The need for an receive elastic buffer is shown in Requirement for the Receive Elastic Buffer. The analysis included in this appendix shows that for standard Ethernet clock tolerances (100 ppm) there can be a maximum difference of one clock edge every 5000 clock periods of the nominal 125 MHz clock frequency.
This slight difference in clock frequency on either side of the buffer accumulates and either starts to fill or empties the receive elastic buffer over time. The receive elastic buffer copes with this by performing clock correction during the interframe gaps by either inserting or removing Idle characters. The receive elastic buffer always attempts to restore the buffer occupancy to the half full level during an interframe gap. See Clock Correction.