FAS is the frame alignment overhead with a length of 6 bytes. The 1st to 3rd bytes are 3 OA1 bytes, each OA1 is always "1111 0110"; the 1st to 6th bytes are 3 OA2 bytes, each OA2 is always "0010 1000". OA1 and OA2 are the frame alignment bytes of this frame signal, which are equivalent to A1 A2 in the SDH frame structure.

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