half duplex operation

Unanswered Question
Dec 14th, 2008

hi all, can someone please clarify the technical aspects of half duplex, does half duplex use both pairs of the ethernet cable to send and receive? or 1 and 2 to transmit and 3 and 6 to receive like normal?

cheers

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Giuseppe Larosa Sun, 12/14/2008 - 09:28

Hello Carl,

in half-duplex the algorythm used is CDMA = Collision Detection multi-acess

each device senses the rx pair to see if any frame in on the cable.

If the wire is free it will try to send.

During tranmission rx is compared to tx if there is any difference in the first 64 bytes that is classified as a collision and the frame retransmitted up to to 16 times using a random exponential backoff timer.

This is needed if connecting to an old hub that acts as a repeater of signal out all of its ports.

So even if both pairs are used it is different the logic

Hope to help

Giuseppe

carl_townshend Wed, 12/17/2008 - 03:18

hi there

So really there is no such thing as tx and rx pair on the wire in this case,it just uses whatever pair is free ?

fjcardenas-1 Wed, 12/17/2008 - 06:38

It uses the pairs of TX and RX as normal but it can not transmit and receive at the same time. It either transmits (using the TX pair) or receives (using the RX pair) at certain point in time.

As mention, CSMA/CD is used to minimize the probability of collisions by sensing if the media is in use (wait a random time before transmitting) or if the media is free (start transmitting).

carl_townshend Wed, 12/17/2008 - 07:52

so basically in the hub etc, the tx and rx pair connect to the same wire on the hub, so basically all traffic gets sent along everyones cable ?

scottmac Wed, 12/17/2008 - 08:58

Carl:

Remember that Ethernet started out on a single chunk of coax (first thick, then thin). Ethernet began as a "bus" technology - every host connected to the same single pair of conductors; the shield and the center conductor. One pair of conductors; half-duplex, either transmitting or receiving

When 10Base-T (originally LatticeNet) showed up, it was to emulate the operation of a bunch of bus-connected hosts in a system connected in a star topology and physically attached with unshielded twisted pair (not even Cat 3 at the time).

10BASE-T (Ethernet over UTP)is emulating the original Ethernet over coax. Optionally, 100Base-tx can be made to operate in half duplex.

Even though there are two discreet paths (two pair of copper) it is still operating in half-duplex becasue both hosts are configured to only transmit after listening to the wire and determining that no other communication is occurring. Half-duplex also implies that the host listens to its own transmission to make sure that no collisions occur during the transmission. That's why when one host is set for full duplex, and the other host end of the connection is set to half-duplex, you will see a rapidly incrementing collision counter.

Hubs are / were just a big multi-port transceiver ... everything coming in one port was repeated out, clock for clock, all the other ports.

The hub is the emulation substitute for the coax cable.Repeaters, not soldering all the wires together.

Good Luck, Happy Holidays

Scott

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