ously, the DSLAM must interface with the local switch so that it can pass voice calls on to the PSTN. However, it often interfaces with a number of other devices as well. For example, on the customer side, the DSLAM may connect to a standard ATU-C, directly to a PC with a built-in NIC, to a variety of DSL services, or to an integrated access device of some kind. On the trunk side (facing the switch), the DSLAM may connect to IP routers as described before, to an ATM switch, or to some other broadband service provider. It therefore becomes the focal point for the provisioning of a wide variety of access methods and service types. ADSL2 ADSL2 is a newer standard that has superseded preexisting ADSL standards. It is interoperable with preexisting ADSL deployments and is critical for service providers and customer alike; it means that they can continue to use their installed base of equipment with minimal disruption. ADSL2+ ADSL2+, on the other hand, is an extension of ADSL2. It is capable of doubling the transmission speed of typical ADSL installations to over 20 Mbps downstream on loops shorter than 8,000 feet. Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. Access Technologies Chapter 6 One major proponent of so-called Next-Generation DSL is BellSouth, which has the largest installed base of fiber among all the ILECs. The company believes that the best path to better residential broadband delivery is through faster DSL. Its strategy, which is a good one, is focused on scaling up existing DSL facilities with ADSL2+ and using DSL bonding techniques to scale DSL bandwidth to as high as 24 Mbps -high enough to support HDTV-quality video. Its plan is to implement ADSL2+, which will boost its available DSL bandwidth to 12 Mbps. Early next year the carrier also will test DSL bonding, which allows mul- tiple DSL connections to be aggregated into a si