DFW APCH CTLR RPTS LOSS OF SEPARATION WHEN B757 DSNDS FROM CLRED ALT PRIOR TO PENNY INTXN ON APCH TO RWY 17C.

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ACR X WAS ESTABLISHED ON THE LOC FOR RWY 17C AT DFW AT 4000 FT. ACR Y WAS ON A VECTOR TO RWY 18R AT 3000 FT. ACR X WAS CLRED FOR ILS APCH TO RWY 17C TO CROSS PENNY INTXN AT 4000 FT. THE ACFT CREW READ BACK APCH CLRNC; CONFUSED ABOUT THE SPD ASSIGNMENT; AND STATED SOMETHING ABOUT 3000 FT. THE ACFT DSNDED OUT OF 4000 FT AND SEPARATION WAS LOST BTWN ACR X AND ACR Y. THE THINGS THAT CAUSED THIS WERE: THE FLT CREW MISSED THE 4000 FT XING RESTR AND I DID NOT HEAR THE STATEMENT ABOUT 3000 FT. IN ORDER TO HELP PREVENT THIS SITUATION FROM RECURRING: FLT CREWS MUST KNOW THAT WHEN YOU ARE WITHIN 15 MI FROM THE ARPT OF INTENDED LNDG (ESPECIALLY A BUSY ONE LIKE DFW); THAT THEY MUST PAY ATTN TO WHAT THEY ARE INSTRUCTED TO DO. IF ONE OF THE CREW MEMBERS IS BUSY DOING SOMETHING THAT THEY ARE UNABLE TO LISTEN TO ATC; THE ONE THAT IS RECEIVING ATC INSTRUCTIONS MUST NOT ASSUME THEY HEARD SOMETHING THEY DID NOT. IN THE CLRNC THAT WAS ISSUED TO THIS ACFT; THE SPD ASSIGNMENT AND XING RESTR WERE CLRLY STATED. IF THERE IS ANY QUESTION OF WHAT WAS SAID; ASK FOR A CLARIFICATION. DO NOT CHOOSE ALTS OR SPDS ON YOUR OWN. ALSO; AS AN ATC SPECIALIST; I MUST ALSO LISTEN MORE CLOSELY TO WHAT FLT CREWS READ BACK. A CTLR ISSUES MANY INSTRUCTIONS HRLY AND THE MAJORITY OF THEM ARE READ BACK CORRECTLY. WHEN ACR X READ BACK ITS CLRNC; I CAUGHT THE STATEMENT ABOUT THE SPD BEING INCORRECT; BUT NOT THE PART ABOUT ALT. THE INCORRECT SPD WAS CAUGHT AND I KNEW I HAD TO CORRECT THIS. POSSIBLY THIS TRIGGERED A REFLEX THOUGHT ACTION THAT MOMENTARILY MADE ME NOT HEAR THE INCORRECT ALT. STANDARD PHRASEOLOGY MUST BE USED BY ATC AND INSTRUCTIONS MUST BE READ BACK BY FLT CREWS IN A LOGICAL SEQUENCE. READ BACK THE CLRNC AS IT WAS STATED. IF THERE ARE ANY QUESTIONS ABOUT THE CLRNC; ASK FOR CLARIFICATION. SUPPLEMENTAL INFO FROM ACN 676213: VECTORS FOR ILS RWY 17C AT 4000 FT CLRED TO 3000 FT AT 190 KTS. THOUGHT WE WERE CLRED TO 3000 FT AND BELIEVED THAT WE WERE BY PENNY INTXN AND CLRED TO 3000 FT.