of 4.3125 kHz over which a 4.0 kilo symbol per second transmission is transmitted. Each subcarrier is individually evaluated for its signal to interference plus noise ratio (SINR) to decide the multiple QAM modulation level. Channel Broadband Telecommunications Technologies and Management Narrowband Modulator Narrowband modulator Narrowband modulator Frequency Figure 9.5 Data transmission using subcarriers Frequency domain Time domain Symbols Digital to Transmit input analogue Figure 9.6 An inverse discrete Fourier transform (IDFT) DMT transmitter Frequency domain Time domain Symbols Analogue to Receive output digital Figure 9.7 A DFT DMT receiver capacity is calculated by the number of subcarriers allocated to the upstream and downstream as well as the system noise and distortions from other wires in the same cable bundle (cross- talk) as a function of the length of the medium (Figure 9.8). As shown in Figure 9.2, twisted pair copper wire has a low pass frequency response, that is, the lower frequencies experience a lower path loss compared with higher frequencies. It follows that signals transmitted at lower frequencies will arrive at a higher signal power level at the receiver compared with higher frequencies and therefore with a higher SINR as illustrated in Figure 9.9. Shannon's theorem shows that channels with higher SINR can support higher transmission rates, that is, more bits per second can be transported for each hertz of spectrum as shown in Figure 9.10 (note the new term I in the denominator, representing interference power.) This means that lower frequency ADSL carriers can support higher transmission rates. Multi-level QAM techniques are employed to take advantage of good channel conditions. Figure 9.11 Fixed Broadband Communications Systems Used for DSL Used for DSL upstream downstream Voice Guard DSL bandwidth Frequency 0 Hz 4 kHz 25 kHz 1.1 MHz Figure 9.8 ADSL frequency band allocation with DMT modulation Used for DSL upstream Used for DSL downstream Voice Guard Channel frequency response DSL ban