From the SCSI-3 MMC draft document the read commands (e.g. Read10 and Read12) for reading a DVD only take a remapped sector address (LBA) and not a physical address (PSN).
From MMC2 draft:
LBA 0 shall correspond with the sector address of 030000h on DVD-ROM media. Only the Data Area is generally addressable using an LBA.
Meaning mapping is like this:
LBA 0 = PSN of start of user data
so for DVD-ROM and DVD-R:
LBA 0 = PSN 30000h
LBA 1 = PSN 30001h
LBA 2 = PSN 30002h
This means LBA is always after the lead-in area. The lead-in is not readable as sectors on the software side, but you can get decoded info of lead-in data using the MMC2 READ DVD STRUCTURE command and later superseded by READ DISC STRUCTURE - as RichMan mentioned.