g signal to cancel it. Matching the signals in crosstalk refers to a method of exactly replicating these extra- neous signals to eliminate them. Noise from other sources such as AM radio trans- missions can be eliminated by injecting signals into each cable pair that have certain opposite characteristics of the noise. According to Bhavani Rao, product marketing manager at Alcatel-Lucent: Manufacturers have improved methods of detecting and measuring noise. They continually monitor for noise from sources such as crosstalk, electric motors, and lightening storms. It is extremely difficult to distin- guish noise surrounding the signals (ambient noise) from the actual data signals. Improvements in detection and cancellation have resulted from years of work and experience, and are a key reason that DSL is able to support TV, high-speed Internet access, and packetized voice. The other major improvement is DSL's capability to transmit signals over higher frequencies. High frequencies carry more bits in shorter time spans than lower frequen- cies. Think of it in terms of a commuter subway system: If the trains don't come very often (low frequencies), fewer people are moved in a given span of time; if the trains arrive more often (higher frequencies), more people will be moved through the system in the same time period. Copper cabling and wireless services carry signals that made up of wavelengths (frequencies). Low frequencies have longer wavelengths that can carry signals over greater distances. High frequencies have shorter wavelengths that do not travel as far, but they can squeeze more bits into each transmission. Thus, to achieve greater capacity, DSL must use high frequencies. However, the shorter wavelengths render high-frequency signals more vulnerable to damage from environmental sources such as magnetic interference. At the same time, vulnerability to attenuation (weakening and fading) means that high speeds are supported only on short lengths of copper cabling. The terms megahertz and gigahertz express types o