The more recent Synchronous Digital Hierarchy (SDH) standard built on the experience of the development of SONET. Both SDH and SONET are widely used today; SONET in the U.S. and Canada, SDH in the rest of the world.
Synchronous networking differs from PDH in that the exact rates that are used to transport the data are tightly synchronized to network based clocks. Thus the entire network operates synchronously. SDH was made possible by the existence of atomic clocks.
The basic SONET signal operates at 51.840 Mbit/s and is designated STS-1 (synchronous transport signal one). The STS-1 frame is the basic unit of transmission in SONET.
The two major components of the STS-1 frame are the transport overhead and the synchronous payload envelope (SPE). The transport overhead (27 bytes) comprises the section overhead and line overhead. These bytes are used for signalling and measuring transmission error rates. The SPE comprises the payload overhead (9 bytes, used for end to end signalling and error measurement) and the payload of 774 bytes. The STS-1 payload is designed to carry a full DS-3 frame.
The entire STS-1 frame is 810 bytes. The STS-1 frame is transmitted in exactly 125 microseconds on a fibre-optic circuit designated OC-1 (optical carrier one). In practice the terms STS-1 and OC-1 are used interchangeably.
Three OC-1 (STS-1) signals are multiplexed by time-division multiplexing to form the next level of the SONET hierarchy, the OC-3 (STS-3), running at 155.52 Mbit/s. The multiplexing is performed by interleaving the bytes of the three STS-1 frames to form the STS-3 frame, containing 2430 bytes and transmitted in 125 microseconds. The STS-3 signal is also used as a basis for the SDH hierarchy, where it is designated STM-1 (synchronous transmission module one).
Higher speed circuits are formed by successively aggregating multiples of slower circuits, their speed always being immediately apparent from their designation. For example, four OC-3 or STM-1 circuits can be agregated to form a 622.08 Mbit/s circuit designated as OC-12 or STM-4.
The current state of the art is the OC-192 or STM-64 circuit, which operates at rate of just under 10 Gbit/s. Speeds beyond 10 Gbit/s are not currently technically viable; however multiple OC-192 circuits can be carried over a single fibre pair by means of Dense Wave Division Multiplexing (DWDM). Such circuits are the basis for all modern transatlantic cable systems and other long-haul circuits.
Due to the fortuitous similarity in bit rates, 10 Gigabit Ethernet has been designed with a capability to interoperate with OC-192/STM-64 equipment.