The 24 symbols of one data block end up distributed over 109 data blocks. The advantage of this technique is that physical defects on the disc do not eliminate complete data blocks, but instead, parts of many blocks. These partially bad blocks can then be reconstructed using the parity information. CIRC error correction is done in two stages referred to as C1 and C2, with deinterleaving of the data taking place between the stages. The error correction chip typically can correct two bad symbols per block in the first stage, and two bad symbols per block in the second stage.
Some chips can correct four bad symbols in the second stage in some cases.
The extra error correction capability can correct errors that are not correctable by the CIRC because it adds additional parity bytes and additional scrambling of the data. If a CD-ROM drive encounters an error which is uncorrectable even by the layered ECC, it will try again to read the sector. This is the failure mode most familiar to the CD-ROM user. Obviously, having to re-try slows down access to the data, but sometimes a second try is successful. If the drive cannot recover the data within a certain number of retries, the disc is unusable.