so, not relevant to a PRS, be- cause it has no input reference. Wander: Long-term variations of the signal (applies to the output of a PRS) where the fre- quency of the variation is less than 10 Hz. Phase transients: The characteristic of the clock's time interval error (TIE) during network re- arrangement or switch over between reference signals. For stratum 1 clocks, those based on cesium oscillators are the preferred choice as an autonomous source; however, deploying cesium clocks across the network is not usually done due to the high price as well as the excessive maintenance and operational costs. Consequently, the most common PRS clocks deployed are those based on GNSS receivers (in combination with a good OCXO inside the receiver). Stratum 2 clocks (G.812 Type II clocks in ITU-T nomenclature) are high-quality clocks that provide accurate time distribution and synchronization in digital networks and are most often found in spe- cialized timing equipment such as BITS systems for ANSI networks and SSU for ITU-T networks. Often, stratum 2 clocks are based on a rubidium oscillator. Stratum 3E (ITU-T G.812 Type III) and stratum 3 (G.812 Type IV) clocks are generally deployed in better-quality network equipment that is expected to give timing information to devices at the bot- tom of the hierarchy, with stratum 3E having better performance than stratum 3 (they are required to have better holdover performance and tighter requirements on the filtering of wander). Stratum 4 and 4E clock systems are deployed on the end device in the synchronization hierarchy. For this level, holdover stability is not defined. Stratum 4 or 4E clock systems are not recommended as a source of timing for any other clock system. Refer again to Figure 3-11 for a summary of the overall clock types and where they are defined and used. 3.2.1 Clock Types As well as ANSI (or ATIS), the ITU-T also defines a synchronization hierarchy for timing net- works. They are grouped into frequency clocks, which includes the legacy TDM circuits such as SDH 