In SDH/SONET system, receivers recover clock based on incoming signal. Insufficient number of 0-1 transitions causes degradation of clock performance. In order to avoid this problem and to guarantee sufficient transitions, SONET/SDH employ a scrambler.
All data except first row of section overhead is scrambled . Scrambler is 7 bit self-synchronizing X7 + X6 + 1 . Scrambler is initialized with ones
This type of short scrambler is sufficient for voice data. But this is not sufficient for data which may contain long stretches of zeros. So, while sending data an additional payload scrambler is used.
This modern standards use 43 bit X43 + 1. It run continuously on ATM payload bytes (suspended for 5 bytes of cell tax) . Run continuously on HDLC payloads