Comparison of SONET and SDH hierarchies

Overview

SONET and SDH are compatible digital hierarchies that support identical transmission rates with similar framing structures. However, SONET and SDH standards differ in the way they multiplex and map tributary payloads into higher digital levels. Because of these differences, some SDH framing blocks do not have equivalent blocks in SONET.

SONET provides one STS sub-channel, the STS-1, for mapping lower-capacity, deep-channel payloads while SDH provides two alternative sub-channels, the AU3 and the AU4. After an AU3 sub-channel is created, an AU4 cannot be created on that port in the case where the port is an OC-3 (STM-1). Conversely, if an AU4 sub-channel is created, an AU3 cannot be created on that port in the case where the port is an OC-3 (STM-1).

SONET and SDH transmission rates converge at 155.520 Mb/s where the SONET STS-3c is equivalent to the SDH STM-1 bit rate. For lower-capacity payloads, such as DS1, E1, DS3, and E3, SONET provides one unique mapping path for each payload while SDH permits two alternative paths for each payload as shown in the following figure.

Figure 16-7: Supported SONET/SDH multiplexing structures