While searching how the PHY LINK status is determined, I stumbled across this answer :

In 100BaseTX Ethernet, link pulses aren't used, but instead, link status is detected and maintained via the continuous reception of Idle ("/I/") 4B/5B codewords (Figure 13 in AN1120).

Where the idle code message is coming from? is it sent directly by the PHY (layer one protocol) or it's coming from the MAC (layer 2) or even above?

Should both parties continuously send the Idle code ?

1 Answer 1


IDLE is a PCS code symbol in 100BASE-X (from the 4b/5b set) on the physical layer, see IEEE 802.3 Clause 24 for details. As most of Fast Ethernet's physical layer, it's been borrowed from ANSI X3T12 aka FDDI.

IDLEs are continuously sent when there's no data transmission:

The Idle code-group (/I/) is transferred between streams. It provides a continuous fill pattern to establish and maintain clock synchronization. Idle code-groups are emitted from, and interpreted by, the PCS.

Since continuous signaling on an idle link wastes energy, low-power idle (LPI) was introduced with Energy-Efficient Ethernet (EEE) in 802.3az-2010.

FE does use Fast Link Pulses, but only for negotiating/establishing a link, not for maintaining it.

  • Are both parties sending the idle code? Mar 6, 2021 at 14:20
  • 1
    Yes, even in half-duplex mode (where at least one side must be transmitting IDLEs, everything else is a collision) - IDLEs do not assert CRS (carrier sense)
    – Zac67
    Mar 6, 2021 at 15:27

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