SONET and SDH are two standards evolved from the traditional PHD. Both of them are time division multiplexing (TDM) techniques. SONET is the North America TDM standard, while SDH is the TDM standard commonly used in Europe and Asia. SDH has become a global standard and SONET is now a subset of SDH.

Both SDH and SONET define digital multiplexing hierarchies, which are compatible in their data rates. They support both nonsynchronous TDM and synchronous TDM hierarchies. SONET’s nonsynchronous TDM hierarchy is based on the DS1 signal (1.54 Mbps), which is made up of 24 DS0s (56 Kbps per DS0). SDH’s nonsynchronous TDM hierarchy is based on E1 signal (2.048 Mbps), which is made up of 32 E0s (64 Kbps per E0). Table below shows the nonsynchronous TDM hierarchies for both SDH and SONET.

Both SONET and SDH also define synchronous multiplexing hierarchies to enable the two types of networks to interconnect each other. SONET starts from STS-1 (corresponding to OC-1 in the optical domain) to include STS-3 (OC-3), STS-12 (OC-12), STS-24 (OC-24), STS-48 (OC-48), and STS-192 (OC-192). SDH starts from STM-1 to include STM-4, STM-16 and STM-64. At OC-3 and STM-1, SONET and SDH match in their data rates. Table below shows the interface rates of the SONET/SDH multiplexing hierarchies. In real networks, the data rate of STS-24 (OC-24) is seldom used.