State the process of calculate the drift rate

A drift rate of 10^-9 means 1 second in 109 seconds or 1 nsec every second. At OC-1 speed, say 50 Mbps for simplicity, a bit lasts for the 20 nsec. This means it needs only 20 seconds for clock to drift off by one bit. Consequently, clocks should be continuously synchronized to keep them from going too far apart. Certainly every 10 sec, if possible very much often.