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The Development of SONET

Some people view SONET as a new technology, and it is only in the last decade that SONET has been deployed extensively. However, SONET did not just appear suddenly on the scene. Extensive research has been underway for well over a decade on many of the features that are found in SONET. One notable achievement began in 1984. It focused on the efforts of several standards groups and vendors to develop optical transmission standards for what is known as the mid-span meet (also known as transverse compatibility). The goal was to publish a specification that would allow different vendors' equipment to interoperate with each other at the fiber level.

In addition, due to the breakup of the Bell System in 1984, there were no standards developed beyond T3 technology. Prior to the divestiture, all equipment was built by AT&T's manufacturing arm, Western Electric (WECO), which ensured that there would be no compatibility problems in any network components.

After the breakup, there was little incentive for the other carriers (such as MCI and Sprint) to purchase AT&T-based equipment. Indeed, there was no incentive to purchase AT&T equipment, since AT&T, MCI, and Sprint had begun competing with each other for long distance services. This situation led to the rapid growth of alternate equipment vendors (such as Nortel Networks), who were developing advanced digital switching technologies.

The 1984 divestiture paved the way for alternate long distance carriers through the equal access ruling. The alternate carriers were given equal access to the local exchange carrier (LEC) infrastructure and connections to AT&T for end-to-end long distance service. The LEC could connect to MCI, Sprint, and others through their switching facilities at an interface in the LEC or long distance carrier offices called the point of presence (POP).

During this time, higher capacity schemes beyond T3 became proprietary, creating serious compatibility problems for network operators who purchased equipment from different manufacturers. In addition, the early 1980s witnessed the proliferation of incompatible and competing optical fiber specifications.

Precursors to SONET

We interrupt the discussion on divestiture to explain some of the technology that was being developed during the early 1980s. A landmark project that contributed to SONET was Metrobus, an optical communications system developed at AT&T's Bell Labs in the early 1980s. Its name was derived from its purpose: It was situated in a metropolitan area to serve as a high-speed optical transport network.

Metrobus demonstrated the feasibility of several new techniques that found their way into SONET. (They are explained in this chap- ter and subsequent chapters.) Among the more notable features were (a) single-step multiplexing, (b) synchronous timing, (c) extensive overhead for network management, (d) accessing low level signals directly, (e) point-to-multipoint multiplexing, and (f) the employment of multimegabit media for achieving high bandwidth network transmission capacity (of approximately 150 Mbit/s).

This latter decision along with the ensuing research and testing was important, because a 150 Mbit/s signal rate can accommodate voice, video, and data signals, as well as compressed high definition television (HDTV). Moreover, these techniques permitted the use of relatively inexpensive graded-index multimode fibers instead of the more expensive single mode fibers, although single mode fiber is now the preferred media for SONET.

The various standards groups began the work on SONET after MCI send a request to them to establish standards for the mid-span meet. The SONET specifications were developed in the early 1980s, and Bellcore submitted its proposals to the American National Standards Institute (ANSI) T1X1 Committee in early 1985,1 based on a 50.688 Mbit/s transfer rate. The initial SONET work did not arouse much interest until the Metrobus activity became recognized.

Later, using the innovative features of Metrobus, the SONET designers made modifications to the original SONET proposal, principally in the size of the frame and the manner in which T1 signals were mapped into the SONET frame.

From 1984 to 1986, various alternatives were considered by the ANSI T1 Committee, who settled on what became known as the synchronous transport signal number one (STS-1) rate as a base standard. Finally, in 1987, the ANSI T1X1 committee published a draft document on SONET.

Participation by ITU-T

During this time, the international standards body now known as the International Telecommunication Union-Telecommunication Standardization Sector (ITU-T) had rejected the STS-1 rate as a base rate in favor of a base rate of 155.520 Mbit/s. For a while, it appeared that the North American and European approaches might not converge, but the SONET frame syntax and structure were altered one more time to a rate of 51.84 Mbit/s which permitted this rate to be multiplexed (concatenated) by an integer of three to the European preference of 155.52 Mbit/s. This work has resulted in almost complete compatibility between the North American and ITU-T approaches. The ITU-T Recommendations are now considered the "official" standards and are collectively called the Synchronous Digital Hierarchy (SDH).

Once the major aspects of the standards were in place, vendors and manufacturers began to develop SONET and SDH equipment and software. These efforts came to fruition in the early 1990s and, as of this writing, SONET and SDH have been deployed throughout the United States and other parts of the world.

Key ITU-T Documents

Listed below are some of the most commonly cited SDH standards available from the ITU-T.2

  • ITU-T G.707: Network Node Interface for the Synchronous Digital Hierarchy (SDH)

  • ITU-T G.781: Structure of Recommendations on Equipment for the Synchronous Digital Hierarchy (SDH)

  • ITU-T G.782: Types and Characteristics of Synchronous Digital Hierarchy (SDH) Equipment

  • ITU-T G.783: Characteristics of Synchronous Digital Hierarchy (SDH) Equipment Functional Blocks

  • ITU-T G.803: Architecture of Transport Networks Based on the Synchronous Digital Hierarchy (SDH)

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