QUESTION: On a SONET NE that is optioned for line timing from an OC-N signal, how is the synchronization extracted from the optical signal? Is it calculated from the input line signal in total, or does it use a specific bit (ie bit 1 of the A1 byte) or byte to establish an 8000 hz reference?
ANSWER: All optical NEs that are optioned for line timing extract the clock from the received optical signal. That is, there is the step of recovering the electrical signal, such as one of the STS-n (typically STS-1) and recovering the clock from that. Often the recovered clock is first in a "raw" form, such as a 155.52 MHz, or in the form of 19.44 MHz corresponding to the recovered "byte-clock". This signal is then used as the reference for PLLs in the local clock card in the NE. If an 8 kHz reference is called for, this is obtained simply by dividing down the recovered clock (155.52 MHz or 19.44 MHz, or equivalent).
It is not tied to a specific bit, such as a bit in the A1 byte as you have mentioned. Note that the framing format is based on a 125 micro-sec (8 kHz) interval. Thus, just to establish the A1 byte (or any other, for that matter), the NE must achieve frame synchronization and therefore does, indeed, have an 8-kHz reference without examining the content of any specific byte. However, there are techniques such as ATDS (see US Patent Nos. 5,638,379 and 5,828,670) that do use information in a specific byte (in particular the F1 byte) that can be used to link the recovered clock (19.44 MHz) plus the recovered frame-sync with an external timing reference.
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