 The DAB+ standard incorporates Reed-Solomon ECC as an "inner layer" of coding that is placed around the byte interleaved audio frame but inside the "outer layer" of convolutional coding used by the original DAB system, although on DAB+ the convolutional coding uses equal error protection (EEP) rather than UEP since each bit is equally important in DAB+.This combination of Reed-Solomon coding as the inner layer of coding, followed by an outer layer of convolutional coding – so-called "concatenated coding" – became a popular ECC scheme in the 1990s, and NASA adopted it for its deep-space missions.One slight difference between the concatenated coding used by the DAB+ system and that used on most other systems is that it uses a rectangular byte interleaver rather than Forney interleaving in order to provide a greater interleaver depth, which increases the distance over which error bursts will be spread out in the bit-stream, which in turn will allow the Reed-Solomon error decoder to correct a higher proportion of errors.