C172 VIOLATES LAS MODE C XPONDER VEIL.

Narrative

I ATTEMPTED TO FOLLOW THE BLUE VFR RTE AROUND THE W SIDE OF LAS VEGAS. I WAS INFORMED BY THE VGT TWR THAT I HAD VIOLATED THE CLASS B AIRSPACE OF LAS INTL. I WAS TOLD TO CONTACT APCH ON AN ASSIGNED FREQ; WHICH I DID. MY PLAN WAS TO FOLLOW THE RTE OVER THE CEILING OF 4500 FT FOR VGT AND UNDER THE FLOOR OF 5000 FT FOR THE CLASS B AIRSPACE ABOVE. I GOT THE ALTIMETER SETTING FROM VGT ATIS OF 30.03 AND FLEW AT 4700 FT TO GO THROUGH THE WINDOW. I TUNED TO VGT TWR FREQ. MY XPONDER WAS SET TO 1200 AND THE LIGHT WAS FLASHING AS I WAS 'PAINTED.' THE XPONDER HAD BEEN ON THIS SETTING THE WHOLE TRIP WITH THE LIGHT FLASHING. AS I WAS DIRECTLY OVER VGT; THE TWR ASKED THE CESSNA DIRECTLY OVER THE FIELD TO GIVE ITS CALL SIGN. I DID. THE REPLY WAS THAT I SHOULD CONTACT APCH ON THE ASSIGNED FREQ; WHICH I DID. A CTLR ASKED ME TO SQUAWK A CODE AND IDENT. CTLR SAID SHE COULD NOT SEE ME. I TURNED THE KNOB BACK TO 'ALT.' CTLR THEN SAW ME AND ASKED MY ALT. I HAD BEGUN MY DSCNT TO GET BELOW THE 4000 FT FLOOR. CTLR ASKED MY ALT AGAIN; I REPLIED 4000 FT AND SHE SAID SHE SHOWED ME AS 4100 FT SO I WENT LOWER. CTLR ASKED ME MY DEST AND I REPLIED THE LOS ANGELES AREA. CTLR GAVE ME A TELEPHONE NUMBER TO COPY DOWN; AND SAID FOR ME TO CALL IN. BARSTOW-DAGGETT WAS THE NEXT PLACE THAT WAS ATTENDED; SO I LANDED THERE AND TELEPHONED. I SPOKE WITH A GENTLEMAN. HE SEEMED FAMILIAR WITH WHAT HAD HAPPENED AND SPOKE WITH THE FEMALE CTLR WHO HAD HANDLED ME. HE TOOK NOTES ON WHAT I SAID. HIS ADVICE; THAT I NOW WILL ALWAYS FOLLOW; IS THAT APCH WOULD ALWAYS PREFER THAT WHEN I REACH THE MODE C VEIL THAT EVEN THOUGH I AM MAKING A VFR TRANSIT; I SHOULD ASK FOR A SQUAWK CODE AND THEY WOULD VECTOR ME THROUGH THE AREA. HE ALSO SAID; WHEN I APCHED LOS ANGELES I SHOULD ASK FOR A XPONDER CHK. I DID THAT AND LAX APCH RPTED THAT THEY COULD SEE MY OK. THEY SAID THEY WOULD CHK THE ALT SENSING OF THE XPONDER.