. And it doesn't help matters when the marketplace value is tepid or weak. Synchronous Optical Network/Synchronous Digital Hierarchy Transmission SONET/SDH evolved from a US industry initiative began in the mid-1980s and aimed at establishing a set of standards to support multi-vendor capability to transmit with one vendor's equipment and receive with another's over fiber media. SDH standards followed and, while they are different, the two are far more compat- ible than earlier T-carrier standards. SONET/SDH is truly synchron- ous. In fact it converts "plesiochronous" T-carrier streams to true synchronous transmission. In terms of backbone speed, it picks up where T-carrier leaves off and scales to around 40x-that's 40 Gbs. The basic SONET/SDH transmission unit is the synchronous trans- port stream running at 51.84 Mbs. Most SONET/SDH transmission runs over light wave medium using fiber conductors; however, noth- ing prevents it from running on other baseband or carrier type medium as long as sufficient bandwidth is available. SONET/SDH is a technology and method of assembling network equipment from diverse manufacturers to realize a connection between two points, also called a link, span, or transmission facility. The transmission facility interfaces with cross connect, cell, circuit, and packet switching facilities. With a given combination and proper configuration, SONET/SDH provides the layer 1 or physical layer connections used to support point-to-point private line, switched voice, data circuits, Internet access, or other services across town or around the globe. 4 NETWORK TECHNOLOGY AND METHODOLOGY SONET and SDH are very similar in their high level framework, function, and other respects. Attempting to differentiate between the two becomes an unnecessary tedious exercise for the purposes of this book. Therefore, from this point forward, the focus is on SONET and where appropriate and necessary to be specific, SONET/SDH, or SDH as a stand-alone term will be used. SONET is an international standard; however,