 Gbps) interfaces, which may be of interest. However, to read the complete recommendation, you will need all three docu- ments, which are accessible at the following link: https://www.itu.int/rec/T-REC-G.825/en 8.2.1 Node and Clock Performance These recommendations cover the timing requirements for a hierarchy of clocks in the physical tim- ing distribution network. The network builds a hierarchy of clocks using a master-slave relationship between each level of the hierarchy. Each clock is synchronized to a higher level over a synchroniz- ation distribution network, with the highest level being the PRC. The hierarchical levels and their associated recommendations are PRC in G.811 Slave clock (transit node) in G.812 Slave clock (local node) in G.812 SDH network element clock in G.813. Figure 8-2 illustrates the hierarchy. G.811 Up to 10 G.812 Up to 10 G.812 total slave clocks total slave clocks G.812 G.812 G.813 G.813 G.813 G.813 G.813 Transit Transit Up to 20 G.813 Up to 20 G.813 Up to 20 G.813 network element clocks network element clocks NE clocks Figure 8-2 Synchronization Network Reference Chain (Based on Figure 8-5 of ITU-T G.803) The further restriction on that topology is that the total number of SDH network element clocks in a chain is limited to 60. This does not mean a limit of 60 network elements, simply a limit of 60 in a single chain. For further information on many aspects of the synchronization chain, see clause 8, "Architecture of synchronization networks," in G.803. The recommendations for these different clock types are each addressed in the following sections. Because traditional frequency timing is not the focus of this book, this is the limit of the coverage of the topic. There are many other resources available that cover PDH, SDH, and SONET timing in far more detail. So, the following section covers the different TDM clocks types, and then the coverage moves on to packet timing. G.811: Timing Characteristics of Primary Reference Clocks ITU-T G.811 defines the (timing) characteristics of PRC