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I don't underastatnd late collisions at all (unfortunately the internet didn't help me). Please describe in as much detail as possible what is late collision, how are they different from normal collisions, how NICs detect it.

And the most important: how duplex mismatch and late collision are connected.

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There's a time window at the beginning of a frame when (for half-duplex mode) collisions may happen. That's also why there's a minimum frame size - collisions need to be detected reliably. The general problem with late collisions or undersized frames is that some nodes detect a collision while others don't. If the sender doesn't detect the collision the frame is lost.

Late collisions are those that happen after that time window at the beginning has passed, when there mustn't be any collision any more. They are a sign of some misdesign or malfunction - overlong segments, too many chained repeaters, duplex mismatches, NIC problem.

With a duplex mismatch - one link partner using half-duplex, the other using full-duplex mode - the full-duplex side (FDX) starts sending it any time it's got data ready, without checking for a collision (there can't be a collision in FDX). Depending on what the half-duplex side (FDX) is currently doing, there are different outcomes:

  • HDX is idle: frame is received correctly
  • HDX has just started sending: it detects a collision and aborts transmission - FDX receives a short frame (a runt) with FCS error
  • HDX has been transmitting more than 512 bits into the frame: it detects a late collision and aborts transmission - FDX detects an incomplete frame (not a runt) with FCS error

You should note that half-duplex mode is all but obsolete, originating from inherently half-duplex coax cabling and obsolete repeater hubs. For the last two decades, business networks have been using switched networks in full-duplex mode throughout.

Duplex mismatches can only happen when you force a specific mode on a port (e.g. 100 Mbit/s, full duplex), disabling Auto Negotiation, which in turn makes the link partner fall back to half-duplex mode. Gigabit Ethernet and faster variants use full duplex exclusively (half duplex was standardized for GbE but isn't supported in real hardware) and Auto Negotiation is mandatory.

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  • Wouldn't the FDX side detect the Jam signal, and detect the collision? – Ron Trunk Feb 22 at 20:40
  • No. For FDX, collisions don't exist. – Zac67 Feb 22 at 20:42
  • I'm not convinced. Whenever I've seen a duplex mismatch on a host, the switch (which is full duplex) reports late collisions. I know there shouldn't be late collisions, but I think the switch detects the jam signal nonetheless. – Ron Trunk Feb 22 at 21:13
  • It's possible that a NIC even in full-duplex mode interprets a jam signal as (likely always late) "collision". However, according to IEEE 802.3 clause 1.1.2.2, "Full duplex operation does not require that transmitters defer, nor do they monitor or react to receive activity, as there is no contention for a shared medium in this mode." Since there's no way a collision can happen, the NIC shouldn't actually report those. – Zac67 Feb 22 at 21:51
  • Annex 4A Simplified full duplex media access control states "This MAC does not support the half duplex mode of operation so there is no need for collision detection or handling." - so, it's not prohibited but simply not necessary to report a detected jam signal. – Zac67 Feb 22 at 22:13
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A late collision is one that occurs at least 512 bits after the start of the frame.

A device that is full duplex assumes that there will never be a collision, so it can transmit data at any time.

If a full duplex device (A) is connected to a half-duplex device (B), A does not check to see if B is transmitting before it starts sending. If B is transmitting at that time, B will send a jam signal, which is detected by A as a late collision.

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  • So device (A) detect jam signal as a late collision only if device (A) already transmitted 512 bits? If device (A) (for example) transmitted 256 bits then jam signal recognized as "common" collision? – Alexander Feb 22 at 18:03
  • That is correct – Ron Trunk Feb 22 at 18:16
  • And for device (A) it can be late collision, but for device (B) it can be "common" collision, am i right? It all depends on which device to look at – Alexander Feb 22 at 18:44
  • @Alexander A port in full-duplex mode doesn't detect any collisions. It just receives aborted transmission attempts with FCS errors. – Zac67 Feb 22 at 19:55

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