SONET
and
SDH
Authors: Freeman Roger L. 
Published: November 2003

SONET and SDH are similar digital transport formats which were developed for the specific purpose of providing a reliable and versatile digital structure to take advantage of the higher bit rate capacity of optical fiber. SONET is an acronym standing for
synchronous optical network
. In a similar vein, SDH stands for
synchronous digital hierarchy
. We could say that SONET has a North American flavor, and SDH has a European flavor. This may be stretching the point, because the two systems are very similar.

The original concept in developing a digital format for high‐bit rate capacity optical systems was to have just one singular standard for worldwide application. This did not work out. The United States wanted the basic bit rate to accommodate DS3 (around 50 Mbps). The Europeans had no bit rates near this value and were opting for a starting rate around 150 Mbps. Another difference surfaced for framing alternatives. The US perspective was based on a frame of 13 rows by 180‐byte columns for the 150‐Mbps rate reflecting what is now called STS‐3 structure. Europe advocated a 9‐row by 270‐byte column STS‐3 frame to efficiently transport the E1 signal (2.048 Mbps) using 4 columns of 9 bytes, based on 32 bytes/125 µs.

The ANSI T1X1 committee approved a final standard in August 1988, with CCITT following suit, and a global SONET/SDH standard was established. This global standard was based on a 9‐row frame, wherein SONET became a subset of SDH.

Both SONET and SDH use basic building block techniques. As we mentioned, SONET starts at a lower bit rate, at 51.84 Mbps. This basic rate is called STS‐1 (synchronous transport signal layer 1). Lower rate payloads are mapped into the STS‐1 format, while higher rate signals are obtained by byte interleaving
N
frame‐aligned STS‐1s to create an STS‐N signal. Such a simple multiplexing approach results in no additional overhead; as a consequence, the transmission rate of an STS‐N signal is exactly
N
 × 51.84 Mbps where
N
is currently defined for the values 1, 3, 12, 24, 48, and 192.

The basic building block of SDH is the synchronous transport module level 1 (STM‐1) with a bit rate of 155.52 Mbps. Lower rate payloads are mapped into STM‐1, and higher rate signals are generated by synchronously multiplexing
N
STM‐1 signals to form the STM‐N signal. Transport overhead of an STM‐N signal is
N
times the transport overhead of an STM‐1, and the transmission rate is N × 155.52 Mbps. Presently, only STM‐1, STM‐4, STM‐16, and STM‐64 have been defined by the ITU‐T Organization.

Both with SONET and SDH, the frame rate is 8000 per second, resulting in a 125‐µs frame period. There is high compatibility between SONET and SDH. Because of the different basic building block size, they differ in structure: 51.84 Mbps for SONET and 155.52 Mbps for SONET. However, if we multiply the SONET rate by 3, developing the STS‐3 signal, we do indeed have the SDH initial bit rate of 155.52 Mbps. Figures 8‐1 and 8‐2 compare the multiplexing structures of each.

Besides differing in basic building block bit rates, SONET and SDH differ with respect to overhead usage. These overhead differences have been grouped into two broad categories: format definitions and usage interpretation. As a result, we have opted to segregate our descriptions of each.