday November 21, @04:43AM

    by Reziac (2489) on Tuesday November 21, @04:43AM (#1333694) Homepage

    Yeah, I have no idea what I was dialing into -- for the ISP, whatever was on Earthlink's end, but it was probably the latest and greatest. However, I got the same speed dialing into the more modern BBSs... so that would have still depended on the phone line, right? (Some of the little hole-in-the-wall BBSs didn't max out the old 14.4.)

    I may dig up a couple modems and see if I can get them to talk, just for shits and giggles. The things we do for science... :) Not something I ever had reason to do. Perhaps nothing any sane man has reason to do. :O

    The phone line where I got the shit speeds was some hideous routing. It had a giant loop that went several miles out of the way (per the guy at GTE, er, Verizon who tried in vain to fix it). WAAAAY out of range for even the poorest DSL. Eventually was able to get fixed wireless, but it maxed out at 1.5Mbps, cut out under anything less than perfect conditions, and after a couple years decided it could only do 300k (I stopped paying, it kept working at that unspeed, I guess everyone was happy??)

    In the present, my rural DSL supposedly can do 7 unstable, was doing 5.125 very stable for several years, but a year ago dropped to a spotty 3 with lots of dropout (which given today's web bloat, is slower than dialup was in its heyday) and no amount of complaining or resets by the tech fixes it for very long. "It's all working" but something died somewhere. Think it's gonna be back to fixed wireless, tho hopefully with better results this time.

    And there is no Alkibiades to come back and save us from ourselves.
  • (Score: 2, Informative) by pTamok on Tuesday November 21, @10:48AM

    by pTamok (3042) on Tuesday November 21, @10:48AM (#1333711)


    you could read the V.92 standard. It is freely available. As is the V.90 standard it updates.

    ITU-T: V.90 : A digital modem and analogue modem pair for use on the Public Switched Telephone Network (PSTN) at data signalling rates of up to 56 000 bit/s downstream and up to 33 600 bit/s upstream []
    ITU-T: V.92 : Enhancements to Recommendation V.90 []

    5 Digital modem
    5.1 Data signalling rates
    Synchronous channel data signalling rates from 28 000 bit/s to 56 000 bit/s in increments of 8000/6 bit/s shall be
    supported. The data signalling rate shall be determined during Phase 4 of modem start-up according to the procedures
    described in 9.4.
    5.2 Symbol rate
    The downstream symbol rate shall be 8000 established by timing from the digital network interface. The digital modem
    shall support the upstream symbol rates 3000 and 3200. It may also support the optional upstream symbol rate 3429 as
    defined in Recommendation V.34.
    6 Analogue modem
    The characteristics of the analogue modem described herein apply when in V.90 mode. After fallback to V.34 mode, the
    analogue modem shall have characteristics as defined in Recommendation V.34.

    6.1 Data signalling rates
    The modem shall support synchronous data signalling rates of 4800 bit/s to 28 800 bit/s in increments of 2400 bit/s, with
    optional support for 31 200 bit/s and 33 600 bit/s. The 200 bit/s V.34 auxiliary channel is not supported. The data
    signalling rate shall be determined during Phase 4 of modem start-up according to the procedures described in 9.4.

    6.2 Symbol rates
    The analogue modem shall support the symbol rate 3200. It may also support 3000 and the optional symbol rate 3429 as
    defined in Recommendation V.34. The other V.34 symbol rates, 2400, 2743 and 2800, shall not be supported. The
    symbol rate shall be selected by the analogue modem during Phase 2 of modem start-up according to the procedures
    described in 9.2.

    5 Digital modem
    The data signalling rates, symbol rate, scrambler and encoder for the digital modem shall be the
    same as those given in clause 5/V.90.

    6 Analogue modem
    6.1 Data signalling rates
    The modem shall transmit synchronously at data signalling rates of 24 000 bit/s to 48 000 bit/s in
    increments of 8000/6 bit/s. The data signalling rate shall be determined during Phase 4 of modem
    start-up according to the procedures described in 9.6.
    6.2 Symbol rate
    The upstream symbol rate shall be 8000 symbol/s derived from the digital network.

    If the service provider was connected to the PSTN with a digital connection (ISDN or T- or E- carrier), then that end is a 'digital modem', and you can rely on the least significant bits of pulse-code modulation encoded signal being correct. This gives you a full 56 knit/s of capacity. The main reason it is 56 kbit/s and not 64 kbit/s is that fact that the PSTN in North America uses/used 'robbed-bit' signalling [], that used the least-significant bit of a PCM encoded channel for in-band signalling (this is an oversimplification, but it'll do),

    If your modem was connected to an analogue line (which it would be), then there was a conversion at the service provider using a modem/codec/DSP that sampled the incoming signal on the analogue line 8000 times a second and encoded it using pulse-code modulation (PCM []) in an 8-bit value, giving you a data rate of 64000 bits per second; otherwise written as 64 kilobits per second or a '64k channel'. It isn't a linear encoding, North America and Japan using a mapping function (compander) called 'ยต-law' [] ('meuw-law') and the rest of the world using 'a-law' [] (using the wrong companding function on companded data caused a noticeable drop in quality).

    The FCC set a maximum power to be used on analogue phone lines in the USA in order to limit the effects of crosstalk between circuits, and that constraint reduces the set of points in the modulation constellation that can be used (certain combinations exceed the power levels), which means that fewer symbols can be used to carry the data*.

    More background: Electronic Products: The standards behind V.90 and ADSL modems []

    More (mathematical details here: PDF: Voiceband modems and DSLs [].

    ADSL uses analogue POTS lines, but gets higher data rates because it operates at higher frequencies over the distance between the DSLAM and the end-ADSL modem. Also, many modems had data compression, so could get better effective throughput on compressible data.

    Yes, I used to work in the industry, and have accumulated a lot of (now useless) knowledge. I didn't have to deal with understanding the mathematics behind convolutional and trellis codes: I 'just' used the products. And T-carriers, and E-carriers. And...

    Now I'm just a dinosaur.

    *A simple example - assume you have a signalling system that uses two tones Low and High. Each tone can be off or full power. The possible combinations that can be sent down the wire simultaneously are: Both off, one on, the other on, and both on. If there is a restriction on the maximum total power that can be sent, then you can't use the 'both on' combination, so you can use only three symbols (off,off), (off, on), and (on, off) because (on,on) is disallowed. Being able to use only three symbols instead of four cuts your data rate. The trellis code [] used for V.92 has more possibilities, but some of the combinations would send too much power down the line, so can't be used in the USA, cutting the data rate.

  • (Score: 1, Interesting) by Anonymous Coward on Tuesday November 21, @03:03PM

    by Anonymous Coward on Tuesday November 21, @03:03PM (#1333736)

    Go ahead and try it. Grab two V.90/V.92 modems connected to your (whatever local terminal devices') serial ports, connect them together with a phone cord, type ATA on one modem, and ATD on the other and see what it says it connects at. You WILL get a CONNECT 33600/ARQ.

    Note that just using a phone cord to connect two modems probably won't work. You will need to simulate the off hook condition by providing an approximately 20mA current source in series with the phone line (a battery and a resistor in series will do).

    Also ATX3 (I think) to disable dial tone detection on the dialing modem, as you obviously won't have a dial tone in this setup.

    When I tried this myself (with two USR modems) it was a bit finicky, I had to sequence the dial and answer commands properly (sending ATA only after the dialing modem had finished dialing), otherwise the modems ended up just screaming at each other without actually connecting.

    But indeed, you will only ever get 33.6K with two analog modems.