e clock is extracted from the preamble of the frame (refer to the section "Asynchronous Net- works" in Chapter 3), which is used to transfer packets over optical or electrical interfaces. Thus, SyncE does not use packets (the packet layers of Ethernet) to distribute a frequency signal; hence, there is no impact on the quality of SyncE when experiencing high rates of packet traffic. Note that the legacy 10-Mbps Ethernet is not capable of a synchronization signal transfer over the physical layer because the clock edges (or pulses) that are used to extract timing information are not sent continuously, and these signals are stopped during times when there is no data to transmit (called idle periods). And if there is no data during these periods, there is no timing information carried across physical links. Within the node as well, SyncE operates in a similar method to SDH/SONET, whereby the slave node extracts timing information from the physical interface and routes it to its own phase-locked loop (PLL). This PLL then locks to this recovered frequency and drives all the other Ethernet or non-Ethernet interfaces with the same frequency, allowing the timing information to be passed to other downstream nodes. Obviously, enabling SyncE on a platform requires hardware that supports these functions as well as the signal traces (wires) to carry the frequency signals between the timing subsystem and the Ether- net ports. Besides performing normal data transmission, equipment supporting SyncE also has the ability to transfer frequency, and such devices are referred to as Ethernet equipment clocks (EEC). Figure 6-7 shows some EEC nodes that are timed externally from the SSU/BITS and the line timing mode using SyncE between EEC nodes. SSU/BITS SSU/BITS Derived clock SyncE SyncE SyncE Switch A Switch B Switch C (EEC node) (EEC node) (EEC node) Figure 6-7 SSU/BITS Distributing Timing to EEC Nodes Note the similarity of EEC nodes to SEC nodes or NEs. Like the SEC node, the EEC node can take an input reference from PRC or SSU/BITS