ide long distance voice services, but most of them are resellers and therefore are not considered IICs. The large IXCs that own and operate national and global networks are also Chapter 1 - Introduction to IICs and OSS NSPs. NSPs provide the Internet's core and backbone - the fiber optic networks that carry high capacity traffic across vast distances. They also provide the private, high capacity pipes - or "leased lines" - many corporations use to connect their geographically disparate locations. Some NSPs do not offer any voice services to end-users, though most will carry some voice traffic along with their data traffic; thus lines between NSPs and IXCs are rather blurry. What's most important to know about these companies is that they provide the high capacity pipes that allow all other IICs to move their customers' voice, data, and video traffic from place to place. Some of their network infrastructure is part of the Internet, but much of it is reserved for more private or dedicated use. Examples of IXCs and backbone providers include AT&T, WorldCom, Sprint, Qwest and Level 3. Competitive Local Exchange Carriers (CLECs). The term CLEC is often used as a catchall phrase to describe any emerging, competitive communications service provider. For our purposes, CLECs are considered competitive - as opposed to incumbent - local providers that offer mainly voice services to business customers. In many cases CLECs utilize traditional time division multiplexing (TDM) technologies and circuit switching equipment. It's arguable that a company offering only voice services using circuit technologies is not really an IIC. The number of companies offering such limited services is shrinking rapidly, however, as most providers adopt newer cell, frame and IP-based network technologies. (Even if they only use TDM, chances are their customers dial-up to the Internet over their networks, SO technically they are still IICs). Companies that employ "voice over" technologies, such as voice over DSL (VoDSL) or voice over IP (VoIP) are