CHAPTER 3 Network Link Technologies 95 DSL can operate in a duplex (symmetrical) fashion, offering the same speeds upstream and downstream. Others, mainly targeted for residential Internet browsing customers, offer higher downstream speeds to handle relatively large server replies to upstream mouse clicks or keystrokes. However, standard VDSL and VDSL2 have much less asymmetry than other methods. For example, 100-Mbps symmetric operation is possible at 0.3 km, and 50 Mbps symmetric at 1 km. The DSLAMs connect to a high-speed service provider backbone, and then the Internet. DSLAMs aggregate traffic, typically for an ATM network, and then connect to a router network. On the interface to the premises, the DSLAM demultiplexes traffic for individual users and forwards it to the appropriate users. In order to support traditional voice services, most DSL technologies require a sig- nal filter or "splitter" to be installed on the customer premises to share the twisted-pair wiring. The DSLAM splits the signal off at the central office. Splitterless DSL is very popular, however, in the form of "DSL Lite" or several other names. In Table 3.2, various types of DSL are compared. The speeds listed are typical, as are the distance (there are many other factors that can limit DSL reach) and services offered. VDSL requires a fiber-optic feeder system to the immediate neighborhood, but VDSL