Смотрите appendix 1 по ссылке http://www.itu.int/rec/dologin_pub.asp?lang=e&id=T-REC-V.27-198811-I!!PDF-E&type=items
цитата (без символов, лучше смотреть в pdf):
The transmitted bit sequence is continuously searched over a span of 45 bits for sequences of the form where ai = 1 or 0 and ai = ai+ 9 or ai+ 12 If such a sequence occurs, the bit immediately following the sequence is inverted before transmission.