Communication channels - TCP-IP

This is a discussion on Communication channels - TCP-IP ; hello The following questions are not related to Multiplexing, where single channel combines multiple signals into one signal ( and at the other end the original channels are extracted by the process called demultiplexing ). For the sake of argument, ...

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Thread: Communication channels

  1. Communication channels

    hello

    The following questions are not related to Multiplexing, where single
    channel combines multiple signals into one signal ( and at the other
    end the original channels are extracted by the process called
    demultiplexing ). For the sake of argument, let's pretend that there
    is no such thing as multiplexing

    1)
    When talking about broadband medium the word channel is often used. A
    single Broadband medium can have more than one channel, where each
    channel transmits signals with certain frequencies. This way one
    broadband medium can transmit multiple signals at the same time.

    * Now does the word channel apply to individual wire in a broadband
    medium and thus each channel is actually a wire, capable of
    transmitting signals with certain frequency? If that is the case then
    for broadband cable to have 20 channels, it must have 20 wires?

    * Or are channels only a logical construct, meaning that word channel
    only refers to a certain frequency? Thus a single wire can at the same
    time transmit more than one signal by having each signal transmitted
    at different frequency? That's a bit hard to believe.



    2)
    * To my understanding, if each channel has its own wire, then
    broadband cable with only one wire would not be able to send signals
    in both directions at the same time ( I think we call this simplex
    transmition )?!

    * But if word channel only describes different frequencies, then a
    single wire can transmit signals in both directions at the same time.
    I find that hard to believe, since ( as little as I know about
    physics ) I know that a wire can only have current flowing in one
    direction at the time ( as such voltage can also have only one
    direction at the time )


    3)
    I'm not saying it would be practical, but if each channel has its own
    wire, then why can't all channels have the same frequency? I know
    cable TV operates by sending each program with different frequency and
    we can tune TV to certain frequency and watch a particular program
    ( that was send with that frequency ). Since each channel has its own
    wire, then each cable program would be send through different wire and
    we could tune into specific program simply by connecting TV receiver
    ( or something ) with that certain wire ( this way all channels could
    send signals with same frequency )?!


    thank you


  2. Re: Communication channels

    wrote:

    > hello
    >
    > The following questions are not related to Multiplexing, where single
    > channel combines multiple signals into one signal ( and at the other
    > end the original channels are extracted by the process called
    > demultiplexing ). For the sake of argument, let's pretend that there
    > is no such thing as multiplexing
    >
    > 1)
    > When talking about broadband medium the word channel is often used. A
    > single Broadband medium can have more than one channel, where each
    > channel transmits signals with certain frequencies. This way one
    > broadband medium can transmit multiple signals at the same time.
    >
    > * Now does the word channel apply to individual wire in a broadband
    > medium and thus each channel is actually a wire, capable of
    > transmitting signals with certain frequency? If that is the case then
    > for broadband cable to have 20 channels, it must have 20 wires?
    >
    > * Or are channels only a logical construct, meaning that word channel
    > only refers to a certain frequency? Thus a single wire can at the same
    > time transmit more than one signal by having each signal transmitted
    > at different frequency? That's a bit hard to believe.


    Well, you can easily picture multiple frequency channels operating over
    the air, right? Radio channels, TV channels. They can be half duplex
    (share the same frequency, but take turns transmitting and receiving )
    or full duplex (transmit and receive simultaneously, using seperate
    frequencies). So, why not do the same thing over a cable?

    Sometimes "channel" refers to a single media stream, and of course media
    streams can be separated within any medium, using packet headers to
    differentiate the streams.

