QUESTION:
SONET clocks have a drift rate of about 1 part in 10^9. How long does it take for the drift to equal the timeframe of 1 bit?
ANSWER:
The time-frame to drift 1 bit depends on the actual bit rate. The manner in which to assess this time frame is the following. Assuming that two clocks are 1 part in 10E9 different, then they will "move" by 1 nsec every second with respect to each other. At a 50 Mbps rate (approximately OC-1), each bit time is about 20 nsec and therefore it will take about 20 sec for one clock to move by 1 bit time with respect to the other. At OC-3 (~150 Mbps) the time to move by 1 bit is about 6.67 seconds, and so on. I am assuming that the "drift rate" you talk about is the frequency difference (in fractional units) between the two clocks. The time to move 8 bits is about 160 seconds at the OC-1 rate.
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