Synchronous Digital Hierarchy (SDH) is a CCITT standard for a hierarchy of optical transmission rates. Synchronous Optical Network (SONET) is a US standard that is largely equivalent to SDH. Both are widely used methods for very high speed transmission of voice and data signals across the numerous world-wide fiber-optic networks.
SDH and SONET use light emitting diodes or lasers to transmit a binary stream of light-on and light-off sequences at a constant rate. At the far end optical sensors convert the pulses of light back to electrical representations of the binary information.
In Wavelength Division Multiplexing (WDM), light at several different wavelengths (colors to a human eye) is transmitted on the same fiber segment, greatly increasing the throughput of each fiber cable.
In Dense Wavelength-Division Multiplexing (DWDM), many optical data streams at different wavelengths are combined into one fiber.
The basic building block of the SONET/SDH hierarchy in the optical domain is an OC-1; in the electrical domain, it is an STS-1. An OC-1 operates at 51.840 mbps. OC-3 operates at 155.520 Mbps.
A SONET stream can consist of discrete lower-rate traffic flows that have been combined using Time-Division Multiplexing (TDM) techniques. This method is useful, but a portion of the total bandwidth is consumed by the TDM overhead. When a SONET stream consists of only a single, very high speed payload, it is referred to as operating in concatenated mode. A SONET interface operating in this mode has a "c" added to the rate descriptor. For example, a concatenated OC-48 interface is referred to as OC-48c.
This chapter discusses the following SONET/SDH interface properties that you can configure: