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When a SONET equipment is first connected to the network, how does the equipment find the start of the SONET frames it is receiving?

I know that T-1 equipment is initially guessing where the framing bit is, and slides over by one bit if the pattern it finds is wrong. Is there something similar going on with the A1 and A2 frame synch bytes in the SONET header, or do I have the wrong idea altogether?

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closed as not a real question by Chopper3, HopelessN00b, Zoredache, Magellan, Scott Pack Nov 7 '12 at 14:36

It's difficult to tell what is being asked here. This question is ambiguous, vague, incomplete, overly broad, or rhetorical and cannot be reasonably answered in its current form. For help clarifying this question so that it can be reopened, visit the help center.If this question can be reworded to fit the rules in the help center, please edit the question.

Even though I'm tempted to just leave a comment that homework questions are off topic, I guess I'll link to this fairly decent SONET primer - use .pdf download link on that page - and suggest looking at the framing structure breakdown and related material starting on page 5. Unless your question is actually about the layer 1 interactions of how the equipment parses individual bits, in which case, I'd suggest asking your Electrical Engineering professor. – HopelessN00b Sep 12 '12 at 23:42
up vote 3 down vote accepted

It's a bit more complicated than "the start" as the receiving end has to figure out the skew and the frame lock. Without going through the whole PDF Hopeless linked, an OC-3, called STM-1, contains 3 STS-1 Frames.

Each frame has a variety of headers, but A1 and A2 contain a fixed pattern that allows the equipment to recognize the start of a frame.

The three frames are interleaved, so one byte from each frame, then the next byte from each frame and so on. Headers E1 and E2 contain data for only the first frame in each interleaved set. The others will contain no data.

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Yes, I found that the idea is similar to T-1.

The two first frame header bytes in a SONET frame, A1 and A2, contains a fixed bit pattern 0xF628 (1111 0110 0010 1000).

When a receiver is observing a stream of data, it will look for this pattern in the stream to detect the beginning of a frame. A1 and A2 are not scrambled (unlike the rest of the frame).

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Good, except that multiple frames are interleaved, and you didn't cover how the equipment figures out which frame is the "first" of the set. – Chris S Sep 13 '12 at 0:10

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