I have a question about each bit fields(bit 5 to bit 8) of PHY register 5. The following is the description page of a PHY.

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I know that these fields are Technology Ability Field which is the link-partner advertised during the auto-negotiation.

The following is the table of the IEEE802.3 specification. enter image description here

In this table, A3 is 100BASE-TX full duplex but A2 is 100BASE-TX without describing duplex. I think that the A2 is the 100BASE-TX half duplex. But there are no comments in the standard.

I have questions at this point:

  • Why didn't the specification specify the "duplex type" for the A2 field?
  • Is the half duplex is the default duplex for the 100Base-TX?
  • Are there any comments about it in the IEEE802.3 standard?
  • Do you have any other opinion?

Thank you in advance.


1 Answer 1


Yes A2 corresponds to half duplex; this is implied by A3 being explicitly specified to be full duplex.

I have a copy of the 2008 version of 802.3. Annex 28.B.3 specifies a prioritization scheme as follows (from highest to lowest):

  • 10GBASE-T full duplex
  • 1000BASE-T full duplex
  • 1000BASE-T
  • 100BASE-T2 full duplex
  • 100BASE-TX full duplex
  • 100BASE-T2
  • 100BASE-T4
  • 100BASE-TX
  • 10BASE-T full duplex
  • 10BASE-T

The spec provides the (performance based) rationale for this, stating: Full duplex solutions are always higher in priority than their half duplex counterparts.


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