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ADSL frequency multiplexing

  1. Nov 1, 2004 #1
    As some may know ADSL technology multiplexes different electrical signal frequencies in the cable to achieve high data rates. Why do these frequencies have to be spaced by a certain amount to avoid interference? Couldn't an arbitrary number of electrical signals of different frequencies be multiplexed to achive very high data rates? In the same way as a continuous spectrum of colors are multiplexed in a beam of white light.
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  3. Nov 1, 2004 #2


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    I will guess harmonics and sidebands, but I know very little of this technology.
  4. Nov 1, 2004 #3


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    In principle you could use an infinite number of closely-spaced frequencies, but the modulating and demodulating circuitry becomes more and more complicated the closer the frequencies are spaced.

    - Warren
  5. Nov 1, 2004 #4


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    edit:I reread your post, perhaps the link I posted to the Cisco site addresses what you are referring to. There is VDSL technology, which sounds like what you are suggesting?

    I think I have the answer to that back at the office, but I am off for a couple of days.

    Here is a good link http://www.cisco.com/univercd/cc/td/doc/cisintwk/ito_doc/adsl.htm

    You need to be posting in the Technology sub forum.

    If I have time I will try to find you some other links. If you are planning to learn about networks, I highly recommend Newton's Telecom Dictionary.

    There are limits to what can be done over existing phone lines. Often the limits aren't necessarily physical, but legal.
    Last edited: Nov 1, 2004
  6. Nov 2, 2004 #5
    Hi Evo, I appreciate your suggestion about the cisco article. But I know the essential details of DSL and instead of becoming an expert in this technology I wanted to clarify that particular issue. If you are interested in networking we could discuss any topic together. Indeed I'm trying to engineer a secure LAN messenger for my dissertation.
  7. Nov 2, 2004 #6


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    If you double the number of carriers, you have to wait twice as long to be able to resolve them all (techincal term - so that they are orthogonal).

    Consider trying to resolve 1000 hz from 1010 hz. You have to wait a whole .1 second to get 100 complete cycles of 100hz and 101 complete cycles of 1010 hz.

    Now if you want to resolve 1000hz and 1001 hz, you have o wait 1 second to get 1000 cycles of 1000hz and 1001 cycles of 1001 hz.

    It turns out that any communications channel (with a finite transmitter power) has a limit as to it's error-free capacity in bits/second, called the Shannon limit. So regardless of how complicated you make your encoding and decoding apparatus, you'll never achieve a bit rate that's greater than the channel limit. This limit is proportional to the bandwidth, multiplied by the the logarithm of the signal/noise ratio.
  8. Nov 2, 2004 #7


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    I thought you were questioning guard bands and types of multiplexing to achieve higher bandwidth. That is why I brought up VDSL, utilizing DMT, it addresses both issues.

    Guard bands are utilized to prevent signal overlap and can be varied in size. The VDSL standard requires very minimal guard bands and with DMT you're talking about a 7.8% loss as opposed to SCM which has a 20% loss.

    Dense Wave Division Multiplexing (DWDM) which is used to transmit at different wavelengths over optical fiber is similar to Frequency Division Multiplexing (FDM) which divides the bandwidth on the analog lines (utilized for DSL) into narrower bands.

    This is where utilizing DMT for VDSL becomes interesting in response to your question. This is an exerpt from a white paper on it.

    Digital Duplexing Techniques Achieve Higher Spectrum Utilization

    For full duplex operation, the upstream and downstream signals can be separated through frequency division multiplexing (FDM). In traditional modulation techniques, such as SCM, FDM is achieved via filtering. It is known that the spectral confinement and time confinement of a signal are inversely proportional to each other. Therefore, SCM systems must use an engineering trade-off between their filter length (time confinement) and excess bandwidth (efficiency in frequency confinement). For example, a higher filter order may reduce the frequency spectrum overhead and increases the frequency utilization, but it also results in severe penalties, and therefore, higher cost. First the filter implementation requires a higher order, requiring more multiplications. Second, to take advantage of the reduction in overhead, the timing requirements are more stringent, to avoid introduction of ISI caused by a longer filter impulse response.

    Typical SCM systems have a loss or excess bandwidth of 20% in the spectrum to achieve a reasonable filter size in the time domain.

    In DMT solutions such as VDSL, FDM can be implemented by utilizing techniques such as digital duplexing, where the transmitter and the receiver are synchronized, obviating the need for filters. In digital duplexing, the separation of upstream and downstream bands is achieved by using the orthogonal property of IDFT/DFT, with the resulting higher spectrum utilization. However, DMT systems have a loss in bandwidth that is about 7.8% for VDSL systems. This loss is due to the use of a cyclic extension in the time domain in order to simplify the equalization and to provide orthogonality of the up and down stream directions. This loss is significantly less than the 20% loss in SCM systems used for VDSL.

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