e 6.19 Bins and their usage in ADSL. Data Communications because every sixty-ninth symbol carries synchronization information instead of user data). The DMT equipment determines the S/N for each of the bins separately and, based on the results, allocates the information bits to be sent to each tone or bin. Then the bins with better S/N transmit more bits per symbol than bins with worse S/N, as shown in Figure 6.20. The ADSL access system is set to operate at a data rate that the cus- tomer has ordered by setting bins to be used and the average number of bits transmitted in each symbol (in each bin). The system may automatically, according to Figure 6.20, adapt its operation into line conditions. At the time ADSL was specified, asynchronous transfer mode (ATM), which is introduced in the end of this chapter, was expected to be the major backbone network technology. To transmit data efficiently end-to-end ADSL was defined to split data into ATM cells for transmission over the sub- scriber line. Now when most of the traffic is IP packets, the IP packets are rebuilt at the other end from ATM cells transmitted over ADSL. One of the major problems with ADSL is that installation to the cus- tomer premises often requires the network operator's maintenance personnel to visit the site. To make installation SO easy that a customer can manage it himself, a light version of ADSL, known as G.Lite or ADSL Lite, was devel- oped by the ITU. It is does not require a filter in the customer premises, its maximum downstream data rate is 1.5 Mbps, and its maximum upstream direction is 640 Kbps. 6.4.3.6 Symmetric DSL The SDSL system transmits the data in both directions just as HDSL but it uses a single pair. Because both transmission directions operate at a high data rate, the near-end crosstalk is higher and the data rate lower than in ADSL Frequency, tone index Figure 6.20 Bit allocation to tones or bins. Introduction to Telecommunications Network Engineering (see Table 6.1). ITU's standard G.shdsl contains an integrated 64- Kbps vo