Medium Transport; Low Wing; 2 Turbojet Eng
Air Traffic Control Fully Certified
Air Traffic Control Time Certified In Pos 1 (yrs) 20.2
Confusion; Communication Breakdown; Distraction; Troubleshooting; Situational Awareness
Conflict Airborne Conflict; Deviation - Altitude Excursion From Assigned Altitude; Deviation / Discrepancy - Procedural Published Material / Policy
Air Traffic Control Separated Traffic
ATC Equipment / Nav Facility / Buildings; Aircraft; Human Factors
I was working aircraft out of a holding pattern for ATL.  I thought I had used proper vertical separation.  I was certain Aircraft X was out of 150 and assigned Aircraft Y 160 and a reverse heading of 340 to sequence them behind Aircraft X.  There was a jet that had just diverted to HSV; and I was in a conversation with them about going to ATL since we had just started running aircraft into ATL.  The next thing I knew; conflict alert was going off and Aircraft Y was asking me to verify their assigned altitude.  I saw Aircraft Y leaving descending through 169 and Aircraft X at 165 right on top of each other.  I told Aircraft Y to level off at whatever altitude they were at; and turned them 40 degrees left.  The next update showed Aircraft Y at 163 and Aircraft X's altitude was Xed out.  At that point I just wanted to make sure everyone was separated.  Pilots were stepping on transmissions and I could not get replies right away.  I then saw Aircraft X out of 150. I think I may have told them to expedite their descent at some point; but I don't remember for certain.  I could not think straight at this point and asked the CIC to get me out.  I have since heard it was a Mode C swap.  I don't know if anything can be done about them; but it was an extremely upsetting experience to go through. Anything that can be done about Mode C swaps.
ZTL Controller reports of an incident where he is trying to separate two aircraft in a conflict due to altitude separation.  The Mode C's of the aircraft swap and cause the Controller problems.