MD88 CREW RESPONDS TO TCASII RA WHILE CLBING TO FL290.

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WE WERE ON J84 APPROX 35 NM S OF NORTHBROOK; IL; CLBING TO ASSIGNED FL290 AT APPROX 2000 FPM WHEN TFC CONFLICT DEVELOPED. OUR TCASII TA SOUNDED; FOLLOWED IMMEDIATELY BY RADIO XMISSION FROM THE ACFT AT FL290 TO ZAU STATING THAT HE HAD TFC AHEAD. WITHIN SECONDS OF OUR TA AND THE RADIO CALL WE MADE VISUAL CONTACT WITH THE OTHER ACFT AND SAW THAT HE WAS AT FL290 APPROX 2 O'CLOCK MOVING R TO L. WITHIN A FEW SECONDS; AND AS OUR TCASII RA ADVISED 'DSND;' WE STARTED AN IMMEDIATE DSCNT AND MAINTAINED VISUAL CONTACT WITH THE TFC. IN APPARENT REACTION TO THE RADIO CALL FROM THE OTHER ACFT; ZAU XMITTED 'ACFT X; CONFIRM FL280.' BECAUSE WE HAD ABANDONED OUR CLB TO FL290 AND INITIATED A DSCNT IN RESPONSE TO THE RA; WE ELECTED TO LEVEL AND REMAIN AT FL280 AND RESPONDED 'ROGER; ACFT X; FL280.' BECAUSE OF THE NATURE OF THE CONFLICT AS WELL AS THE ABSENCE OF ANY OTHER CLRNC OR XMISSION FROM ZAU; IT WAS APPARENT THAT THE INTENT WAS FOR US TO REMAIN AT FL280. THE INITIAL CLRNC TO CLB TO FL290 WAS ISSUED WITH NO TA'S; AND THE ALT WAS PROPER FOR OUR DIRECTION OF FLT. THROUGHOUT THE TIME WE WERE UNDER THE CTL OF THIS SECTOR OF ZAU. RADIO TFC WAS RELATIVELY LIGHT; ESPECIALLY WITH RESPECT TO THE PREVIOUS LOW SECTOR; WHICH HAD GIVEN US A 360 DEG TURN TO FACILITATE OUR CLB TO THE TOP OF HIS SECTOR. NO TA'S WERE ISSUED WITH REGARD TO THE CONFLICT TFC OR ANY OTHER TFC WHILE CLBING IN THIS HIGH SECTOR. HAD A CLRNC TO FL280 BEEN ISSUED SHORT OF THE STANDARD FL290 OR FL330; IT WOULD HAVE BEEN PRUDENT TO ISSUE A REASON FOR THE NONSTANDARD ALT IN THE FORM OF A TA.