CD-ROMs do not have a fixed number of sectors in a fixed-arm position. Instead, an inward spiral of records is arranged to maintain minimal latency from record to record.
CD-ROMs are not random access, but rather sequential access, which is why they tend to be slow, because the head hunts to find the desired record. The sectors are indexed by track in the same way cylinder, head, and sector indices are used in a hard-disk drive. With SCSI, these sectors are hidden by logical address translation in the drive's controller. Unlike with hard drives, a separate head is not used for timing information, so each time a CD-ROM is moved for random access it must search up and down the spiral in the vicinity the head let down to find the desired sector.
CD-ROM sector sizes are large, usually 2048 bytes per sector (larger sizes are possible) — the concept of logical sectors is introduced. Each logical sector, with a constant logical sector size (= the sector size), starts in a different sector from any other logical sector.