Shown here is an example of a synchronous clock tree. Take a look at the clock generator clock on the left. The reference source of the clock generator is a reference clock, and NOT a crystal or crystal oscillator. The reference clock is a clock from the network upstream and is used to synchronize all the other clocks which are generated from the clock generator or jitter attenuating clock in this example. Synchronous clock trees are typically used in Optical Transport Networks (OTN), Remote Radio Head, SONET/SDH, mobile backhaul, synchronous Ethernet, and 3G-SDI, 12G-SDI video transmission.