Lock a clock to a 8B10B data stream?

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To ask a possibly ignorant question: What's the purpose of "looping back" the received signal clock? It must be expected to result in worse jitter performance, even with a high quality external PLL.

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I didn't mean to "loop back" the clock to the upstream. The network is a chain of equipments. The receiver receives 8B10B data stream from the upstream, (hopefully) sync itself up and pass the data stream down the line with the transimitter. The objective is to sync all the equipments with clock info embedded in the 8B10B data stream.