I have this basic question.

Gigabit ethernet has two things - 1000BASE-T and 1000BASE-TX. And Fast Ethernet has 100BASE-T.

Ethernet has 4 twisted pairs. Can someone tell me which are half duplex and full duplex and how to identify them?

In many articles, they are confusing. Some say Ethernet is half duplex. But in some articles 1000BASE-T is mentioned as full duplex.

Can someone tell me how to identify which is half and full duplex?

And in the below image of 1000BASE-T (which I understand is Full-duplex_, can someone tell how is the 125M calculated value arrived for each pair?

  • \$\begingroup\$ 100Base-T does not exist. Do you mean TX, T1, T2 or T4? \$\endgroup\$
    – Justme
    Mar 10 at 7:16
  • \$\begingroup\$ Sorry, I meant 100Base-Tx \$\endgroup\$
    – Newbie
    Mar 10 at 7:23

2 Answers 2


1000Base-T and 1000Base-TX are both full duplex only. T uses all four pairs bidirectionally, while TX uses two pairs to transmit and two pairs to receive.

100Base-TX can be full or half duplex. It uses one pair for transmission and one pair for reception.

There are also many other variants in both 100Base-T and 1000Base-T family of copper interfaces.

As per the picture you posted, 1000Base-T sends 1000 Mbit/s using 8-bit symbols, so 1000/8 equals symbol rate of 125 Mbaud. If it helps think that 8 bit symbol is divided to 4 lanea so each lane transmits 2 bits per symbol.

As to the question how to figure out how to identify these differences, for example Wikipedia articles have quite detailed information with explanations how different Ethernet standards work.

  • \$\begingroup\$ Thank you for the answer. Could you also tell me difference between T and TX (in either 100base-t and 1000base-t)? \$\endgroup\$
    – Newbie
    Mar 10 at 8:01
  • \$\begingroup\$ I've been messing with computers since about 1990 (= I've come pretty late), at the receiving end of the global supply chains - and speaking of metallic Ethernet, the only media flavours that I've seen practically available on the market were 10Base-2, 10Base-T, 100Base-TX and 1000Base-T. Regarding 1000Base-TX - is this supported by any common hardware on the market? \$\endgroup\$
    – frr
    Mar 10 at 8:04
  • 1
    \$\begingroup\$ @Newbie You accepted my answer but did you even read it? Because it already explains the difference between 1000Base-T and 1000Base-TX for you, and how 100Base-TX works, and based on my comment you should also know that 100Base-T does not exist. It also mentions how you can find more info yourself. \$\endgroup\$
    – Justme
    Mar 10 at 8:05
  • \$\begingroup\$ @Justme, sorry. Yes I did read it. Just wanted to understand whether the same difference between the 1000BASE-T and 1000BASE-TX is the same for 100BASE-T and 100BASE-TX. \$\endgroup\$
    – Newbie
    Mar 10 at 8:09

The legacy of 10Mbit twisted-pair Ethernet is just two pairs, at RJ45 pins 1+2 and 3+6. These already allow for full duplex operation, provided that the active elements support it. This wiring has been inherited by 100Base-TX.

Half-duplex operation is inherent to coax media (10Base-2), where full duplex is not possible at all.

Twisted pair Ethernet (10/100 Mbps), by virtue of having one pair in each direction, is inherently capable of full duplex. But, the old active element, called a hub (a dumb electrical repeater/amplifier) turns the network into a multi-exit half-duplex medium - not capable of full duplex. To make use of full duplex on twisted pair media, you need the network to contain a "non-blocking data stream cross-connect element" in the active element role = you need a bridge/switch. I'd almost say packet-level "store and forward", except that the other mode is "cut-through switching", which is also compatible with L1 full duplex operation. A special case is a point-to-point twisted pair connection (no hub/switch) that is also inherently full duplex capable.

I.e. the 10/100 metallic media, albeit twisted pair, with two independent pairs, is still capable of running in half duplex, for various legacy-compatibility reasons. The PHY transceivers are still capable of running the CSMA-CD over twisted pair.

Gigabit Ethernet over metallic wiring is running over all 4 pairs, using full duplex and echo cancelation on each pair. Just the initial auto-negotiation handshake still runs over the two pairs: 1+2 and 3+6. Thus, if you have a broken patch-cord, where only 1+2 and 3+6 are functional, and you use it to connect two gigabit PHY ports, the link handshakes at 1 Gbps, but then fails to function...

If auto-negotiation fails, the fallback mode is the lowest rate at half duplex, i.e. typically 10/half.

  • \$\begingroup\$ The answer is very generalized and assumes certain standards that are just commonly used, but there are many standards, e.g. 100M can be sent over one, two or four pairs, and 1Gbps can be sent over 4 or 1 pairs, depending on which exact standars is used. \$\endgroup\$
    – Justme
    Mar 10 at 7:54

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