Physical Layer Errors in Ethernet – Common Issues

errorsethernet

What happens if an error is detected at the lowest possible layer of the stack ISO/OSI for Ethernet?

For example, considering 100BASE-TX, it is implemented with a 4B5B coding this means that there are 16 combinations of bits are invalid. What happens if a receiver "reads" from the physical medium an invalid combination of bit, e.g. "00001"?

  • It discards that bit (invalidating the entire frame)
  • It select the "most similar" combination
  • or something else?

I know that upper layers can detect the error, e.g. the MAC layer via the FCS field, but I don't know how the transreceiver works with an error detected in the signal.

Best Answer

For the specific case of 100BASE-TX and the 100Mbps MII interface: there is a receiver signal RX_ER.

The Physical Coding Sublayer (PCS) specification says:

24.2.2.1.7 Invalid code-groups

...The PCS indicates the reception of an Invalid code-group on the MII through the use of the RX_ER signal.

The Reconciliation Sublayer and Media Independent Interface specification says:

22.2.1.5 Response to RX_ER indication from MII

If, during frame reception, both RX_DV and RX_ER are asserted, the Reconciliation sublayer shall ensure that the MAC will detect a FrameCheckError in that frame.

So the PHY part indicates the error out of band, and the MAC does what it likes, as long as it ends up generating a FrameCheckError.

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