t and the value that it brings to the table; and mas- sive, unstoppable demand for more bandwidth. To those, add these: increasing demand for high-speed routing capability to work hand-in- glove with transport; deployment of DS1, DS3, and E1 interfaces directly to the enterprise customer as access solutions; growth in demand for broadband access technologies such as cable modems, the many flavors of DSL, and two-way satellite connectivity; the ongoing replacement of traditional circuit-switched network fabrics with packet- based transport and mesh architectures; a renewed focus on the SONET and SDH overhead with an eye toward using it more effectively; and convergence of multiple applications on a single, capable, high-speed network fabric. SDH Nomenclature Before launching into a functional description of SDH, it would be good to first cover the differences in naming convention between the two. This will help to dispel confusion (hopefully!). The fundamental SONET unit of transport uses a nine-row by 90- column frame that comprises three columns of section and line overhead, one column of path overhead, and 87 columns of payload. The payload, which is primarily user data, is carried in a payload envelope that can be formatted in various ways to make it carry a variety of payload types. For example, multiple SONET STS-1 frames can be combined to create higher-rate systems for transporting multiple STS-1 streams, or a single higher-rate stream created from the combined bandwidth of the various multiplexed components. Conversely, SONET can transport subrate pay- loads, known as virtual tributaries, which operate at rates slower than the fundamental STS-1 SONET rate. When this is done, the payload envelope is divided into virtual tributary groups, which can in turn transport a variety of virtual tributary types. In the SDH world, similar words apply, but they are different enough that they should be discussed. As you will see, SDH uses a fundamental transport "container" that is three times the size of its SONET cou