dlbAthPhyOfdmIllegalLength is a read-only Counter32 tallying OFDM illegal length errors for this Atheros interface's radio (PHY) hardware (row keyed by ifIndex) since the interface (or peer entry) came up, and it wraps back to zero once it overflows 32 bits. Because it is a PHY-layer condition reported directly by the Atheros radio chip (below the 802.11 frame-parsing logic), a rising count points to a physical-layer problem such as RF noise, a bad antenna connection, or a failing radio rather than a configuration mistake higher up the stack.