A11. Jitter is defined as the magnitude of phase variation (with respect to a reference clock or data signal) whose frequency of variation is greater than 10Hz. Jitter is measured in unit intervals (UI), where 1 UI is equal to one data bit-width. For an E1 signal, 1 UI is equal to 488ns, and for a DS1 signal, 1 UI is equal to 648ns. However, if the rate of change in phase is less than 10Hz, then this phenomenon is known as wander and is measured in nanoseconds.