Suppose I have the following situation...

  • PC with the NIC hard-coded to 100Mbps, Full-duplex
  • RJ45 cable, pinned EIA-568B (not that the colors in the pinout matter)
  • Cisco Catalyst 3560 Switch, using IEEE 802.3-1998 autonegotiation (10/100Mbps, only)

Since the PC's NIC is locked at 100/full, the Cisco's Ethernet autonegotiation fails and it falls back to 100/half. Now there is a duplex mismatch on the line. Now, let's assume the PC and the Cisco both transmit at exactly the same instant.


                      Tx                  Tx
  100/full          ----->              <-----      100/half
  PC        -------------------------------------  Cisco Catalyst 3560


                    Tx D1              Tx on Rx D2??
                    ----->              <------
      568B                                       568B
      Pin  Signal                                Pin  Signal
      1   TX+ D1    ---------------------------  3   RX+ D2
      2   TX- D1    ---------------------------  6   RX- D2
      3   RX+ D2    ---------------------------  1   TX+ D1
      6   RX- D2    ---------------------------  2   TX- D1

                                           Tx D1

CSMA/CD's flowchart looks like this...

My questions are specifically at the decision block for "Is some other station transmitting?" How exactly does this work at the physical layer?


  • Consider the PHYSICAL LAYER diagram. When the Cisco transmits at 100/half, is it only transmitting on pins 1 and 2 (marked Tx D1)? Intuition says this is true, but I want to be sure I understand how a 100BaseT Ether PHY's CSMA/CD works. I consider it possible that half-duplex would configure to transmit on all four pins (Tx D1 and Rx D2) since I have heard that the tx line driver inspects the transmitted signal to look for collisions.
  • If the Cisco only transmits on 1 and 2 (Tx D1), does the Ether PHY register the collision because it senses the PC's transmit on pins 3 and 6 (Rx D2)? To me, it's counter-intuitive to register a collision if you have a dedicated Rx pair.

Please cite standards references for answers.


1 Answer 1


Half-duplex base-T Ethernet does not mean that the same wires are used for both directions of transmission, only that the two ends cannot transmit simultaneously.

Full-duplex means that both ends can transmit and receive at the same time, which only became possible with the shift from the shared coaxial cable to point-to-point unshielded twisted pair cabling.

There are never any physical collisions with base-T Ethernet, only "logical" collisions within a hub or switch. Even with Gigabit Ethernet, in which all four pairs are used in both directions simultaneously, each end uses a "digital hybrid" to separate incoming data from outgoing data on each pair.

  • \$\begingroup\$ Could you cite standards references for which pairs are used to Tx in 100/half, and for how 100-BaseT detects collisions? \$\endgroup\$ Commented Feb 1, 2014 at 17:56

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