I wanted to know what made a Duplex system work? I always thought it was the Switch that made it work. But I just read on my Cisco course 2 things that made me question it. It said this, and made me believe it was the cable's properties that made Duplex work.

The frames sent by two connected devices cannot collide, since these use two independent circuits inside the network cable.

And also

Full Duplex connections require a switch that supports Full Duplex configuration or a direct connection between two devices using an Ethernet cable

3 Answers 3


Cisco is being a bit literal. 10/100 "T" (twisted pair) does have independent TX and RX conductors. In a point-to-point situation, the link is fundamentally full-duplex. However, when a hub is involved, everyone's TX is connected to everyone else's RX. As a result, it's impossible for more than one node to transmit at a time, thus half-duplex, but it's the same cable. A switch returns us to the p-t-p model; a node isn't connected to any other nodes, just the switch, and the switch has buffers to store frames so they don't collide.

In the end, it's a combination of things. The cable is part of the equation, but does not inherently make something full-duplex. It can rule it out, though. 10base-2, for example, is literally a shared wire, so full-duplex cannot be done here. Two nodes could, in theory, talk at the same time and make sense of each other -- using echo cancellation -- but all others on the line will hear nonsense.

  • 1
    The bit about full-duplex over point-to-point 10BASE2 is actually true but it should be noted that it's only hypothetical - there are no Ethernet devices supporting that.
    – Zac67
    Apr 8, 2021 at 7:01
  • 1
    @Zac67 I must have a vampire tap around here somewhere. Let me look up in the attic :)
    – Ron Trunk
    Apr 8, 2021 at 18:56
  • @RonTrunk Good luck drilling that into 10BASE2... :-D
    – Zac67
    Apr 8, 2021 at 19:28
  • It's probably right next to the box of Arcnet terminators
    – Ron Trunk
    Apr 8, 2021 at 19:46
  • While absolutely worthless today, I still have cable, tee's, terminators, and AUI converters in the archives. I also have a small gathering of token-ring. And a "100/10" ISA network card. (using a chip from National Semiconductors for which there are zero records. and yes, 10M is a huge ask for ISA.)
    – Ricky
    Apr 8, 2021 at 20:57

Does duplex depends on the cable or the interface?

A bit of both.

There are two kinds of duplex in media (cabling): dual simplex, with a dedicated channel per direction (e.g. 100BASE-TX, 1000BASE-SX), and full duplex using echo compensation and hybrids (e.g. 1000BASE-T, 10GBASE-T). Some media are inherently half-duplex since there's only a single channel (e.g. 10BASE5). Outside the distinct media context, dual simplex is called full duplex as well.

For full-duplex communication (at the top of the physical layer), the media, the interfaces (NICs), and everything in between need to be capable of full duplex.

So, you can link two capable NICs with a direct connection that supports full duplex, like twisted pair or fiber. If you use a network concentrator (hub) in between, a switch in full-duplex mode is required because a repeater hub only supports half duplex.


duplex is measured by capability of the device the duplex is auto set by the devices max output and other factors, however todays devices run full duplex and are gigabit proven ..thats one reason 5g Is soo favorable for computers mobile and mini devices

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