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Speed and rate of data transfer

  1. Feb 22, 2013 #1
    suppose i want to transfer digital signals over 100 meter wire, the loic is implemented using two level, a +5 volts for 1 and 0 volts for logic 0. I want to transfer the data with speed 1000 Mbps which is 10^9 bits per second.

    now i consider the worst case scenario that my data consist of series of 01010101... or 10101010.... this mean the digital signal must change from +5 volt to 0 volts in 1/2(10^9) seconds. now given that the rate at which the signals are changed, will they not make to the end of the 100 meter wire, if they do, then how?

    I do not think that repeaters are a better solution here, because if i consider that the signal travels 1 foot in 1 nanoseconds (as light would do), which is not possible, it would be like putting up thousands repeaters over 100 meters wire.

    please help me, if i am wrong, and where?
     
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  3. Feb 22, 2013 #2

    mfb

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    There is no fundamental problem with multiple bits traveling in a wire at the same time - as long as they all move with (roughly) the same velocity, you can receive the bit stream at the other end. Dispersion will degrade the signal a bit, if the cable is too long.
     
  4. Feb 22, 2013 #3
    Couple of considerations as the standard for your goal on bit rates is pretty high.

    What I mean by that is if you look at the ethrnet standards It'll become readily apparent some of the difficulties on obtaining the transfer speed you desire.
    http://en.wikipedia.org/wiki/List_of_device_bit_rates.

    Yes it is possible but you will need quality components. The link above may guide you to the type of components will work for you.

    The other consideration is that 100m is typically the maximum distance for ethernet without repeaters. At least for Cat5, Cat5E with sheilded ends or Cat6 standard may be able to help gain your distance.

    Location of wiring will also play a factor on digital signals you will want a routing of least EM interferance.

    Good luck with your project what I posted above is just a few considerations. Others may have more to offer.
     
  5. Feb 22, 2013 #4

    but i have shown above that the phase of the digital signal reverses before the charges moving in line could actually move even a foot
     
  6. Feb 22, 2013 #5
    have you even read what i asked for
     
  7. Feb 22, 2013 #6
    He did read what you stated. Pulses can be sent down a line. In communiciations its not a steady state 1 or 0 throughout a line. Its multiple 1's and zero's or rather its differential voltages used to represent those 1's and zero's. Just like a sinusoidal wave form can travel down a line so can digital representatin signals.
    Provided all your devices can send and receive data at 1 gigabit/sec then the line itself can handle it up to a maximum distance= to the line rating. Exceeding the line rating will slow the rate down through propogation delays. Hence why I posted the different standards to show what class of devices and wiring you should be looking at. Aslo keep in mind data can also be sent in various compression techniques. To increase data transfer one such is FSK
     
  8. Feb 22, 2013 #7
    When you say you want to send digital signals over 100 meter wire, do you mean a transmission line of some sort with a defined impedance or literally over a wire with the return path through the ground? If you are talking about a transmission line I think you will find that 10101010... is not your worst case. Your worst case will likely be something like 11111111101111 or its inverse. This type of transmission is called NRZ for non-return to zero. The transmission line is essentially a low pass filter consisting of series inductance and shunt capacitance. If you send a long series of digits of the same value, the transmission line will tend to charge up to that value. Then when you try to send the opposite value, at high data rates, the transmission line may not be able to discharge enough to cross the threshold of the level detector at the receiver. This problem can be worsened by employing hysteresis in the detector.

    The solution is to use RTZ (return to zero) coding. The best known of these is Manchester encoding. For every bit transmitted, there is both a 1 and a 0. This means that as the signal attenuates with distance, the average value stays about halfway between a 1 and a 0 and the level detector doesn't get fooled by a long string of 1s or 0s.

    But there is a problem with Manchester encoding and that is that it requires double the bandwidth for the same data rate. That could easily erase any advantages from going to RTZ. However other RTZ coding schemes have been developed that do not require a larger bandwidth.
     
    Last edited: Feb 22, 2013
  9. Feb 22, 2013 #8

    jim hardy

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    @ N'Zilch:

    Maybe this is an easier question than we think.

    You seem to be under the misconception that the sending end of the wire must be held "high" until the pulse reaches other end.

    That's simply not so.

    Imagine how a radar works - sends out a pulse and waits for the echo.
    Same can be done with a wire, it's called Time Domain Reflectometry.

    Send a narrow pulse down a wire and it'll get to the other end just fine.

    I once stumbled across a computer memory that used a mechanical analog to that concept.
    It was a coil of wire, perhaps fifty feet, with a loudspeaker voice coil at both ends.
    The sending end applied a series of little twists to the wire, which propagated down it according to its stiffness and mass , per Newton's laws. When they got to receiving end they were detected and re-applied to sending end.
    So this little memory could store hundreds of bits as a long serial "word".
    Novel application of a mechanical delay line. What's most remarkable is, it actually worked.

    old jim
     
  10. Feb 23, 2013 #9
    how do you send a pulse down the wire, as much as i know, the signal is electrical, you would need a potential difference to so the same, and for that the potential must be maintained unless the signal has reached the other end.

