ible over the same sub- scriber line simultaneously. A downstream data rate of 1.5 or 2 Mbps can be used over 6-km-long subscriber loops. ADSL terminals modulate the video signal and control signal to a higher frequency band that the telephone or ISDN basic rate signal does not use as shown in Figure 6.18. As an example the frequency band up to 410 kHz is in use and the transmission distance is restricted to approximately 5 or 6 km in the case of a 1.5- or 2-Mbps data rate. Figure 6.18 shows the spectrum allocation used in some VoD field trials. A standardized ADSL G.dmt technology supports downstream data rates up to 8.1 Mbps at a 3-km distance as shown in Table 6.1. G.dmt ADSL ADSL, Asymmetrical digital subscriber line Subscriber line ISDN or telephone Telephone channel exchange DSLAM terminal Video channel Lowpass Control channel Television filter Splitter, lowpass filter Video server Figure 6.17 Video-on-demand and ADSL. Introduction to Telecommunications Network Engineering Upstream data Downstream data, 1.5 Mbps video Frequency Figure 6.18 Spectrum of 1.5-Mbps ADSL use in VoD. uses discrete multitone (DMT) modulation in which the entire frequency band is divided into 4.3125-kHz-wide subbands, called bins. Bins are num- bered from 0 to 256, and the upper cutoff frequency of each bin is given as X 4.3125kHz, where k is the bin index. Then the upper cutoff frequency of the ADSL band is 256 X 4.3125 kHz = 1.104 MHz. Figure 6.19 shows allocation of the bins when an ordinary telephone is used over the same sub- scriber loop simultaneously. If ISDN is used over the same subscriber loop, the lowest bins of upstream data are not used. DMT ADSL uses a fixed symbol rate, that is, each bin transmits a sym- bol for a fixed length and then all of them simultaneously change to the next symbol. The symbol rate is 4,000 bauds and each symbol carries 0 to 11 bits of information using QAM (actual symbol rate on line is slightly higher Guard band Not used Upstream Downstream data Pilot tone Frequency Frequency Bin number Figur