The Sonet is by far the most widely used optical data-transmission network in the U.S. It's an American National Standards Institute (ANSI) standard as well as a subset of the broader international standard known as the Synchronous Digital Hierarchy (SDH). The latter was developed and sanctioned by the International Telecommunications Union (ITU) and then used throughout the rest of the world.

Sonet is a physical layer (PHY) standard that defines the method for formatting and transmitting data in sync with a master timing system keyed to an atomic clock. The standard defines different optical-carrier (OC) speeds, from 51.84 Mbits/s to 39.812 Gbits/s. The electrical signal to be transmitted is called the Synchronous Transport Signal (STS) and can exist at several levels or speeds, although the base speed (STS-1) is 51.84 Mbits/s. In the SDH system, the signal also is called the Synchronous Transport Mode, or STM. The table lists the most common channel speeds.

Earlier systems used OC-3 and OC-12 levels, but today a majority of Sonet systems have been upgraded to operate at OC-48. The OC-192 and OC-768 levels aren't far behind.

Sonet is a time-division multiplexing (TDM) transmission scheme that sends time-interleaved data in fixed-length frames of 810 bytes. The frame format consists of nine 90-byte rows transmitted consecutively. Three bytes in each row are for overhead and 87 bytes per row are for data payload. Furthermore, the overhead bytes contain framing, control, parity, and pointer information for managing the payload.

While Sonet can operate in a point-to-point link, the most common topology is a ring. Multiple nodes comprise the ring with an add-drop multiplexer (ADM) at each node. One of the key features and benefits of a Sonet system, the ADM permits data from multiple sources to be added to or extracted from the data stream as required. The ring topology also offers data-transmission redundancy, which ensures survivability in situations when there's a cable cut.

Most of today's long-distance telephone service as well as the Internet backbone is made up of a hierarchy of large and small Sonet rings. Even though Sonet was designed to carry synchronous voice traffic in a circuit-switched environment, its primary function is data transportation with no regard to its content. As a result, Sonet can carry asynchronous, packet-switched data from Asynchronous Transfer-Mode (ATM), frame-relay, Ethernet, or Transport-Control Protocol/Internet Protocol (TCP/IP) equipment.

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