    > 2)
    > * To my understanding, if each channel has its own wire, then
    > broadband cable with only one wire would not be able to send signals
    > in both directions at the same time ( I think we call this simplex
    > transmition )?!
    >
    > * But if word channel only describes different frequencies, then a
    > single wire can transmit signals in both directions at the same time.
    > I find that hard to believe, since ( as little as I know about
    > physics ) I know that a wire can only have current flowing in one
    > direction at the time ( as such voltage can also have only one
    > direction at the time )


    If you consider the use of passband filters, then you can conceive of a
    cable being able to send data in both directions over the same coax or
    twisted pair, at the same time. In different frequency bands, energy can
    flow in either direction. For example, Transmitter A will see a very
    high impedance on all frequencies except Frequency 1. So Transmitter A
    will only be able to send energy out on the Frequency 1 band. At the
    same time, Receiver A connected to that same cable will be blocked from
    receiving any energy from the cable, except on the Frequency 2 passband.
    So the same cable will be used to send energy in both directions at the
    same time, and Transmitter A will not damage Receiver A by blasting it
    with too much energy.

    Should not be hard to believe. You can certainly do this with cell
    phones over the air, right?

    Alternatively, the system can share the same band, switching quickly
    between transmit and receive, so that in use, a human won't notice. This
    describes the original coax Ethernet, for example. Not so easy to do
    with voice, though.

    > 3)
    > I'm not saying it would be practical, but if each channel has its own
    > wire, then why can't all channels have the same frequency? I know
    > cable TV operates by sending each program with different frequency and
    > we can tune TV to certain frequency and watch a particular program
    > ( that was send with that frequency ). Since each channel has its own
    > wire, then each cable program would be send through different wire and
    > we could tune into specific program simply by connecting TV receiver
    > ( or something ) with that certain wire ( this way all channels could
    > send signals with same frequency )?!


    If each channel has a dedicated twisted pair, or dedicated wire and
    common ground, in principle you would not need to send the signal over
    different RF carriers. For example, twisted pair Ethernet, like
    10BASE-T, has separate, dedicated transmit and receive twisted pair
    cables between each host and each hub or switch. The signals are
    transmitted at baseband. Which means, they use the same carrier
    frequency, if you will.

    Bert


  3. Re: Communication channels

    wrote:

    > hello
    >
    > The following questions are not related to Multiplexing, where single
    > channel combines multiple signals into one signal ( and at the other
    > end the original channels are extracted by the process called
    > demultiplexing ). For the sake of argument, let's pretend that there
    > is no such thing as multiplexing


    Oh, and by the way, what I decribed in the previous reply *is*
    multiplexing. It's called "frequency division multiplexing," or FDM, as
    opposed to "time division multiplexing," or TDM, but multiplexing
    nevertheless.

    And what is commonly referred to as "wave division multiplexing," WDM,
    is the same as FDM exactly. It simply applies to higher frequencies in
    the EM spectrum, up in the visible light range, or near there.

    All are forms of multiplexing. Different techniques used to share a
    single medium among many data streams.

    Bert


  4. Re: Communication channels

    On Jun 4, 1:10 pm, kaja_love...@yahoo.com wrote:
    > hello


    > * Now does the word channel apply to individual wire in a broadband
    > medium and thus each channel is actually a wire, capable of
    > transmitting signals with certain frequency? If that is the case then
    > for broadband cable to have 20 channels, it must have 20 wires?
    >
    > * Or are channels only a logical construct, meaning that word channel
    > only refers to a certain frequency? Thus a single wire can at the same
    > time transmit more than one signal by having each signal transmitted
    > at different frequency? That's a bit hard to believe.


    When thinking about something like cable TV, it is the latter. It is
    very possible for a wire to transmit more than one frequency at the
    same time. If you take an oscilloscope and look at the signal on the
    wire, and and discover that it is not a pure sine wave, but some
    warbling thing that does not make any sense (is not periodic), then
    technically, that thing is a bunch of sine waves added together that
    are changing in time. This was the thesis of the most his eminence,
    J.B.J. Fourier, of Fourier Theory http://en.wikipedia.org/wiki/Joseph_Fourier).

    > * To my understanding, if each channel has its own wire, then
    > broadband cable with only one wire would not be able to send signals
    > in both directions at the same time ( I think we call this simplex
    > transmition )?!


