ne code that is used in ISDN basic rate subscriber lines for 160- Kbps bidirectional transmission and each symbol carries two bits of informa- tion. That reduces the symbol rate on the line to half of the binary rate and the lower transmission rate decreases attenuation and crosstalk. Taken together, these developments double the transmission distance compared to the distance of conventional systems. The HDSL system transmits the same data rate to both directions just as conventional 1.5/2-Mbps copper cable transmission systems. It will replace them in other applications in the telecommunications network, such as in ISDN PRI connections, because it requires fewer intermediate repeat- ers, which reduces costs. HDSL is not a consumer access technology because it is symmetrical, uses two pairs, and does not allow a voice-band telephone connection to coexist in the same subscriber loop. Data Communications 6.4.3.5 Asymmetrical DSL and G.Lite symmetrical connection has the same capacity in both transmission direc- tions. The conventional T1 and E1 (1.5- and 2-Mbps) transmission systems and HDSL systems are symmetrical in this sense. However, many applica- tions do not require as much capacity from a subscriber to the network as from the network to a subscriber. One example of this type of application is video-on-demand (VoD), which transmits one video program to a subscriber via an ordinary telephone subscriber pair. A subscriber needs only a narrow- band channel to the network that enables her to select and control the video program. ADSL was originally developed for VoD. This service has not been successful, but ADSL has ideal characteristics for residential Internet users. ADSL uses a single pair and transmits downstream at a high data rate and at a lower data rate in the upstream direction. Figure 6.17 shows how the ADSL technique is used for VoD service. In this application the downstream video channel capacity is 1.5 or 2 Mbps, the upstream control channel is 16 or 64 Kbps, and an ordinary telephone call is poss