This is because not only those unknown words cannot be recognized correctly, but the words surrounding them will be affected.
many efforts have been made to deal with the issue of OOV words (Martins et al., 2006;Galescu, 2003;Bazzi and Glass, 2001), and various model units smaller than words have been examined to recognize OOVs from speech, such as phonemes (Bazzi and Glass, 2000a), variable-length phoneme sequence (Bazzi and Glass, 2001), syllable (Bazzi and Glass, 2000b) and sub-word (Galescu, 2003).