ARTCC RADAR CTLR CLBED AN LR31 TO FL200 ON A CONFLICTING COURSE WITH A C414 AT FL210. THE LR31 MISINTERP HIS ALT ASSIGNMENT AND CLBED THROUGH THE C414'S ALT WITH LTSS. RPTR FEELS THAT PHRASEOLOGY USED BY THE LR31 CONTRIBUTED TO THIS ERROR.

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SMT X WAS ENRTE THROUGH THE N DEP (#38) SECTOR AT HIS CRUISE ALT OF FL210. LTT Y WAS CLBING OUT OF AN ATLANTA SATELLITE ARPT; POSSIBLY FULTON COUNTY (FTY). THE CTLR ON THE SECTOR WAS EXTREMELY BUSY WITH A MIX OF OVERFLTS AND SATELLITE/ATL JET DEPS. TO FURTHER COMPLICATE THE SIT; THERE WAS CONVECTIVE WX ACTIVITY IN THE NORTHERN PORTION OF THE SECTOR NECESSITATING DEV FROM ASSIGNED ROUTING BY SEVERAL ACFT. ON INITIAL CONTACT; LTT Y WAS ISSUED A CLB CLRNC TO FL190 WHICH HE ACKNOWLEDGED BY READING BACK 'UP TO 1-9-0.' THE RTES OF FLT PUT LTT Y AND SMT X IN CONFLICTION NECESSITATING ALT SEPARATION. WHEN SUBSEQUENTLY CLBED TO FL200; LTT Y ACKNOWLEDGED WITH 'UP TO 2-ZERO.' LTT Y CLBED TO FL220 AND LOST SEPARATION WITH SMT X AT APPROX XA34 ON JUL/XA/97. WHEN QUESTIONED BY THE CTLR; LTT Y STATED THAT HE WAS ISSUED FL220 AND ACKNOWLEDGED THE ALT. CURIOUSLY; SMT X WAS ALSO ON THE FREQ AND ACKNOWLEDGED EACH OF HIS ALT CLRNCS WITH 'UP TO' THE ASSIGNED ALT. I BELIEVE BOTH OF THESE ACFT ARE FROM THE SAME COMPANY; POSSIBLY A COMPANY BASED IN INDIANAPOLIS. AFTER REVIEWING THE TAPES; THE CTLR WAS IDENTED AS PRIMARY IN THE ERROR FOR FAILURE TO INSURE A PROPER READBACK. FAR SHOULD BE AMENDED SO AS TO REQUIRE PLTS TO COMMUNICATE VIA STANDARD PHRASEOLOGY WHEN ACKNOWLEDGING CLRNCS/CTL COMS. NEITHER OF THESE ACFT WERE EQUIPPED WITH TCASII. A LARGE PERCENTAGE OF SYS ERRORS ARE DUE TO 'READBACK' DISCREPANCIES. A MIDAIR COLLISION DUE TO CONFUSION OVER A CLRNC; REGARDLESS OF WHO IS THE INCORRECT PARTY; IS UNACCEPTABLE IN LIGHT OF THE 'EASY' REMEDY AVAILABLE. I HAVE NO DOUBT THAT THE PLT NOT IN COMMAND ON LTT Y (OR WHOMEVER WAS ON THE RADIO) WAS NOT THE PLT THAT SET IN THE ALT. HE HEARD 'UP 2-2-ZERO' BEING ACKNOWLEDGED AND PLUGGED IT IN.