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SONET (Synchronous Optical NETwork) was the first attempt to develop a set of fiber-optic system standards. As each manufacturers products were designed according to proprietary specifications systems of disparate origin were incompatiblethey simply did not interconnect, much less interoperate. Therefore, each fiber optic link (e.g, from CO-to-CO) was required to have equipment of the same origin at both ends. This limitation effectively forced the carriers to select a single equipment vendor, limiting the ability of other manufacturers to compete and stifling technical creativity. Additionally, the interconnecting carriers had to limit fiber optic systems either to a single vendor, or to optoelectric interfaces which limited the capacity to far less than that actually supported by the individual systems. (The lowest common denominator always rules.) Therefore, the economic and technical benefits of a multivendor market were limited, and largely unrealized.
Initial standardization efforts began in 1984, when MCI proposed the development of connectivity standards, or mid-span fiber meets, to the Interexchange Carrier Compatibility Forum (ICCF). The ICCF then requested that the Exchange Carriers Standards Association (ECSA) develop those standards [10-15]. In 1985, Bellcore proposed SONET to the ANSI T1X1 committee; the process continued until final approval for a much-revised SONET was gained in June 1988. The resulting ANSI specification for SONET Phase 1, were released in T1.105-1988 and T1.106-1988, specifying a basic transmission level of 51.84 Mbps, which will carry a T3 signal of 45 Mbps plus additional overhead.
SDH (Synchronous Digital Hierarchy) is the international version of SONET standards. The initial SDH efforts, driven by the SONET impetus, were begun by the CCITT in 1986. In 1988, the SONET standards were accepted by the CCITT, (now ITU-T) with modifications which were mostly at the lower multiplexing levels. These differences are largely due to the requirement to accommodate the complexities of internetworking the disparate national and regional networks. The ITU-T (nee CCITT) Recommendations referenced are G.707, G.708 and G.709.
Standards work continues on SONET/SDH, with the involvement of standards bodies and associations including ANSI, EIA, ECSA, Bellcore, IEEE and ITU-T (nee CCITT) [10-16].
SONET standards were developed in three phases. Phase I (1988) defines transmission rates and characteristics, signal formats, and optical interfaces. Phase I also defines Optical Carrier (OC) levels and Data Communications Channels (DCC), used for network management purposes in support of mid-span meet at the payload level. While Phase I does not support network management from end-to-end, neither does it preclude that potential. Phase II refined the physical portion of the standards and defined protocols used on data communications channels DS-1 to DS-3. Phase II also defines interoperability parameters for mid-span meets, network management, OSI CMISE (Common Management Information Service Elements) and add/drop multiplexer capabilities. Phase II further defines OAM&P (Operations, Administration, Management and Provisioning) procedures and connectivity to B-ISDN. Phase III provides all network management (OAM&P) requirements for midspan meet. Phase III also defines all network management standard message sets (alarm state, circuit-pack failure, and intermittent failure) and addressing schemes for interconnection. Finally, Phase III provides for ring and nested protection switching standards [10-15] and [10-16].
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