e. Often, business customers who send as well as receive large files select symmetrical service. In the United States, Verizon is the provider with the largest installed base of FTTH connections. Using Digital Loop Carrier Systems to Extend Fiber Prior to the introduction of PONs in the early 2000s, providers installed the more costly Digital Loop Carriers (DLCs) when they extended fiber closer to customers. Carriers ran fiber to the neighborhood and terminated it in DLCs. In addition to being more expensive, DLCs take up more space, require electricity, and are less flexible than PONs. DLC systems have multiplexing and demultiplexing functionality. At the central office, traffic is combined into high-speed optical streams. The DLC separates out (demultiplexes) the traffic and sends it to end users in analog format on twisted-pair copper cabling. It also converts the optical pulses to electrical signals, and vice versa. Most DLCs support Plain Old Telephone Service (POTS), T1, E1, and Integrated Ser- vices Digital Network (ISDN) technologies. DLCs are also used to improve speeds on Digital Subscriber Line (DSL) systems by facilitating shorter copper cabling runs. Access Networks in Cable Operators' Networks DSL speeds decrease as copper cabling distances increase. See Chapter 5 for informa- tion on ISDN, T1/T3, and DSL technologies. ACCESS NETWORKS IN CABLE OPERATORS' NET- WORKS Access networks in cable operators' and traditional telephone companies' networks use a different type of customer modem. However, both telephone companies and cable Multiple System Operators (MSOs) face the challenge of upgrading access net- works to increase capacity. Access networks are those portions of a network between a provider's equipment and that of the customer. MSOs are large cable TV providers with service areas that extend over many states. They are referred to as MSOs because they grew by pur- chasing other cable companies and merging operations with these often- smaller companies' systems. Comcast Corporation, Cox Cable, a