 quency distribution over Ethernet never emerged. Ethernet itself was originally intended for trans- mission of asynchronous data, and either did not need to synchronize the components in an Ethernet circuit or needed the synchronization only on a link-by-link basis. As you read in Chapter 3, to achieve link link synchronization, Ethernet selects one end of the link to be the master and the other end becomes the slave. The master end uses its local free-running oscillator to transmit data to the slave; and the slave turns around the frequency of the incoming signal and uses that same frequency to clock its transmission back toward the master. However, these Ethernet nodes do not need to pass timing information from one node to another, which is re- quired to achieve timing distribution across the whole network. Due to its simplicity, low cost, and bandwidth flexibility, Ethernet soon became the default choice of physical medium for data transmission and the service providers started migration from SDH/SONET or T1/E1 to Ethernet. While the data transmission (and TDM to packet) was easily migrated, a need appeared for the tim- ing network to also migrate to Ethernet and packet technology. Therefore, Ethernet needed to sup- port frequency distribution in the same or a similar way to SDH/SONET networks. So, the gaining popularity of Ethernet and demand for the benefits of synchronization in networks led to the devel- opment of Ethernet to carry frequency. The technology that was proposed to carry frequency using the packet network is synchronous Ethernet (SyncE). SyncE uses the physical interface to pass timing from node to node-meaning that it uses the frequency of the pulses of light (or electrical waves in copper), much like SDH/SONET or E1/T1. Because the frequency is carried over a physical medium, SyncE can provide at least equivalent reliability and accurate frequency distribution as does SDH/SONET or T1/E1. SyncE uses line coding of the Ethernet interface to transfer timing. The timing information on the slav