B737-300 PIC TAXIES IN TO THE RAMP AFTER HE HAD BEEN TOLD TO HOLD SHORT OF THAT AREA BY GND CTLR AT IAH; TX.

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CLRNC WAS RECEIVED FROM IAH GND CTL BY MY FO AFTER CLRING RWY 26 ON THE HIGH SPD TXWY; TO TAXI VIA 'L TURN NA; R TURN NK AND HOLD SHORT NB; CONTACT RAMP CTL BEFORE REACHING NK.' FOR CLRNC INTO THE INTL RAMP AREA. I ALSO HEARD AND UNDERSTOOD THE CLRNC. I TAXIED E ON NA WHILE MY FO ACCOMPLISHED THE AFTER LNDG CHKLIST. I THEN TURNED DOWN THE VOLUME TO A LOWER LEVEL ON THE #1 RADIO. THIS WAS THE RADIO IN WHICH I WAS MONITORING GND CTL IN ORDER TO ALSO HEAR THE CONVERSATION BTWN MY FO AND THE RAMP CTLR. AS I TURNED S ON NK TOWARDS THE RAMP; I HEARD THAT WE WERE CLRED INTO THE RAMP BY THE RAMP CTLR. THIS CLRNC WAS RELAYED VERBALLY TO ME BY MY FO AS WELL. I WAS OF THE UNDERSTANDING THAT ARPT TFC BTWN THE RAMP AND THE LCL GND CTLR WAS COORD. I AM MISTAKEN ON THIS ACCOUNT BECAUSE AS I MANEUVERED THE ACFT ACROSS NB; I OVERHEARD THE GND CTLR ADVISING AN ACR ACFT EXITING THE RAMP AREA CAUTION FOR THE INBOUND B737 THAT WAS NOT TALKING TO HIM (GND CTL). THERE IS CONFUSION IN MY MIND AS TO WHAT AREA IS CTLED BY EACH CTLR AND WHEN AND WHERE FREQ CHANGES ARE TO BE TAKING PLACE. THERE IS NOTHING ON OUR ARPT DIAGRAMS THAT DEPICT WHERE THE DIFFERENT JURISDICTIONS ARE LOCATED. AS MORE AND MORE ARPTS ARE USING RAMP CTLRS; THIS TYPE OF SIT HAS THE POTENTIAL OF CREATING A GND COLLISION. ARPT DIAGRAMS THAT DEPICT AREAS THAT ARE CTLED BY LCL GND CTL AND RAMP CTL WOULD BE HELPFUL AS WELL AS SPECIFIC INSTRUCTIONS BY GND CTLRS AND RAMP CTLRS WHEN OR WHERE TO CHANGE FREQS. SITS WHERE THE CAPT MUST MONITOR 2 RADIO CONVERSATIONS IS UNSATISFACTORY. POSSIBLE CONTRIBUTING FACTORS MAY INCLUDE MY DESIRE TO ACCOMPLISH A TIMELY GATE ARR. I ALSO SHOULD HAVE TAKEN THE OPPORTUNITY TO CALL THE TWR AND EXPLAIN MY CONFUSION AND ACTIONS. I DON'T BELIEVE FATIGUE WAS A FACTOR; HOWEVER; I HAD JUST FLOWN FROM FLL TO LGA AND THEN TO IAH; WITH A DUTY TIME OF JUST OVER 9 HRS. SUPPLEMENTAL INFO FROM ACN 511833: APPARENTLY (I DIDN'T HEAR IT; SINCE I WAS ON COM #2); GND SAID SOMETHING TO THE EFFECT THAT HE HAD NOT CLRED US ACROSS NB. THERE WAS NO TFC IN SIGHT TO US ON NB.