Transmission Lines: Frequency Limitations & Attenuation

In summary, landline connections have limitations on the frequencies that can be used due to velocity dispersion, frequency-dependent attenuation, and impedance mismatches. In wireless networks, different frequencies may have lower attenuation over long distances due to atmospheric composition or objects in between points A and B. The use of different carriers helps to avoid interference, and new modulation schemes like OFDM make it possible to use multiple signals of the same frequency. The material of the wire does not make a significant difference in attenuation. The limitation in using phone lines for high-speed transmission is often due to the cost and complexity of upgrading equipment. Crosstalk can also affect performance, and longer processing times can introduce delays and additional costs. The maximum capacity of a landline connection is dependent on factors
  • #1
quantumlight
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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
 
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  • #2
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
 
  • #3
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
 
  • #4
quantumlight said:
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?)
The attenuation of coax tx lines get quite high when frequency goes up into GHz range.
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.
New modulation scheme can make many signals of same modulation frequency possible. Modulation like OFDM make possible use receivers that only work if you have the right key. BTW, this is invented by a beautiful actress called Hedi Lamar in WWII
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?
I don't think the material is going to make a big difference if it is copper, gold, silver type of good conductor. Some maybe, the conductivity is not that big a difference between the ones I gave, it is not like 100% better or what!

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
Some phone lines are not gear for high speed, but slowly they are upgrading them. Theoretically, you can get much higher frequency.
 
  • #5
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!
 

1. What is the frequency limitation of a transmission line?

The frequency limitation of a transmission line refers to the maximum frequency at which the line can effectively transmit a signal without significant attenuation. This is determined by the physical properties of the line, such as its length, diameter, and material composition, as well as the surrounding environment and any external interference.

2. How does attenuation affect the performance of a transmission line?

Attenuation refers to the loss of signal strength as it travels through a transmission line. The higher the attenuation, the weaker the signal will be at the end of the line, which can result in errors or distortion. It is important to minimize attenuation in order to maintain the integrity of the transmitted signal.

3. How can frequency limitations and attenuation be mitigated in a transmission line?

There are several techniques that can be used to mitigate frequency limitations and attenuation in a transmission line. These include using high-quality materials, minimizing the length of the line, using proper shielding to reduce external interference, and implementing signal amplification or equalization techniques.

4. What factors can cause attenuation in a transmission line?

Attenuation can be caused by a variety of factors, including resistance, capacitance, and inductance in the line itself, as well as external factors such as temperature, humidity, and electromagnetic interference. These factors can cause the signal to lose energy as it travels through the line, resulting in attenuation.

5. How does the frequency of a signal affect the attenuation in a transmission line?

The frequency of a signal can have a significant impact on the attenuation in a transmission line. Higher frequencies are more susceptible to attenuation due to their shorter wavelengths and higher energy levels. This is why it is important to consider the frequency limitations of a transmission line when designing a system, and to use appropriate techniques to mitigate attenuation at higher frequencies.

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