POTENTIAL CONFLICT AS CORPORATE JET CLIMBED ABOVE ALT EXPECTED BY CTLR. ALT ASSIGNMENT CONFUSION RESULTED FROM FLT CREW MISTAKING FL200 ALT ASSIGNMENT FOR FL290; WHICH THEY WERE EXPECTING. SITUATION EXACERBATED BY HIGH CTLR AND FLT CREW WORKLOAD; TRAFFIC VOLUME; FREQ CONGESTION BLOCKED AND SIMULTANEOUS TRANSMISSION.

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ON CLBOUT OF IAD; BWI APCH HANDED US OFF TO ZDC JUST NE OF THE BAL VOR AND INSTRUCTED US TO CLB TO FL190. COM WAS ESTABLISHED WITH ZDC. ON THE HDOF BOTH PLTS THOUGHT THE CENTER CTLR CLRED US UP TO FL290. THE ALT WAS SET IN THE ALT ALERTER AND READ BACK 'ROGER CLBING TO 290;' AND CALLING THE ACFT'S CALL SIGN. THERE WAS NO RESPONSE FROM THE CTLR. AT FL208; THE CTLR INSTRUCTED US TO TURN RIGHT TO 180 DEGS AND DESCENDED IMMEDIATELY TO 20000'. WHEN QUESTIONED ABOUT THE TURN AND DSCNT; THE CTLR SAID SHE HAD CLRED US TO 20000'. WHEN WE TOLD HER WE HAD UNDERSTOOD AND READ BACK THE CLRNC AS CLBING TO FL290; SHE SAID THERE MUST HAVE BEEN A MISUNDERSTANDING. END OF ISSUE. IN ANSWER TO YOUR QUESTIONS: A) THE PROB WAS DISCOVERED BY ALT READOUT ON THE CTLR'S SCOPE. B) FACTORS THAT CONTRIBUTED TO THE SITUATION: 1) CREW AND CTLR WERE UNDER A HEAVY WORKLOAD. 3 HDOFS IN THAT SHORT A DISTANCE CAUSES CONFUSION AND INCREASES WORKLOAD IN THE COCKPIT. AREA CONGESTED WITH AIR TFC INCREASES THE CTLR'S WORK (AND NEITHER THE CREW NOR CTLR LISTENED CLOSELY ENOUGH). ALSO; THERE WERE A LOT OF BLOCKED XMISSIONS ON THE FREQ. C) SOLUTION: MORE FREQS; A BETTER ATC PLAN IN THE WASHINGTON AREA; BETTER EQUIP AND MORE CTLRS.