ity level. If more than one source signal is available at that quality level, the slave selects the available reference with the highest priority (say, priority 2). The slave will then start to acquire this new input signal and continue to provide an output signal but now aligned to the new reference. This switchover, referred to as reference clock change, is known as a transient event and the change in timing characteristics of a clock during such an event is referred to as transient response. This is a very different event from a clock moving into the holdover state, because the change in reference clock happens very quickly, resulting in the slave clock being in the holdover state for only a very brief time. These transients are thus divided in the ITU-T recommendations into two categories: long-term transients and short-term transients. Switching between two reference inputs is an example of a short-term transient. Long-term transient is another name for holdover. Transient response specifies the noise that a slave clock generates during such events. Slave clocks are unlikely to be transparent during such events and could either transfer or generate additional noise, which is measured as the transient response. Again, for each clock type, there are MTIE and TDEV masks in the ITU-T standards to specify the accepted transient response. For example, you can find MTIE and TDEV masks for transient response for clocks supporting SDH (E1) and SONET (T1) in clause 11 of ITU-T G.812. 5.5 Measuring Time Error This chapter explores several definitions and limits of time error, and numerous metrics to quantify it, as well as examining the impact of time error on applications. In summary, time error, essentially the difference between a slave clock and a reference or master clock, is primarily measured using four metrics: cTE, dTE, max|TE|, and TIE. From those basic measurements, engineers have statist- ically derived MTIE and TDEV from TIE, in combination with the application of different filters. ITU-T recommendations s