SL arrived, it provided the offload required to save the network. The typical ADSL installation is shown in Figure 6-4. No change is required to the two-wire local loop; however, minor equipment changes are required. First, the customer must have an ADSL modem at their premises. This device allows both the telephone service and a data access device, such as a PC, to be connected to the line. The ADSL modem is more than a simple modem, in that it also pro- vides the frequency-division multiplexing process required to separate the voice and data traffic for transport across the loop. The device that actually does this is called a splitter, in that it splits the voice traffic away from the data. It is usually bundled as part of the ADSL modem although it can also be installed as a card in the PC, as a stand-alone device at the demarcation point, or on each phone at the premises. The most common implementation is to integrate the splitter as part of the DSL modem; this, however, is the least desirable implementation because this design can lead to crosstalk between the voice and data circuitry inside the Figure 6-4 Router Layout of Data Traffic typical Digital Subscriber Line Voice (DSL) Switch Voice Traffic DSL Modem 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 device. When voice traffic reaches the ADSL modem, it is immediately encoded in the traditional voice band and handed off to the local switch when it arrives at the central office. The modem is often referred to as an ADSL transmission unit for remote use, or ATU-R. Similarly, the device in the central office often called an ATU-C (for "central office"). When a PC wishes to transmit data across the local loop, the traffic is encoded in the higher frequency band reserved for data traffic. The ADSL modem knows to do this because the traffic is arriving on a