HDMI lanes explained

The short explanation

Under the older TMDS scheme there are three data lanes plus a separate clock lane. HDMI 2.1's FRL repurposes the clock lane as a fourth data lane and embeds the clock in the data, which adds to the extra bandwidth that faster per-lane rates provide.

A readout such as 4 lanes / 40Gbps means four lanes at 10Gbps each.

Where you run into it

The lane count tells you which scheme the link is really using. Four lanes always means FRL; three lanes means TMDS or one of the two lowest FRL rates. If you expected full-bandwidth 4K120 and see three lanes, the two ends never reached the higher FRL rates — and that is usually an EDID or device question rather than a cable one.

All four lanes also have to be electrically sound. A cable with one marginal conductor can pass at a lower rate and fail at a higher one, which is exactly the intermittent behavior that gets blamed on everything else.

How the DR HDMI 8K handles it

The DR HDMI 8K reports the FRL rate, the number of lanes and the per-lane bandwidth on both the OLED and the web interface. That single line is usually the fastest answer to “did my chain actually reach HDMI 2.1 speeds”.