er of magni- tude compared with the desired signal as both travel relatively equal distances to opposite ends. NEXT and FEXT are shown in Figure 9.13. The impacts of both NEXT and FEXT are cumulative. Each two users induce a certain amount of interference on each other's signals as a function of signal power level, proximity, line gauge and insulation, and frequency of operation. As many lines are bundled together in the same physical cable each user signal induces interference on all its neighbours to varying Fixed Broadband Communications Systems 100 pair cable (4 X 25 pair binders) 400 pair cable (4 X 100 pair super binders) Figure 9.12 A typical copper binder Cable Insulated wires Transmitter Far end receiver Transmitter Near end receiver Figure 9.13 FEXT and NEXT degrees. The cumulative crosstalk interference adds a significant amount of noise to the ADSL system, and is a major cause of reduced capacity. Since ADSL and VDSL standards specify the use of different frequency bands for upstream and downstream, interference from upstream will not impact downstream reception significantly. As a result the NEXT process does not contribute much to ADSL performance and capacity cal- culation. The FEXT process on the other hand contributes significantly to the total noise and interference in ADSL/VDSL systems, and is a major reason for capacity degradation. Digital Subscriber Line Capacity Calculations Maximum possible transmission rates of ADSL/VDSL systems, or systems capacity, are sim- ilarly calculated from Shannon's theorem based on available bandwidth and SINR. DSL sys- tems experience noise and interference from a number of sources. Thermal additive white Gaussian noise (AWGN) is generated by electrical devices and has a constant power spectral density. Impulse noise is generated by devices active in the vicinity of communications and have various sources such as electrical switching. Amplitude modulated (AM) radio, DSL Broadband Telecommunications Technologies and Management signals of other operatorstravellin