d QL TLV. This is done to allow interoperability of ESMC and eESMC within the same network. While eEECs are always required to support eESMC, there could be EECs in the same synchronization chain that either support eESMC or do not; mandating the first TLV as a normal QL TLV makes sure that the EECs that do not support eESMC can coexist in the same synchronization chain. The main difference between the enhanced SSM codes compared to normal SSM codes is that the enhanced version is extended to 8 bits of value as compared to 4 bits of SSM code value as spe- cified in ITU-T G.781 and ITU-T G.8264. Table 6-6 shows the eSSM codes for the enhanced clock types. Table 6-6 Enhanced SSM Codes for SyncE Clock Quality Level SSM Code QL-EEC1 QL-EEC2 As per [ITU-T G.781]/[ITU-T G.8264] QL as per [ITU-T G.781]/[ITU-T G.8264] QL-PRTC ePRTC QL-ePRTC QL-eEEC QL-ePRC Source: ITU-T G.8264, Table 11-6 6.7 Synchronization Network Chain There are many network deployment models for timing distribution that can exist, and one such de- ployment is shown in Figure 6-10. The figure shows a sample network with a chain of nodes carry- ing timing information; however, there can be different network models as well. PRC/PRS Stratum 1 level "K" G.812 nodes - maximum 10 Stratum 2 level SSU/BITS SSU/BITS Stratum level > 3 "N" G.813 nodes maximum 20 Maximum 60 Figure 6-10 Synchronization Reference Network Chain The following points explain the different elements of the network chain and cover the variations that can be seen in those chains: The network consists of several quality levels related to the height of the clock within the syn- chronization hierarchy and its position along the chain. The highest level contains the PRC/PRS of the network, and there could be additional PRC/PRS sources for backup purposes. Note that there are alternative network models that allow multiple PRC/PRSs to be active at the same time. On level 1, the PRC/PRS provides distribution of the synchronization reference signal to one or more SSU/BITS clocks, which forms the 