uch as telephone voice channels). Synchronous networks (such as SDH and SONET) send data throughout a network with the phys- ical layer of the network synchronized to a common frequency. For this to work, engineers need to provide a source of accurate frequency to all elements across the whole network because oscillators need help to stay accurately aligned. This allows a data receiver in a router to read the incoming data stream at the same frequency as the transmitter used when sending it. Engineers achieve this alignment by using the data circuit itself as a source of clock (line clocking), or they built a dedicated, independent network using specialized timing devices with stable oscillat- ors as an external frequency source for each of the routers. Figure 2-1 gives a conceptual view of how the frequency is distributed across a network to align every device. In the interests of clarity, references to the ITU-T and ANSI (and now ATIS) standards that define each of these components has been omitted. Frequency source Timing links SDH/SONET SDH/SONET SDH/SONET Data links Data links Data links Figure 2-1 Frequency Distribution Network You must also understand that SDH and SONET use different terminology for equivalent items in each system. As an example, note that the standards refer to the source of frequency (for example, an atomic clock) as a primary reference clock (PRC) in SDH and as a primary reference source (PRS) in SONET. Similarly, an intermediate node is known as a synchronization supply unit (SSU) in SDH and a building integrated timing supply (BITS) in SONET. For clarity, where items from both systems are being discussed, this book uses the SDH term before the slash and the SONET after it (for example, SSU/BITS for an intermediate timing node). The PRC/PRS sources of frequency are either atomic clocks or a device combining a global navig- ation satellite system (GNSS) receiver with a stable oscillator. PRC/PRS devices based on atomic clocks can keep frequency accurately over a long period of time beca