SONET and SDH
On a larger scale, the telecommunications industry uses the Synchronous Optical Networking (SONET) or Synchronous Digital Hierarchy (SDH) standard for optical transport of TDM data. SONET, used in North America, and SDH, used elsewhere, are two closely-related standards that specify interface parameters, rates, framing formats, multiplexing methods, and management for synchronous TDM over fiber.
The figure displays SONET, which is an example of STDM. SONET/SDH takes n bit streams, multiplexes them, and optically modulates the signals. It then sends the signals out using a light emitting device over fiber with a bit rate equal to (incoming bit rate) x n. Thus, traffic arriving at the SONET multiplexer from four places at 2.5 Gb/s goes out as a single stream at 4 x 2.5 Gb/s, or 10 Gb/s. This principle is illustrated in the figure, which shows an increase in the bit rate by a factor of four in time slot T.