SONET, Synchronous Transport Signal, and Synchronous Digital Hierarchy

SONET is a fiber- optic -based network specification that is capable of speeds up to 2 Gbps. SONET is based on using optical media, and as such has numerous advantages in comparison to copper conduits . SONET uses STS Level 1 (STS-1) as a basic building block and, like any standard STS-1, assures interoperability of equipment from multiple vendors .

The Optical Fiber Hierarchy of Circuits

The digital hierarchy of SONET is based on Optical Carrier Level 1 (OC-1), which defines transmission rates of 51.84 Mbps. Therefore, the hierarchy starts with data rates of close to T3 speeds of 51.840 Mbps. The transmission rate of 51.840 Mbps is also called STS-1. Every OC-x transmission rate of SONET is the exact multiple of the basic speed, which creates a straightforward way of multiplexing the higher speed formats. The international hierarchy is known as SDH, with starting data rates of OC-3. The optical fiber hierarchy is shown in Table 3-2.

Table 3-2. Hierarchy of Optical Transmission Rates

OC-Level (Optical)	STS Level (Electrical)	SDH Equivalent	Line Rate (Mbps)
OC-1	STS-1		51.840
OC-3	STS-3	STM-1	155.520
OC-12	STS-12	STM-4	622.080
OC-24	STS-24	STM-8	1244.160
OC-48	STS-48	STM-16	2488.320
OC-192	STS-192	STM-64	9953.280