dme frequency usage, channelization and pairing with other navaids (vor, ils, etc.) are defined by icao. 252 dme "channels" are defined by the combination of their interrogation frequency, interrogation pulse spacing, reply frequency, and reply pulse spacing. these channels are labeled 1x, 1y, 2x, 2y, ... 126x, 126y. x channels (which came first) have both interrogation and reply pulse pairs spaced by 12 microseconds. y channels (which were added to increase capacity) have interrogation pulse pairs spaced by 36 microseconds and reply pulse pairs spaced by 30 microseconds.