ule of this master and slave clock relationship is that a slave clock must only be "slaved" to a clock of higher or equal stability. This becomes a hierarchical model, which is sometimes referred to as synchronization hierarchy, where clocks are divided into different levels or quality that define the accuracy and stability for the timing (again, refer to Chapter 3 for a detailed description). Figure 6-5 shows a hierarchy of clocks belonging to different stratum levels mixed with transport elements (SECs/NEs). Stratum 1 level Stratum 2 level SSU/BITS SSU/BITS SSU/BITS Stratum level > 3 Figure 6-5 Clock Hierarchy with Different Stratum Levels The stratum level 1 primary reference clock (PRC) or primary reference source (PRS) is hosting the reference clock for the whole network. SSU/BITS systems are typically stratum 2 systems and the SECs/NEs typically have stratum 3E or stratum 3 clocks (or lower). Note that BITS and SSU could also have stratum 3E clocks, which makes them like SECs/NEs in terms of accuracy and stability. Thus, the performance specifications for such systems are tied to the clock type these systems belong to. For example, a BITS system with stratum 2 is required to com- ply with performance specifications defined by ITU-T for stratum 2 systems, which are Type II clocks as specified in ITU-T G.812, whereas a BITS system with stratum 3E should comply with Type III clocks as specified in ITU-T G.812. Figure 6-6 shows the synchronization hierarchy for SDH and SONET nodes along with the ITU-T specifications that cover the performance requirements for clock types. Note that G.781 refers to SSU-A as Type I or V clocks and to SSU-B as Type VI clocks. The performance specifications for these clock types are defined in G.812. G.811 Stratum 1 SSU-A SSU-A G.812 Stratum 2 SSU-B (Stratum 3E) G.812 Stratum 3 G.813 Stratum 4 SONET Figure 6-6 Clock Hierarchy with Stratum Levels and Specifications 6.4 Synchronous Ethernet For a long time, even though Ethernet became a dominant technology for data transmission, fre-