Abstract: Line cycle adaptation periods having variable duration are disclosed. A powerline cycle may be segmented into a plurality of line cycle adaptation periods having variable duration based upon signal-to-noise (SNR) characteristics measured at various times throughout the powerline cycle. The line cycle adaptation periods may include at least two periods with unequal durations. Each line cycle adaptation period may be associated with one or more tone maps defining physical layer transmission properties to be used by a second device for transmissions occurring during the line cycle adaptation period.