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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.
Figure-5
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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