Ethernet – How does Ethernet driver know if the cable is plugged in

Assume you 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 Switch, using autonegotiation (at 100Mbps)

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. The Cisco switch uses CSMA/CD for access to the link.

Let's assume the PC and the Cisco both transmit at exactly the same instant; the logical diagram and physical layer diagram show the same behavior from two different perspectives, but the physical layer diagram is most relevant to your question.

  LOGICAL DIAGRAM
  ===============

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


  PHYSICAL LAYER PIN DIAGRAM
  ==========================

      PC                                         Cisco Catalyst Switch
      100/full                                   100/half

                    Tx D1             
                    ----->
      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

In the diagrams above, the PC (full duplex) is on the left and the Cisco Switch (half duplex) is on the right. Both sides transmit (Tx) simultaneously on pins 1 and 2, this pair of pins is called D1.

When the NIC on the switch receives the PC's frame on the D2 pair while the switch is simultaneously transmitting on the D1 pair, the switch registers a collision (answer reference). The collision is only registered on the switch, because it is in half-duplex mode.

Notes about GigabitEthernet:

• Half-duplex is called out in the standard^{Note 1}; however, nobody actually uses half-duplex GigE. This means that GE won't use CSMA/CD
• GE uses all 8 pins in the RJ45 mod plug, and the specific TX / RX pins are allocated dynamically.

End Notes:

^{Note 1} Quoting IEEE 802.3-2012 4.1 (Italic emphasis mine):

4.1.2.1.2 Reception without contention

In half duplex mode, at an operating speed of 1000 Mb/s, frames may be extended by the transmitting station under the conditions described in 4.2.3.4. The extension is discarded by the MAC sublayer of the receiving station, as defined in the procedural model in 4.2.9.

Short answer: in theory yes, in practice no.

Long answer: There are SFP+ optics which can also work at 1 GbE, I've talked (sorry, no other source than that) to people who were experimenting with it and were able to get it running. Unfortunately it is highly dependable on the device you plug the optics in, i.e. not every device in the test was able to tell the transceiver to step down to 1 GbE.