CPR X DSCNT THROUGH OCCUPIED ALT HAD LTSS FROM ACFT Y. SYS ERROR.

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I; WORKING YNG/HOP; ACCEPTED HDOF ON CPR X BEACON XXXX DSNDING TO FL120 ENRTE CGF FROM OVER DRI VOR. DKK CTLR COULDN'T GET ACFT TO ANSWER FREQ CHANGE. WHEN CPR X FINALLY CHKED IN (SEVERAL MI SW ERI VOR) LEVEL 12000 FT; ASKING FOR LOWER. ACFT GIVEN 11000 FT (CTR'S LOWEST ALT). CTLR CALLED ERI APCH ON 26 LINE; 10 MI SW ERI VOR CODE XXXX INBOUND CGF. REQUESTED CTLR LOWER. GIVEN 6000 FT BY ERI APCH. (CTLR DID NOT ASK FOR A SPECIFIC ALT.) CPR X ISSUED 9000 FT FOR SAFETY MARGIN IN CASE CLE APCH (OWNS 8000 FT AND BELOW) COULDN'T RELEASE LOWER. (9000 FT IS NORMAL ALT FOR CLE SATELLITE ARPTS TO BE DSNDED TO.) CLE APCH OK'ED 6000 FT. CPR X GIVEN 6000 FT AT THIS TIME. SEVERAL MINS LATER (APPROX 15-18 MI; MAYBE MORE) ERI APCH CALLS; SAYS WHAT ALT CODE XXXX DSNDING TO? REPLIED 6000 FT. ERI CTLR SAYS: 'I SAID 8000 FT.' DO YOU WANT ME TO CLB HIM BACK TO 8000 FT? ERI SAYS 'YES.' JUMPED OFF LINE AND INSTRUCTED CPR X TO STOP HIS DSCNT AND CLB BACK UP TO 8000 FT. PLT DIDN'T LIKE IT BUT COMPLIED (SLOWLY; BUT I UNDERSTAND RECONFIGURING HIS ACFT FOR A CLB). ERI NOW CALLS AND SAID THEY'RE NOT TALKING TO SOME ACFT Y AND THAT I HAD BETTER TURN MINE W. CPR X GIVEN 280 DEG HDG. I STILL DID NOT KNOW OF ANY OTHER TFC IN AREA NOR HAD BEEN INFORMED OF ANY UNTIL TOLD TO TURN 'MINE W.' CLEVELAND APCH HAD BEEN WORKING NEBOUND TFC Y AT 7000 FT AND HAD HIM 4 MI OR SO INSIDE ERI APCH'S AIRSPACE AT THE TIME; I LATER FOUND OUT.