e master clock to the slave clock. This QL information is basically equivalent to the stratum level of the master clock to which the quality level on that physical link can be traced. Table 6-2 shows the SSM codes in an Option II SDH synchronization network. Table 6-2 Clock Quality Levels and SSM Coding in Option II SDH Synchronization Network Qual- GEN2 SSM GEN1 SSM Level SSM coding SSM coding SSM coding SSM coding [MSB..LSB] in [MSB..LSB] in 1544- [MSB..LSB] in [MSB..LSB] in 1544- STM-N signal kbps signal with ESF STM-N signal kbps signal with ESF (BINARY) (HEX) (BINARY) (HEX) abled abled abled abled abled abled abled abled abled abled abled Source: ITU-T 3.781, Table 6 Note that there are shaded fields in Table 6-2 for the first generation (GEN1) SSM codes. As ex- plained previously, GEN1 SSM codes are a subset of the GEN2 SSM codes, and the shaded fields are not differentiated in the GEN1 case, but they are split out in the GEN2 case. For example, SSM coding for both QL-TNC and QL-ST3E are the same as QL-ST3 for GEN1 SSM encoding, whereas these are separated with different codes when using GEN2 SSM encoding. There is one point that the network engineer should be aware of when deploying a QL distribution network. Because the codes for different SSM values vary between Option I, II, and III circuits, it is imperative that the router connecting to these circuits is configured with the correct option code un- der frequency synchronization. This is especially SO in North American markets, as Cisco routers assume Option I is the default encoding unless changed via configuration (because it should be Op- tion II in North America). 6.6.1 Ethernet Synchronization Messaging Channel For SyncE, the clock traceability is achieved via the transmission of special packets, in a method called the Ethernet synchronization messaging channel (ESMC). An ESMC is a logical communica- tion channel running over the Ethernet transport layer. It transmits QL information, which is the quality level traceable from the transmitting synchr