SONET Interfaces

SONET is widely used in the USA for very high-speed transmission of voice and data signals across the numerous world-wide fiber-optic networks.

SONET uses LEDs 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 (or 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 hierarchy in the optical domain is OC1; in the electrical domain, the basic building block is STS1. OC1 operates at 51.840 Mbps. OC3 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 OC48 interface is referred to as OC-48c.