    In fact, it is possible and used every day. This is why, among a many
    other situations, a plain old telephone system (POTS) phone has only
    two wires instead of four. Both ends of the phone line, the end in
    the house as well as the end in the central office, use inductors
    (http://en.wikipedia.org/wiki/Inductor) to induce into the wire the
    signal that they want on the wire. The signals manifest as charge
    distributions along the wire. Normally, if only one side were
    inducing, there would be only one charge distribution. But if both
    sides induce, then you can think of the sides generating charge
    distributions that "fight" with each other (sometimes causing a zero
    at some point along the wire, but not other points). The resulting
    charge distribution after the "fighting" is nothing more than the the
    two signals added together.

    > * But if word channel only describes different frequencies, then a
    > single wire can transmit signals in both directions at the same time.
    > I find that hard to believe, since ( as little as I know about
    > physics ) I know that a wire can only have current flowing in one
    > direction at the time ( as such voltage can also have only one
    > direction at the time )


    The signal can be, indeed, transmitted in both directions at the same
    time. The funny thing about your remark is that you can think of it
    at many different levels. Many electrical engineers take the
    principle of superposition as obvious. But if you think really hard
    about it, you come to realize that electromagnetic waves must be
    implemented as distributions of quantized particles of light, each
    with there own momentum, so you get into quantum mechanics very
    quickly. It is true..if you present a pure DC signal against the wire
    at both ends, assuming no noise, there will be a fixed net voltage,
    fixed electrical field, and therefore a fix flow in one direction.
    When you start jiggling the charges however, you also giggle the flux
    of electromagnetic emission, and their movement result in propagation
    of (time-varying) fields. What's remarkable is that the moment was
    there before, only now, it's time varying, so there is a visible
    manifestation as a result at each end.

    > I'm not saying it would be practical, but if each channel has its own
    > wire, then why can't all channels have the same frequency? I know


    The can. Called mulltiplexing.

    > cable TV operates by sending each program with different frequency and
    > we can tune TV to certain frequency and watch a particular program
    > ( that was send with that frequency ). Since each channel has its own
    > wire, then each cable program would be send through different wire and
    > we could tune into specific program simply by connecting TV receiver
    > ( or something ) with that certain wire ( this way all channels could
    > send signals with same frequency )?!


    Note that there is nothing inherently desirable about using same
    frequency in a coaxial cable. Also note that there are modulation
    schemes were this is done. amplitude modulation, phase modulation,
    quadrature amplitude modulation , etc. http://en.wikipedia.org/wiki/Amplitude_modulation

    As for as many wires...I have about 200 channels on my
    television...and so, the wire would be about the thickness of my arm.
    The cable installer would get very tired lugging such a cable around.

    -Le Chaud Lapin-


  5. Re: Communication channels


    wrote in message
    news:1180980623.296524.29640@q75g2000hsh.googlegro ups.com...
    > hello
    >
    > The following questions are not related to Multiplexing, where single
    > channel combines multiple signals into one signal ( and at the other
    > end the original channels are extracted by the process called
    > demultiplexing ). For the sake of argument, let's pretend that there
    > is no such thing as multiplexing
    >
    > 1)
    > When talking about broadband medium the word channel is often used. A
    > single Broadband medium can have more than one channel, where each
    > channel transmits signals with certain frequencies. This way one
    > broadband medium can transmit multiple signals at the same time.
    >
    > * Now does the word channel apply to individual wire in a broadband
    > medium and thus each channel is actually a wire, capable of
    > transmitting signals with certain frequency? If that is the case then
    > for broadband cable to have 20 channels, it must have 20 wires?


    No
    >
    > * Or are channels only a logical construct, meaning that word channel
    > only refers to a certain frequency?


    Getting warm. This is where frequency division multiplexing comes in


    > Thus a single wire can at the same
    > time transmit more than one signal by having each signal transmitted
    > at different frequency? That's a bit hard to believe.


    Not at all hard to believe.

