as well as the newer ones based on Ethernet and the optical transport network (OTN) system of transport. This second group includes mostly packet clocks that are sometimes frequency only, but increasingly they understand phase and time in addition to frequency. See Chapter 8 for a lot more detail on the ITU-T recommendations Frequency Clocks The ITU-T recommendations G.811, G.812, and G.813 detail the clock performance requirements in their hierarchy of the frequency network. They define PRCs, synchronization supply units (SSU- A and SSU-B), and synchronous equipment clocks (SEC), respectively. The G.811 PRC is much like the ANSI PRS, while G.812 covers the higher quality clocks equival- ent to BITS for ANSI. At the lower level, G.813 covers the standard network element clocks known as SEC (like stratum 4 in ANSI). There is now a new recommendation, G.811.1, which defines a higher accuracy, enhanced primary reference clock (ePRC), specifying an enhanced level of fre- quency accuracy of at least 1 part in 1012 Figure 3-12 captures the synchronization hierarchy of clock quality from different types of syn- chronization networks, with the ANSI stratum level and SONET clock types included for reference. The ePRC on this diagram would sit above the PRC/PRS layer. SyncE SONET G.811 Stratum 1 Max 60 G.812 SSU-A SSU-A Stratum 2 Max 10 SSU G.812 SSU-B Stratum 3 Max 20 EEC/SEC G.813 Stratum 4 Figure 3-12 Time Distribution Hierarchies Compared Note that the old version of G.812 (November 1988) refers to SSU-Transit (SSU-T) and SSU-Local (SSU-L), which are now referred to as type V and type VI clocks, but were then stratum 2 and stratum 3, respectively. Table 3-3 provides a quick reference to the defined quality levels required to maintain the accuracy of the different types of clock when deployed in the network. The field of maximum TIE (MTIE) is a measurement of error in the signal which will be covered extensively in Chapter 5. Table 3-3 Performance of Clocks in Synchronization Chain SI Stratum ITU-T Clock Free-run Holdov