    In case of radar, you use the wave nature of radiation, therefore you can radiate and wait for the echo. however the same does not happen here.

    and that is the reason that we need optical fiber to transmit data at higher rates.

    tell me where i am wrong or answer to the basic question below

    " how do you send a signal over a wire(co-axial or copper) whose frequency is large enough that the signal reverses it phase before the signal could reach its destination"
     
    Last edited: Feb 23, 2013
  11. Feb 23, 2013 #10

    davenn

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    Having to ask that question, infers that you are probably in over your depth (wink)

    Yes the same thing does happen, BOTH the pulse in the cable and the radar radio signal are electromagnetic radiation. They both travel at the speed of light minus appropriate velocity factors of the given mediums ... copper wire, air, or even optical fibre

    So what Jim told you is correct
    you still get that echo effect in a cable. In "another life" I worked for a telecommunications company. We would find cable faults using an instrument called a pulse echo locator and the principle of Time Domain Reflectometry that Jim mentioned, that way we could find out where down a long length of cable the fault was. The fault would provide an impedance bump in the cable and a % 'age of the pulse would be reflected back down the cable to the test instrument and it would give a digital readout of the distance to the fault

    Dave
     
  12. Feb 23, 2013 #11
    i have an ADSL at my home, the wire runs through my telephone line, and what you say suggest that my line carries electromagnetic waves, not particle electron.

    i know flowing electrons are also electromagnetic radiations, but i just want to confirm if the pulses we are talking of here is similar to transmitting current in the wire, under some potential difference, as we do in transmitting ac power to homes.

    or we do it the other way?
     
  13. Feb 23, 2013 #12

    davenn

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    electrons actually travel very slowly

    do a search on these forums or in google and you will find quite a bit of
    background on that subject
    in AC power to the home, electrons do nothing more that cycle back and forward a few cm or so. Cant remember the actual distance but it isnt much
    some one else on here will be sure to fill in the blanks

    Dave

    EDIT ... doing a search of the forums gave an indication that electron drift is ~ 1 mm / second
    so in a 50Hz or 60Hz (depending on your country) AC mains supply, the electrons are oscillating back and forward just a tiny fraction of a mm

    the pulse or other change in signal level is propagated down the wire, through the air or optical fibre cable as an electromagnetic wave, travelling at near the speed of light
     
    Last edited: Feb 23, 2013
  14. Feb 23, 2013 #13

    mfb

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    You are trying to use an approximation usually done in electronics (speed of light is faster than anything else) in a system where this approximation is not valid any more.
    You can send electromagnetic waves along a wire.

    I used this to determine the length of a cable once.
     
  15. Feb 23, 2013 #14

    sophiecentaur

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    There is a delay over any length of line, or Radio link (or even when you have a voice conversation with someone across the room). The delay for EM signals is at least 1ns for every foot. It does not matter at all, in most circumstances, except when there are multiple routes for signals and they may arrive 'out of step'. This needs to be taken into account.
    The fact that the TV picture you're watching is a small fraction of a second later than when it was transmitted doesn't matter at all. The fact that the signals from a GPS Satellite are delayed on their journey is what makes the whole system work.
     
  16. Feb 23, 2013 #15
    In most communications. The process relies on differential oscillations. Essentially waves. Such as the case with your ADSL. They use a variety of communication protocols to compress data. The compression scheme is either an RTZ scheme or a NRTZ. If I recall The 802.11 is a form of RTZ but not positive on that.
    It may be better if you can describe your project in more detail. The reason being the goals you set forth in the OP is not an easily obtainable rate of data transfer. Particularly if your not too familiar with communications. As the link above I posted on data speeds indicate.
    Couple of valuable steps will help you out.
    1) quality of wire is important as the medium your sending the signals down will as noted in everyones posts slow the signals down. Use a good quality low impedance cable.
    Personally I recommend Cat 6
    As we can see by that link above as being capable of those speeds.
    2) the rate of switching of tranmitter and reciever as well as any repeaters.
    3) Em noise. This usually isn't a problem as good cables are well shielded but its still a consideration.
    4) you indicated 100 metres. This length is further than rs 232 signals. Though Rs232 can easily be converted to RS 485.
    If your sending ethernet based then 100 metres is the limit for Cat 5. Although cat5e And cat6 can travel further they stll recommend the 100 metre limit. Without repeaters.
    You will find that 5volt signals do not travel very far without losses. Hence for the distance I would recommend using 12 volt signals similar to RS 485.


    Just some things to consider
    As Jim stated it may be straight forward. However with enough considereations it should be do able
     
    Last edited: Feb 23, 2013
  17. Feb 24, 2013 #16
    the answer to the question below will make my understanding better

    " If i have a wire consisting of only neutrons, connected across the bulb, will the bulb light up when i apply a battery across it?"
     
  18. Feb 24, 2013 #17

    sophiecentaur

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    How would that work? What would be holding the neutrons together, for a start?
     
  19. Feb 24, 2013 #18

    mfb

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    There is no solid material with just neutrons. You can use isolators, however, to get a similar effect. Without movable charges, you don't get current flow.
     
  20. Feb 24, 2013 #19
    but in case you have a charged battery and you connect it to a bulb through insulators, will the bulb glow? if yes, then why, if no, then why not?
     
  21. Feb 24, 2013 #20

    mfb

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    No, it will not, there is no (significant) current flow.
     
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