Packet over SONET/SDH
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Packet over SONET / SDH (POS ) is a protocol that is to incorporate within the ISO / OSI model Layer 2, the data link layer. POS is based on the Point-to- Point Protocol.
The user data are similar to HDLC packaged and in virtual containers of higher order (VC -4) octet way bytesynchron inserted.
To prevent that user data ( payload) accidentally or deliberately by manipulation look like SDH control data ( overhead), the data is sent through a self-synchronizing scrambler.
Scrambler
The scrambler uses a linear feedback shift register that is associated with the user data by the XOR.
Channel:
Unscrambled Data ↓ ----------------------------------------- --- → | → 43 -bit shift register → | → | xor | | ----------------------------------------- --- | | ----------------------------------------------- ↓ Scrambled Data recipient:
Scrambled Data | ----------------------------------------------- | ↓ | ----------------------------------------- --- → | → 43 -bit shift register → | → | xor | ----------------------------------------- --- ↓ unscrambled Data To generate a specific bit sequence in the data stream, both of the 43 -bit initial value of the shift register have to be known, as well as all previously sent data. The probability to guess the current shift register content correctly is, in addition, the probability that the bit sequence in the right place in the VC stands for 1 / 127th This gives a total probability of the occurrence of special bit sequences, which might be confused with control characters, of about 10.9 -16.