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
Scrambler :
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