hood digital loop carrier configured to support DSL, although it can also reside at the customer premises in the case of a large enterprise or an MDU. The service provider gener- ally connects all its DSLAMs, which are essentially massive ATM concentrators, over an ATM backbone network. DSLAMs are designed to concentrate hundreds of DSL access lines onto ATM or IP trunks connecting to ATM switches, routers, or multiservice edge switches that then connect the DSLs to the ISPs. Each DSLAM DSL Technology Voice Voice Gateway Network Integrated Access Device (voice/data) Voice Switch Class 5 DSLAM (incorporates ADSL modem Corporate and voice Network splitter) Switch Internet ATM or IP DSL Subscriber Backbone Other POTS Splitter Corporate Offices Online Information Services DSL Modem Figure 12.2 An ADSL configuration has multiple DSLAM aggregation cards, and each card has multiple ports (typically 24, although the number can vary by manufacturer). DSLAMs typically contain power converters, DSLAM chassis, DSLAM cards, cabling, and upstream links. The upstream links are most commonly either multigigabit fiber-optic links or Gigabit Ethernet. The DSLAM splits the voice and data traffic, sending the voice traffic through traditional local exchanges onto the PSTN and sending the data traffic to the appropriate ISP or enterprise network. Each subscriber's traffic appears as a separate ATM VC (a permanent virtual circuit [PVC]). If the service provider pro- vides access to more than one ISP, an ATM VC is created to provide the subscriber with access to his or her ISP of choice. DSLAMs also support quality of service (QoS) features such as priority queues, contention, and DiffServ. ADSL2 In July 2002, the ITU-T approved G.992.3 and G.992.4, two new standards for ADSL technology collectively called ADSL2. The basic goal of both standards, known as G.dmt.bis and G.lite.bis, respectively, is to increase the transmission rates, increase the range, and improve the overall reliability and manageability of DSL services. ADSL2 adds se