n-I are QL-ePRTC, QL-PRTC, QL-ePRC, QL-PRC, QL-SSU-A, QL-SSU-B, QL-eSEC, QL-SEC, and QL-DNU. Here, QL-SEC represents the same quality level as QL-EEC1 as defined by ITU-T G.8264. Option II synchronization networking: Clock source quality levels of Option II were defined initially as seven quality levels. This set of seven quality levels is also referred to as First Gen- eration, or GEN1. These seven levels were expanded to nine levels, and this extended level set is referred to as Second Generation, or GEN2. Later the GEN2 quality levels were further en- hanced to include enhanced PRTC (ePRTC) and PRTC as well. The GEN1 clock quality set is a subset of GEN2 because GEN1 quality levels do not define QL-ST3E and QL-TNC as separate quality levels, and QL-PROV was identified as QL-RES. Table 6-1 lists the different clock quality levels, in order from highest to lowest levels of clock quality. Table 6-1 Clock Quality Levels for Option II Synchronization Quality Level Description Order QL-ePRTC Enhanced PRTC Traceable Highest QL-PRTC PRTC Traceable QL-PRS PRS Traceable QL-STU Synchronized (and) Traceability Unknown QL-ST2 Stratum 2 Traceable QL-TNC Traceable to Transit Node Clock QL-ST3E Stratum 3E Traceable QL-ST3 Stratum 3 Traceable QL-SMC SONET Minimum Clock Traceable QL-ST4 Stratum 4 Traceable (freerun) QL-PROV Provisionable by the Network Operator QL-DUS Do not Use for Synchronization Lowest Source: ITU-T G.781, Table 2 Option III synchronization networking: The only clock source quality levels that are defined in the synchronization process of an Option III network are QL-ePRTC, QL-PRTC, QL-UNK, QL-eSEC, and QL-SEC, corresponding to four levels of synchronization quality. QL-UNK is used to indicate an unknown clock source. The exact encoding of SSM depends on the physical medium in the network being used and con- figured. For example, encoding of an SSM in T1 is different from encoding in E1, and SO on. But irrespective of the encoding scheme, the only information that an SSM carries is the QL information from th