PA28 INBOUND TO SQL IS INFORMED BY ATC OF CLASS B ENTRY.

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I WAS CONDUCTING A VFR FLT FROM AUBURN; CA; TO SAN CARLOS; CA. THERE WAS MODERATE TURB BELOW 2500 FT MSL DUE TO STRONG NE WINDS. WHILE DSNDING BELOW THE SFO CLASS B AIRSPACE. I ATTEMPTED TO RECEIVE A DSCNT THROUGH THE CLASS B AIRSPACE. THE SIERRA APCH CTLR SAID HE WAS UNABLE. I CONTINUED MY DSCNT BELOW THE CLASS B AIRSPACE; LEVELING AT 2900 FT MSL AT 15 NM FROM SFO VOR. MY PLAN WAS TO REMAIN AT 2900 FT UNTIL I HAD TO BEGIN A DSCNT TO CROSS THE SFO VOR 10 DME FIX BELOW 2100 FT MSL. A DIFFERENT CTLR THAN THE ONE I HAD BEEN IN CONTACT WITH; TOLD ME TO BEGIN A DSCNT TO 2500 FT MSL AND THAT I WAS IN THE CLASS B AIRSPACE. I RESPONDED BY SAYING THAT I WAS LEVEL AT 2900 FT. ALL SHE (THE CTLR) SAID WAS THAT WAS NOT CORRECT. WE DID NOT HAVE TIME TO DISCUSS THIS OVER THIS PARTICULARLY BUSY FREQ. THE ORIGINAL CTLR THEN RETURNED TO THE FREQ AND HANDED ME TO OAK TWR. I HAVE AND WAS USING THE CURRENT AREA CHART FOR VFR USE IN SAN FRANCISCO. BTWN THE SFO VOR 10-15 DME THE CLASS B BASE IS 3000 FT. THUS; I WAS FLYING AT 2900 FT. A COMMON INSTRUCTION FROM SIERRA APCH IS TO MAINTAIN 2500 FT ON THIS RTE. HOWEVER; UPON INITIAL CONTACT WITH SIERRA APCH; THE CTLR DID NOT INSTRUCT ME TO MAINTAIN 2500 FT. SO; I REMAINED CLR OF THE SFO CLASS B AIRSPACE. I THOUGHT THAT THERE MIGHT BE A DIFFERENCE BTWN CTLRS WITH THE 'NEW' COMBINED APCH FACILITIES CALLED 'SIERRA APCH.'