Transmission Lines

  1. I am writing my senior thesis on communication networks and this question sort of popped into my head:

    What determines what frequencies you can use in a landline connection? (e.g. say I have a wire connecting point A to B to send a bunch of signals, what prevents me from using the entire bandwidth from 1 hz to 1 ghz and beyond?)

    In wireless networks I guess some frequencies have lower attenuation over long distances than others due to the composition of atmosphere or objects in between points A & B that absorb. different frequencies. Also you don't want two people to have their cellphones use the exact same carrier in the same zone or you won't be able to separate it out at the receiver.

    However how do these limitations apply if you have a landline, you couldn't possibly be interfering with other landlines....? and how does the attenuation vary depending on whether the wire is copper, zinc etc. on frequency?

    If I don't care about equipment complexity, what limits me from using my phone line to transmit at ridiculous data speeds for everything from internet to voice because I can use all the bandwidth possible?

    Thanks
     
  2. jcsd
  3. Landlines have both velocity dispersion and frequency-dependent attenuation. The attenuation arises from both skin effect losses (√f) in copper and 1/f dielectric losses (at very high frequencies). Impedance mismatches cause reflections, which depend on phase advance between reflection points. Twisted pair lines try to decouple wire pairs in trunk lines. Some lines are full duplex (simultaneous bi-directional transmission). Time division multiplexing uses shared lines.

    Look up quarter wave impedance transformer for example of reflections at http://en.wikipedia.org/wiki/Quarter-wave_impedance_transformer
     
  4. In most places the signals from individual subscribers is multiplexed onto fiberoptic cable. In order to get as many subscribers on one fiberoptic cable as possible, the bandwidth of each subscriber is limited, usually to 300 - 3300 Hz. It is possible to get higher bandwidth channels at a higher price.

    http://computer.howstuffworks.com/question372.htm
     
  5. Some phone lines are not gear for high speed, but slowly they are upgrading them. Theoretically, you can get much higher frequency.
     
  6. sophiecentaur

    sophiecentaur 13,774
    Science Advisor
    Gold Member

    This is a seriously practical-based topic, involving COST as much as anything else - plus History (Much of our traffic can be expected to be going through at least one link of old equipment).
    You assume that there is no crosstalk on landlines. A massive assumption. Where there are bundles of twisted pairs, they 'talk' to each other. There are clever systems for swapping connections every so often, in order to help cancel the effects of crosstalk. But it's always a matter of cramming as much as possible into the available pipe space.
    There is another way to improve performance and that is to take longer and longer to process the signals. That introduces a delay which is an extra 'cost'.

    Your 'bottom line' question about maximum possible capacity has not definite answer but is basically comes down to noise, interference, inter-symbol interference and the effect on 'eye height' (ability to distinguish between digital states).

    Hedi Lamar. Gorgeous actress - reputed to be the first to do a nude scene on a general release film. COR!
     
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