 Broadband ISDN standard and a desire to deliver advanced applications. SDH/SONET is a family of transmission standards designed to achieve compa- tibility between different fiber-optic transport products as well as to provide com- patibility with the existing digital hierarchy, PDH. A lot of fiber-optic systems have been deployed since 1984, but they're not all compatible with one another. They use different forms of cables with different diameters, and they use different types of light sources. And where there are physical incompatibilities, you can't achieve midspan meet-where two carrier services have to come together. A railroad analogy can be used here. Think back to when people had just begun to build railroads. Often, one provider was building tracks going east to west that were a certain width, and another provider was building tracks going west to east that were a different width. Chapter 4 The PSTN When the providers met in the middle, their cars couldn't cross each other's tracks. The same thing happens when there's a lack of physical compatibility in fiber-optic transport, and that means you lose the ability to carry network management infor- mation on an end-to-end basis. You may not be able to do end-to-end monitoring and control of a network that is multivendor or multicarrier in nature if the vendors or carriers use incompatible physical fiber-optic equipment. It's always important to develop and have available very strong network man- agement tools. The goal of network management is not to eliminate downtime because we know that would be impractical; rather, it is to minimize the resolution time. So the ability to do end-to-end testing remotely is very critical to quick recov- erability. SDH/SONET provides the physical layer (i.e., Layer 1) framework for broadband applications. It provides a standardized list of optical parameters that define the allowed types of cables and light sources. It defines a new table of data rates that are much higher than older transmission rates. It redefines how th