ethernet inactivy traffic

An ethernet PHY (serdes) immediately notices whether there is an ethernet link, even when autonegotiation is disabled (on both sides). How exactly does this work, does the PHY continuously send some kind of "background" Fast Link Pulses ?

Is there something like background/inactivity traffic in full-duplex ethernet (say 100BaseT), what about 1000BaseT and 1000BaseX (optical) ? Does the same go for 10/100 half-duplex ethernet (CSMA-CD) ?

I just wonder what goes over the (serial) ethernet wire-pairs when no data is exchanged (temporarily).

I guess the parallel (xMII) link between PHY and MAC never exchanges inactivity like traffic, is this true ?


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PHYs send idle characters when there is no activity. This gives the receive PLL something to lock onto.

These idle characters are generated by the PCS (physical coding sublayer) when idle is indicated on xMII.

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As Marris wrote, there are IDLE symbols going over the wire of a

100Base-TX/FX or 1000Base-T/X link while no data is sent - this is defined by the 4B/5B and the 8B/10B encoding rulesets used in those media systems. The duplex operation mode does not matter. 10Base-* systems do use a different type of encoding (Manchester encoding) not using IDLE symbols and being silent at the DC offset level when no data is sent. However, there are approaches for flow control on half duplex links which actually do congest the link with data to throttle sending activity of the station(s) on the other end of the link.
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