 all international links are based on SDH standards. All SONET specifications comply with the ITU SDH recommendations. In the United States and Canada, service providers and equipment manufacturers rely on ANSI T1 (SONET) specifications. SONET standards evolved in three phases. Phase I was issued initially in 1988 and provided eight discrete line rates between 51 Mbs and 2.4 Gbs (since extended to 40 Gbs). There were several advances over T-carrier standards, including byte- interleaved multiplexing, a monitoring scheme capable of fault alarm at path and section level, and a built-in administrative com- munications channel separate from the payload. Backward compati- bility was ensured by creation of VT mapping to DS1/T1, DS2, and DS3/T3. In SDH, the equivalent to VT is a TU, and T3, and TU mapping to E1, E3, and E4 SDH interfaces. Phase II enabled different manufacturer's equipment to interoperate, included automatic protection switching, and mapping of the North American DS4 (274 Mbs) signal into STS-3c. Phase III continues to evolve in the form of improvements to past work and to address operations, administration, maintenance and provisioning (OAMP) issues, the last remaining and reportedly con- troversial area to be resolved. Synchronous Optical Network/Synchronous Digital Hierarchy Layering SONET/SDH has its own layering model. It shouldn't be con- fused with the OSI stack. Anything and everything that SONET/ SDH does is all at layer 1 of the OSI stack. However, it is easy to see the commonality between layer 1 of the OSI and SONET infra- structure. SONET has five layers: media, section, line, path, and PRACTICAL IP AND TELECOM FOR BROADCAST ENGINEERING AND OPERATIONS virtual tributary. Each of these layers can also be viewed as a level within an end-to-end facility where each level encompasses certain functional parts of the system while excluding others in the end-to- end chain. These layers and levels defined in the layering model and the end-to-end block diagram also have a distinct and direct relation-