    Why would you say that is hard to believe.



    >
    >
    >
    > 2)
    > * To my understanding, if each channel has its own wire, then
    > broadband cable with only one wire would not be able to send signals
    > in both directions at the same time ( I think we call this simplex
    > transmition )?!
    >
    > * But if word channel only describes different frequencies, then a
    > single wire can transmit signals in both directions at the same time.
    > I find that hard to believe, since ( as little as I know about
    > physics ) I know that a wire can only have current flowing in one
    > direction at the time ( as such voltage can also have only one
    > direction at the time )


    Volatge is not the issue here.
    >
    >
    > 3)
    > I'm not saying it would be practical, but if each channel has its own
    > wire, then why can't all channels have the same frequency?


    Interference for starters


    > I know
    > cable TV operates by sending each program with different frequency and
    > we can tune TV to certain frequency and watch a particular program
    > ( that was send with that frequency ). Since each channel has its own
    > wire, then each cable program would be send through different wire and
    > we could tune into specific program simply by connecting TV receiver
    > ( or something ) with that certain wire ( this way all channels could
    > send signals with same frequency )?!


    Cable TV as an example. The cable used carries a frequency range, with
    different discrete frequency channels within that cable
    >
    >
    > thank you
    >




  6. Re: Communication channels

    hiya

    >>The following questions are not related to Multiplexing, where single
    >>channel combines multiple signals into one signal ( and at the other
    >>end the original channels are extracted by the process called
    >>demultiplexing ). For the sake of argument, let's pretend that there
    >>is no such thing as multiplexing

    >
    >Oh, and by the way, what I decribed in the previous reply *is*
    >multiplexing. It's called "frequency division multiplexing," or FDM, as
    >opposed to "time division multiplexing," or TDM, but multiplexing
    >nevertheless.


    If I understood you guys correctly, you are saying that we can for
    example send two signals at the same time through a single wire -
    first signal in one direction and second in the opposite direction.
    But I don't understand why multiplexing would be required for two
    signals to travel in opposite direction. Isn't multiplexing a
    technique which combines signals traveling in same direction, into one
    signal?
    If two signals are traveling in opposite directions, then there is no
    need to combine the two signals, since the meeting of two waves
    ( signals ) along a medium does not alter the shape of individual
    waves?!

    cheers


  7. Re: Communication channels

    wrote:

    > If I understood you guys correctly, you are saying that we can for
    > example send two signals at the same time through a single wire -
    > first signal in one direction and second in the opposite direction.


    Yes. You can even try this in a sink full of water. Fill up a sink. Then
    touch the water on the left with one finger, and at the same time, touch
    the water on the right with one finger. You will see ripples propagating
    at the same time from left to right and right to left. The ripples cross
    each other and stay on their previous course. So you can say that the
    single medium, in this case a pool of water, is simultaneously carrying
    more than one signal.

    Electrons can similarly "bump" each other that way, either absorbing or
    giving off energy, thereby propagating a signal in both directions.

    > But I don't understand why multiplexing would be required for two
    > signals to travel in opposite direction. Isn't multiplexing a
    > technique which combines signals traveling in same direction, into one
    > signal?


    No requirement that the signals be in the same direction. "Multiplexing"
    is a very broad term. Here is the Wikipedia definition:

    "multiplexing (short muxing) is a term used to refer to a process where
    multiple analog message signals or digital data streams are combined
    into one signal."

    I would even more simply say that multiplexing means to share a medium
    among many "channels" or "data streams" or whatever type of connection
    you care to define. The exact technique may vary, but the essence
    remains the same.

    > If two signals are traveling in opposite directions, then there is no
    > need to combine the two signals, since the meeting of two waves
    > ( signals ) along a medium does not alter the shape of individual
    > waves?!


    But it does alter the energy transferred through the medium. From the
    outside looking in, you will have changed the EM patterns, or "signal,"
    on that one cable, for example.

    Bert


  8. Re: Communication channels

    thank you all for your help

    cheers


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