This blog post discusses data synchronization in communication systems. Many digital communication systems, such as synchronous optical network/synchronous data hierarchy (SONET/SDH), switch services, wireless backhaul, and so on, require accurate data synchronization in order to work correctly. Atomic clocks or GPS disciplined oscillators are often used to meet this requirement

[1, 2]. Therefore, frequency synchronization is the key component to make many services in synchronous networks function properly.

Synchronous SONET/SDH networks

For example, two network elements in a SONET/SDH network having their own free running local clock source must be synchronized to the same reference clock source, as illustrated in Figure 1. The recovered data is written to a first-in-first-out (FIFO) memory buffer that is synchronized to the recovered transmit clock. The data is read from the FIFO using the local clock source that is synchronized to the reference clock source. If the write and read frequencies are different, a loss of data occurs that is referred to as data slip. To avoid this problem, synchronous networks such as SONET/SDH require frequency synchronization to guarantee that the write and read frequencies at the FIFO are the same.

 External synchronization in synchronous SONET/SDH networks

Figure 1: External synchronization in synchronous SONET/SDH networks


In SONET/SDH, the reference clock source has the highest accuracy, using either an atomic clock or a GPS disciplined oscillator. It is usually called stratum 1 or primary reference source (PRS), and has a +/- 0.00001 parts-per-million (ppm) free running accuracy. The stratum 1 clock is transmitted to the stratum 2 clock with +/- 0.016 ppm free running accuracy in the building integrated timing supply (BITS) of the hierarchical network. The stratum 2 clock tracks and holds (holdover mode) the last best estimate of the input stratum 1 clock reference during degradation of the reference clock. The local clock source of the transmitter or the receiver is usually referred as the stratum 3 clock, with +/- 4.6 ppm free running accuracy [3]. The stratum 3 clock also tracks and holds the last best estimate of the input reference clock from the stratum 2 source. This approach ensures that all local stratum 3 clock sources are accurately synchronized to the external PRS clock in order to avoid data slip between network elements. In holdover mode, the frequency drift over time must be slow enough for a good reference clock to re-establish.

The above approach is often considered an external timing scheme and it provides the most reliable frequency synchronization method in a SONET/SDH network. As a result, it leads to high costs for deployment and maintenance. Therefore, an alternative approach uses the recovered transmit clock at the receiver, which is already frequency synchronized to the stratum 1 clock from the transmitter as the reference clock for the receiver’s local clock source. In this case, both the transmitter and receiver are synchronized to the same reference clock source, which ensures that no data slip will occur, as illustrated in Figure 2. This method of synchronization is called line-timing distribution in SONET/SDH networks. This approach requires only one BIST device and significantly reduces both deployment and maintenance costs in the network.

 Line timing in synchronous SONET/SDH networks.

Figure 2: Line timing in synchronous SONET/SDH networks


This blog post studied the important role of clock frequency synchronization in synchronous networks. SONET/SDH is a good example of a synchronous network that requires all network elements to be synchronized to a primary reference clock source in order to avoid data slip during operation. The line-timing distribution concept in SONET/SDH networks opens an avenue to synchronous Ethernet (SyncE).

Watch for a discussion of packet-based timing concepts in packet networks, which will be covered in an upcoming blog post.


[1] Wikipedia. Accessed June 2013. “Synchronization in telecommunications.” http:/
[2] Wikipedia. Accessed June 2013. “Synchronous optical networking.” https:/
[3] Last Updated November 28, 1998. “SONET Glossary.”