lity of these circuits, engineers developed and adopted synchronous forms of communication. Synchroniz- ation was also introduced to allow channels (such as individual voice connections) to be multi- plexed onto a single communications medium. Therefore, network engineers needed to have a good understanding of clocking because they configured routers to use such circuits as an everyday task. Compare this to modern network engineers, who might never have had to configure an E1/T1 cir- cuit in their career, and SO are not as familiar with the continued importance of synchronous communications. Efficient operation of digital transport continues to be the most common usage of frequency syn- chronization today in the telecommunications industry, such as in legacy TDM circuits like those based on E1/T1, or SDH and SONET. However, frequency (and sometimes phase) timing is widely deployed in other data transports such as cable, passive optical networking (PON), and mobile, as well as in modern packet networks such as Ethernet (so-called synchronous Ethernet) and optical technologies. 2.1.1 Legacy Synchronization Networks This section addresses the fundamental ideas and history of legacy networks only to provide the ba- sic concepts that are needed to understand the discussion of the standardization behind SyncE. This section also addresses the levels of accuracy used to source and transport frequency. Frequency synchronization provided benefits to engineers designing the then-new optical networks such as SDH and SONET, as it allowed the new networks to transmit more data with less overhead than available in previous schemes such as plesiochronous digital hierarchy (PDH). It was also clearly superior to other methods, such as those that used dedicated Start and Stop bits to indicate the beginning and end of a data byte or other techniques such as bit-stuffing. The difference here was that synchronization was used to implement the transport itself, rather than being a requirement for some application using the transport (s