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Why analog communication over the copper rather than digital ?

  1. Jun 2, 2010 #1
    Hi,

    I wonder if someone could shed light on my question.
    It's kind of related to Fourier Transform and attenuations on signals.

    As everyone knows, in the early days, the communication on the local loop was on dial up which was basically analog communication.The digital data in computer gets modulated to analog signal and transferred over the phone line. You often see the reason of this as the phone line doesn't have enough bandwidth to pass the digital signals which contains very high frequency components that could be easily attenuated. I understand that for the high bit rate communications, but I don't see the reason why this fact restricts the communication at the lower bit rate..

    The max bit rate on dial-up is 56kbps, but couldn't we do the same bit rate without modulation ?
    Let's say serial communication at 56kbps in digital form. The worst case is the data toggles every cycle at 56kHz, and this contains very high frequency components which are prone to the attenuation, however, we actually don't need all the ultra high frequencies. The phone line might be able to pass the first few harmonics (starting at 56kHz sine wave or less, depending on the data pattern) which are enough to reconstruct the original data at the receiving end. Considering the fact that ADSL can run up to 4MHz, it should be able to accommodate a decent number of harmonics within 4MHz(or even 2MHz) frequency band to get the "digital" communication done on phone line without a serious attenuation, shouldn't it ?

    Am I misunderstaning something ?
    Appreciate your opinion.
     
    Last edited: Jun 2, 2010
  2. jcsd
  3. Jun 2, 2010 #2
    At 56k you can indeed send digital data over the phone line. And in fact that's what was done with 56k modem.
    56k modems can send data with 33k and convert the signal to analog to do so. But when they are receiving with 56k the data doesn't get modulated. They receive digital data. That works because analog phone lines are actually digital with a d/a converter at the end and you can trick them into giving you the raw digital data.
     
  4. Jun 2, 2010 #3

    vk6kro

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    In the following diagram,

    500px-ADSL_frequency_plan.svg.png

    of an ADSL signal on a telephone line, the red portion is used for normal speech, while the green and blue areas are used for data.

    If you did manage to send data down such a line, the "red" voice section would be wiped out and the line would only be used for data.

    Also, you can see that giving the upstream and downstream signals different bands gives them a chance to happen simultaneously without interfering with each other.
     
  5. Jun 2, 2010 #4
    But completely wiping out voice band is true for dial-up connection too so that I wouldn't see it as the disadvantage of digital communication against the dial-up.
    However, I see your point. Simple digital communication of baseband data causes interfarence between the upstream and downstream and full duplex isn't possible.
     
    Last edited: Jun 2, 2010
  6. Jun 3, 2010 #5

    vk6kro

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    The telephone network is designed for voice communication and it passes audio signals through transformers.

    If you passed data signals through transformers, the output would be mainly at the rising and falling edges of the data signal.

    Data consists of two levels. 0 volts and some DC voltage which might be +5 Volts. This cannot be passed through a transformer since transformers cannot pass DC and only react to variations in signals.

    Modem signals as used in early dial-up modems consist of two audio frequencies for transmit and two different frequencies for receive. All these frequencies are in the normal speech audio range between 500 Hz and 2000 Hz.
    Later modems used multiple frequencies for transmit and receive and achieved their speed in that process.
     
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