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Introduction to SONET

The Synchronous Optical NETwork (SONET) standard for fiber optic networks was developed in the mid-1980s. It remains in widespread use today. In a nutshell, SONET allows multiple technologies and vendor products to interoperate by defining standard physical network interfaces.

In Europe, the term Synchronous Digital Hierarchy (SDH) refers to essentially the same standard as SONET. This article uses the term "SONET" to refer to the common characteristics of SONET/SDH.


SONET was originally designed for the public telephone network. In the early 1980's, the forced breakup of AT&T in the United States created numerous regional telephone companies, and these companies quickly encountered difficulties in networking with each other. Fiber optic cabling already prevailed for long distance voice traffic transmissions, but the existing networks proved unnecessarily expensive to build and difficult to extend for so-called long haul data and/or video traffic.

The American National Standards Institute (ANSI) successfully devised SONET as the new standard for these applications. Like Ethernet, SONET provides a "layer 1" or interface layer technology (also termed physical layer in the OSI model). As such, SONET acts a carrier of multiple higher-level application protocols. For example, Internet Protocol (IP) packets can be configured to flow over SONET.


SONET commonly transmits data at speeds between 155 megabits per second (Mbps) and 2.5 gigabits per second (Gbps). To build these high-bandwidth data streams, SONET multiplexes together channels having bandwidth as low as 64 kilobits per second (Kpbs) into data frames sent at fixed intervals.

Compared to Ethernet cabling that spans distances up to100 meters (328 feet), SONET fiber typically runs much further. Even short reach links span up to 2 kilometers (1.2 miles); intermediate and long reach links cover dozens of kilometers.

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