dwidth 0 Hz 4 kHz 25 kHz 1.1 MHz Figure 9.9 ADSL carrier received SINR level = log2 Shannon limit S/N (dB) Figure 9.10 Normalised Shannon limit Broadband Telecommunications Technologies and Management 4-QAM 8-QAM 16-QAM 32-QAM Figure 9.11 Some common xDSL QAM signal constellations shows 4-, 8-, 16- and 32-QAM constellations. A symbol in these constellations carries 2, 3, 4 and 5 bits of data, respectively. Higher order modulations (64-QAM, 128-QAM, and up to 32, 768-QAM) levels which carry 6, 7 and up to 15 bits of data per symbol, respectively, have been specified in the xDSL standards. CrossTalk The performance of twisted pair copper wire systems is affected by interference from other telephone connections. In general a home telephone is individually connected to a telephone exchange. However, all telephone lines from a neighbourhood are bundled together in an underground duct and are further bundled with wires from nearby areas before they reach the exchange as shown in a typical configuration in Figure 9.12. Although these wires are insulated from each other, they still act as electromagnetic antennas transmitting their signal to, and receiving signals from, other wires in the ducts. This interfer- ence is generally known as crosstalk, and exists in two main types: near end crosstalk (NEXT) far end crosstalk (FEXT) NEXT occurs when interfering signals are received at nearby devices. For example, two ADSL modems may be sending and receiving data over their respective lines to/from a CO. The transmitted signal of one ADSL modem may induce interference over the other modem, impacting the received signal at the near end. FEXT occurs when the signal from one modem induces interference in the upstream, and impacts on the signal of another subscriber at a CO. The same may happen in downstream transmissions. In general NEXT power level can be quite high if the path length of the interfering signal travel is shorter compared with the desired signal (coming from an opposite end). The FEXT power level is of the same ord