to the households. This allows you to eliminate much of what used to be an analog plant leading up to the local exchange. It also allows you to provide service to customers who are outside the distance specifica- tions between a subscriber and the local exchange. So, in effect, that DLC can be used to reduce the loop length. Traditional DLCs are not interoperable with some DSL offerings, including ADSL and SDSL. For example, about 30% to 40% of the U.S. population is serviced through DLCs. And in general, globally, the more rural or remote a city or neigh- borhood, the more likely that it is serviced via a DLC. For those people to be able to subscribe to high-bandwidth DSL services, the carrier has to replace the DLC with a newer generation of device. Lucent's xDSL Access Gateway, for example, is such a device; it offers a multiservice access system that provides Lite and full-rate Chapter 1 Telecommunications Technology Fundamentals ADSL, Integrated Services Digital Network (ISDN), Asynchronous Transfer Mode (ATM), and plain old telephone service (POTS), which is analog over twisted-pair lines or fiber. Or the carrier may simply determine that the market area doesn't promise enough revenue and leave cable modems or broadband wireless as the only available broadband access technique. Inverse Multiplexing The inverse multiplexer, which arrived on the scene in the 1990s, does the oppo- site of what the multiplexers described so far do: Rather than combine lots of low- bit-rate streams to ride over a high-bit-rate pipe, an inverse multiplexer breaks down a high-bandwidth signal into a group of smaller-data-rate signals that can be dispersed over a range of channels to be carried over the network. A primary appli- cation for inverse multiplexers is to support high-bandwidth applications such as videoconferencing. In Figure 1.18, a videoconference is to occur at 1.5Mbps. A good-quality, full- motion, long-session video requires substantial bandwidth. It's one thing to toler- ate pixilation or artifacts in motion for a