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Full duplex or half duplex?

Discussion in 'Electronic Design' started by Peabody, Apr 6, 2013.

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  1. Peabody

    Peabody Guest

    Ok, I'll admit it up front. I never learned RF. Indistiguishable from
    magic, without even needing smoke, or mirrors.

    But after dropping my traditional Ma Bell twisted pair landline, and trying
    to assess how well its replacment (Ooma) is doing, I've been trying to notice
    which conversations are effectively half duplex and which are full duplex.
    And that has led to the more general question of how all RF devices operate
    in this regard.

    My landline was full duplex. Even though both sides of the conversation were
    on the same twisted pair at the same time, circuits at both ends were able to
    subtract what they were sending from the combined signal, and end up with the
    received signal, at the same time. But that landline didn't involve
    transmitting with RF.

    The replacement Ooma VOIP device, when used with a corded phone, seems to be
    full duplex, but cell phones and cordless phones at the other end often seem
    to be half duplex.

    It seems to me that a device cannot both send and receive at the same time
    over the same antenna. Well, at least not at anywhere close to the same
    frequency, and maybe not at all - because the transmitted energy would
    completely swamp the much weaker received signal. But if that's true, then
    it would appear that the wireless devices we use all the time are actually
    half duplex devices. And that would even be true of a cablemodem, which
    transmits and receives modulated carriers - over coax instead of over the
    air.

    Generally, what would be the actual duplex status of these devices:

    Cell phone
    Cell tower
    Cordless phone
    Bluetooth headset
    Cable modem
    VOIP phone via a cable modem
    Wi-Fi

    And those half-duplex devices which in practice appear to function as full-
    duplex - how do they do that?
     
  2. FatBytestard

    FatBytestard Guest



    Bwuahahahahahaha!

    Funny post.
     
  3. Fred Abse

    Fred Abse Guest

    Look up "Hybrid coupler". You might be surprised.

    You can get around 40dB of isolation between ports at the same frequency,
    just using pieces of coax. There's a 3dB penalty, though.

    Much more with sophisticated designs.
     
  4. Fred Abse

    Fred Abse Guest

    Sorry, FB, I wrongly attributed Peabody's post to you.
     
  5. Guest

    At my PPoE the major product used ten slots for five full-duplex
    channels over a single frequency (at a time, it was frequency-hopping
    spread-spectrum in the ISM bands). The base mixed and assembled all
    of the packets for each recipient. It sounded full duplex but the
    digital data was all half-duplex.
     
  6. Peabody

    Peabody Guest

    Jeff Liebermann says...
    So at least I was right about that.

    I appreciate your response, and will study it to see if some
    of the magic will rub off on me.
     
  7. josephkk

    josephkk Guest

    For the devices mentioned they are mostly half duplex, but due to data
    compression and time division (or code division) multiple access they act
    full duplex (you can get into some interesting issues with quality of
    service (QoS) and traffic priorities versus dropped packets and voice
    quality). The actual data transmission rates on these devices are much
    higher than is required for voice (approximately 9600 baud for decent
    voice fidelity) and most of these is in the high 100 kilobit/s range to 11
    Mbit/s (or even more). The cable modem in particular uses some variants
    of Frequency division multiplexing as well, with the back channels being
    in a different frequency region than the incoming data (including the TV
    channels).

    This is just a general description, others here can fill in details better
    than myself usually. Also many "speaker phones" are internally half
    duplex to avoid having to compensate for room acoustics.

    ?-)
     
  8. josephkk

    josephkk Guest

    That would be worth the effort. Jeff is really spot on at this level. 'E
    wisely left out many more details and variations.

    ?-)
     
  9. josephkk

    josephkk Guest

    This is rather different from the traditional definition. e.g. Ham radio,
    CB, and FRS radio is half duplex, each side can transmit or receive but
    not both at the same time. Thus two way connection, but only one direction
    at a time.

    See:
    http://www.techterms.com/definition/half-duplex
    http://dictionary.reference.com/browse/half-duplex
    http://www.merriam-webster.com/dictionary/half duplex
    http://en.wikipedia.org/wiki/Duplex_(telecommunications)

    Digital cell phones can be described as TDD full duplex, old AMPS cell
    phones were (are) FD full duplex.
    Cool beans. At 1200 baud codec2 voice quality is showing noticeable
    deterioration, though not so noticeable at a mere 1400 baud. All other
    codecs except g.729 show various artifacts, though LPC-10 ain't half bad.

    ?-)
     
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