signal from a reference master clock, and yet every clock (even reference clocks) generates some additional noise on its output. As described in the previous sec- tion, there are metrics that quantify the noise that is generated by a clock. But how much noise can a slave clock receive (tolerate) on its input and still be able to maintain its output signal within the prescribed performance limits? This is known as the noise tolerance of a clock. Figure 5-20 shows the noise tolerance at the input of Network Element-3 (NE-3). It is simply a measure of how bad an input signal can become before the clock can no longer use it as a reference. Going back to the photocopier analogy, the noise tolerance is the limit at which the photocopier can only just read the input document well enough to produce a readable copy. Any further degradation and it cannot produce a satisfactory output. Like noise generation, noise tolerance is also specified as MTIE and TDEV masks for each class of clock performance. Up until the maximum permissible values of noise specified in ITU-T recom- mendations is applied at the input of a clock, the output of the clock should continue to operate within the expected performance limits. These masks are specified in the same ITU-T specifications that cover noise generation, such as G.812, G.813, G.823, G.824, and G.8262. What happens when the noise at the input of a clock exceeds the permissible limits-what should be the action of a clock? The clock could do one or more of the following: Report an alarm, warning that it has lost the lock to the input clock Switch to another reference clock if there are backup references available Go into holdover (operating without an external reference input) and raise an alarm warning of the change of status and quality Noise Transfer To synchronize network elements to a common timing source, there can be at least two different ap- proaches-external clock distribution and line timing clock distribution. An external clock distribution or frequency distribution network