The Masks Defined in the ETSI Standards

 This section presents the form of the specifications of synchronisation quality relevant to the problem at hand. They are discussed in greater detail in section 6.4. The quality of the synchronisation signal has been characterised by graphs known as masks . The masks are defined in terms of two statistical measures known as Timing Deviation (TDEV) and Maximum Time Interval Error which are discussed at some length in section 4. The masks reflect that in order to minimise the frequency of pointer adjustments, clocks must have very good short term stability.

The masks are defined for observation intervals greater than 0.1s. Below 0.1s, input noise is rejected by clocks and so is not relevant to the problem at hand. It will be seen that this noise rejection property is due to the low pass filtering of noise on the input to phaselock loops.

It will be shown in section 6.4 that the asymptotic behaviour of the TDEV produced by the noise model is as is depicted by the solid line in figure 2.6. The process by which the absolute levels were set is rather misleading and this topic is discussed in section 2.4.2.

 

Figure 2.6:   The TDEV wander mask

Thus for a given worst case behaviour a TDEV curve known as a mask is defined (shown by the dashed line in figure 2.6) below which the TDEV produced by a clock must lie for the standard to be satisfied. This process is also carried out for the MTIE of the noise produced by a clock. Similar masks are defined for the TDEV and MTIE at the end of a maximum reference chain.