es are preferred in cases where the sub- scriber wants to connect to his or her enterprise via a VPN or to host a Web server. ADSL allows for simultaneous voice and Internet traffic on the same twisted- pair that used to be a phone line. It reserves the bottom 4KHz of spectrum for the voice traffic, and filters (known as splitters) at each end of the copper pair split the frequency bands. The lower frequencies are sent to the local exchange to switch the voice traffic. The higher frequencies are sent to the DSL modems, and a user is generally connected over a packet-switched backbone to the ISP. Carrierless Amplitude Phase Modulation (CAP) was the de facto modulation scheme for ADSL deployments, used in 90% of the installations, until 1996. How- ever, the DMT modulation scheme was selected for the first ITU-T ADSL stan- dards, G.992.1 (also called G.dmt) and G.992.2 (also called G.lite), and CAP is no longer used. DMT, which is an Orthogonal Frequency Division Multiplexing (OFDM) technique, is a multicarrier scheme in which the spectrum is divided into 256 4KHz carriers. Variable numbers of bits are put on each carrier, and the por- tions of the frequency band that suffer interference from other devices don't have any bits put onto them. The result is improved performance. Compared to CAP, DMT is less prone to interference and can carry data over a longer distance. Figure 12.2 shows an example of an ADSL environment. At the residence, a splitter is splitting off the plain old telephone service (POTS) to the telephone instrument, using the bottom 4KHz of spectrum on the twisted-pair; the remainder of the line is left for the ADSL modem and the data communications. At the top of the figure is a business environment using a DSL line to carry the voice and data traffic, on an integrated basis. In Figure 12.2, numerous DSL lines come in from residential and business pre- mises. Their first point of termination is the DSLAM. The DSLAM is a network device, usually located at the telco local exchange or within a neighbor