I am interested to know if the CSMA/CD protocol is still active in modern Networks and if it is active why and what is his role now?

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2 Answers 2


I am interested to know if the CSMA/CD protocol is still active in modern Networks and if it is active why and what is his role now?

What is your definition of "active"?

If you are talking about is it in common use, then no it isn't. Most modern networks and devices utilize interfaces that are capable of establishing a full-duplex connection. CSMA/CD is not used in full-duplex operation.

However, if by active you are asking if it is present, then the answer is yes. Up through 1000BASE-T, half-duplex operation is supported. CSMA/CD is used for any half-duplex link.

  • Half-duplex communication has been defined for Gigabit Ethernet but there are no devices around actually supporting it. In practice, half-duplex can only be used for 10/100.
    – Zac67
    Commented Sep 3, 2017 at 9:00
  • 1
    @Zac67, since that was where I was defining CSMA//CD as present, there is no denying it is present in 1000BASE-T. It doesn't matter that there are very few cases where it is actually used, it is still present.
    – YLearn
    Commented Sep 3, 2017 at 16:35
  • My point was that it is not present for 1000BASE-T (or -X for that matter) - real hardware simply doesn't support it. It supports HDX for 10M or 100M links but not for 1G.
    – Zac67
    Commented Sep 3, 2017 at 16:43
  • 1
    @Zac67, sorry, but you are wrong. There are a number of devices where you can set 1000/half in the configuration, simply because it is present in the standard. Just because something isn't used doesn't preclude it from being present. I have a bandsaw in my workshop; I can use my workshop without using my bandsaw, but that has no bearing on if the bandsaw is present in my workshop.
    – YLearn
    Commented Sep 3, 2017 at 18:50
  • Care to share which devices? I'd been looking at datasheets for months and couldn't find anything.
    – Zac67
    Commented Sep 3, 2017 at 18:53

Short answer:

The support is still present but it is generally only used when legacy equipment is connected, either hubs or very old (or embedded) network interfaces that operate at 10Mbps only and do not support autonegotiation.

Long answer:

CSMA/CD is a protocol used for communication over half-duplex communications media.

10BASE-5 and 10BASE-2 links are half-duplex by nature but are extremely rare nowadays. Hubs (aka multiport repeaters) are also by their nature half-duplex.

10BASE-T, 100BASE-TX and 1000BASE-T links are full duplex at the electrical level but for backwards compatibility and to allow the use of hubs they support a half-duplex mode where simultanious transmission is treated as a collision.

Autonegotiation allows devices to automatically choose a speed and duplex, it was introduced alongside the 100 megabit standards. If one end supports autonegotiation and the other does not then for compatibility reasons the end with autonegotiation will default to half-duplex mode. As a general rule devices that support 100Mbps and higher support autonegotiation, 10Mbps only devices do not.

In principle you can run full duplex without autonegotiation by manually specifying full duplex mode on both ends of the link but in practice that way lies pain. It is very easy to end up with an inadvertant duplex mismatch which results in a link that works very badly in ways that are diffiult to troubleshoot.

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