Synchronous Optical Network (SONET)

SONET is the American National Standards Institute (ANSI) standard for synchronous data transmission on optical media. The international equivalent of SONET is synchronous digital hierarchy (SDH). SONET provides standards for a number of line rates up to the maximum line rate of 39.808 gigabits per second and beyond. SONET is considered to be the foundation for the physical layer of the broadband ISDN (B-ISDN). Asynchronous transfer mode runs as a layer on top of SONET as well as on top of other technologies.

The network defines optical carrier levels and their electrical equivalents, called synchronous transport signals (STS) for fiber optic transmission. The first step in the process involves multiplexing multiple signals by generating the lowest level or base signal, called STS-1. Its optical carrier counterpart is called OC-1, and it transmits at 51,480 Mb/s. Other levels operate from 155 Mb/s up to 40 Gb/s. The basic network elements include the terminal multiplexer (PTE), a regenerator (as needed for long distance transmissions), an add-drop multiplexer (ADM), for use in point-to-multipoint configurations, wideband digital cross-connects (W-DCS), broadband digital cross-connects, and the digital loop carrier. Together, these elements may be used in a point-to-point, point-to-multipoint (hub), or ring network configuration. Figure 5 illustrates a typical hub network configuration.

SONET provides a number of benefits over asynchronous systems. Its multiplexing technique allows simplified synchronous clocking and reduced back-to-back multiplexing, which reduces circuit complexity and cost. SONET’s optical interconnections meet a number of vendor requirements. The hub configuration adds greater flexibility to the system, allowing the convergence of a number of types of network protocols, ATM, Internet protocol, etc.