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SONET Tutorial: Scrambling and Descrambling

By K. Surya Prakash




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Before getting into Synchronous Payload Envelope (SPE), I feel its better to know about scrambling. 


Every add/drop multiplexer sample incoming bits according to a particular clock frequency. Now this clock frequency is recovered by using transitions between 1s and 0s in the incoming OC-N signal. Suppose, incoming bit stream contains long strings of all 1s or all 0s. Then clock recovery would be difficult. So to enable clock recovery at the receiver such long strings of all 1s or 0s are avoided. This is achieved by a process called Scrambling.

Scrambler is designed as shown in the figure-5 below.



It is a frame synchronous scrambler of sequence length 127. The generating polynomial is 1+x6+x7. The scrambler shall be reset to �1111111� on the most significant byte following Z0 byte in the Nth STS-1. That bit and all subsequent bits to be scrambled shall be added, modulo 2, to the output from the x7 position of the scrambler, as shown in Figure 5.

The framing bytes A1 and A2, Section Trace byte J0 and Section Growth byte Z0 are not scrambled to avoid possibility that bytes in the frame might duplicate A1/A2 and cause an error in framing. The receiver searches for A1/A2 bits pattern in multiple consecutive frames, allowing the receiver to gain bit and byte synchronization. Once bit synchronization is gained, everything is done, from there on, on byte boundaries � SONET/SDH is byte synchronous, not bit synchronous.

 An identical operation called descrambling is done at the receiver to retrieve the bits.



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