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Thread: half duplex

  1. #1

    half duplex

    hello,

    i am a newbie to system administration. what is the difference between half duplex and full duplex? I have seen some admins change the half duplex to full duplex to increase the download speed. Is there any use for keeping the ethernet in half duplex mode?

  2. #2
    anybody can advice me?

  3. #3
    Join Date
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    Quote Originally Posted by tigerlinux
    what is the difference between half duplex and full duplex?
    http://en.wikipedia.org/wiki/Duplex_communication

  4. #4
    hmmm, what is the advantage if we keep the server in half duplex mode?

  5. #5
    Join Date
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    good question think of it like this. half-duplex is like a hand radio you can only speak at any one time, so when the other person on the other end is speaking you have to wait until he's finished.

    full-duplex is when data communication can pass both ways for example a telephone you can speak at the same time as the other person

    network professionals prefer full-duplex becuase data can travel at the same time resulting in no collsions so it conserves bandwidth and network resources.

    i hope that answers your question or helps you to answer it
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  6. #6
    Thank You very much

  7. #7
    Join Date
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    glad to help
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  8. #8
    Join Date
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    Keep in mind that full-duplex Ethernet is only possible with two-node collision domains - you must be connected directly to a switch, bridge or through a crossover cable to achieve full duplex. Otherwise you'll just get a whole lot more collisions if you enable it, obliterating network performance. Also remember that modern network gear will auto-negotiate the fastest available mode. If this autonegotiation is unsuccessful, it is likely that forcing a faster or more capable mode will fail or be unreliable. In general it's not a good idea to modify the transfer mode manually (unless you want to force the other endpoint to, e.g. 10mbps for bandwidth limiting considerations).

  9. #9
    Join Date
    Mar 2004
    Posts
    295
    I wanted to expand on this..

    A collision domain is a datapath for which devices sending data on that datapath could have a collision with other devices using the same datapath/(collision domain).

    A few years back, half duplex was used because multiple machines were on the same collision domain.
    One computer would send data, but it could not receive data.
    This is known as half duplex, it could not receive data because since it was sending data.
    Any other machine on the half duplex network that sent data would cause a collision with the data it was sending itself.
    In half duplex, when you send data you copy that data to the loopback (rx) on your NIC card.
    That data has to be an exact duplicate of what you are sending.
    If it is not, then someone else has sent data on the network, and caused a collision ruining the data you sent to the network.

    Today, switches and routers have a complex matrix for their bus.
    They have dedicated datapaths instead of shared, meaning each port has its own collision domain.
    1 PC connected to the port means that pc has its own collision domain, since no other computers are on that port you will not have collisions.
    As you are the only one on that collision domain.

    Hubs use a shared bus, so every port on a hub is 1 collision domain.
    Any port you connect to, and another pc on a different, are in the same collision domain.
    Thus half duplex is needed.

    Take this scenario for example.
    You connect your dedicated server to a switch in a datacenter.
    The switch port is set to full duplex, and lets say it will not autonegotiate your pc's NIC Speed/Duplex because setting it to full duplex disables auto negotiation.
    So your server boots up and defaults to 10/Half Duplex, or 100/Half Duplex.
    What do you think will happen?

    You will have packet loss.
    Your server will not expect to receive packets when it sends packets.
    And it will not ever send packets when it is receiving, thou sometimes it happens (millisecond difference)
    Lets say you are sending some packets, the switch in full duplex will say.. you know what ? this port is full duplex I dont have to wait to send this packet.
    The packet is sent to the server, only to mess up the data going to your loopback (rx) since you are in half duplex.
    Causing collisions and likely the possibility of packetloss.

    The more data you tx/rx, the more collisions you will have.
    This will highly degrade your performance.
    Each time a collision happens, a timer is set on the server for retransmit.
    Once the timer expires it may send data again.
    But with the duplex mismatch, you will likely have some extra laitency and loss.
    As once it fails so many times of re transmitting, it will discard the data.

  10. #10
    Thank You

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