SL2, VDSL2-30MHz Short Reach (SR-VDSL2-30MHz; the highest-performance DSL standard), it is possible to achieve 100Mbps in both the downstream and upstream directions, but the maxi- mum distance is only 0.3 mile (0.5 km). A number of factors can affect the loop condition for a subscriber, including the following: Attenuation-Attenuation is signal loss, and it is a function of frequency. As the frequency increases, the distance the signal can travel decreases, by the square root of the frequency. So higher frequencies lose power more rapidly, thereby limiting the loop length; it is necessary to use the full- frequency spectrum available on the twisted-pair to carry the promised data rates and hence, the higher frequencies must be used for DSL. Resistance-As signals are transmitted through wires at very high frequen- cies, a phenomenon called the skin effect occurs. As electricity migrates to the medium's skin, resistance increases because less of the wire is used. This increased resistance weakens the signals. Because of the skin effect, there are currently no services above 1GHz over wired media. Crosstalk-When two adjacent wires carry signals, signals from one wire might be able to enter the other wire as a result of electromagnetic radia- tion; this is called crosstalk. Crosstalk increases as frequency increases and is a principal cause of signal degradation at the frequencies required by high-speed services. This can affect how many pairs within a cable can be used to deliver DSL service. The spectral compatibility of two DSL systems depends on the effect of crosstalk that one DSL system has on another inside a single cable bundle. Chapter 12 Broadband Access Alternatives Phase error-Phase error introduces bit errors where modulation tech- niques depend on phase modulation. Loaded pairs-Loaded pairs, which means that loading coils are placed on twisted-pair for purposes of improving performance over 3.5 miles (5.5 km), cannot be used for DSL. A coil is an inductor that acts like a low- pass filter, filtering ou