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The downstream channels occupy a band of frequencies from approximately 50 MHz to 1 GHz, while the upstream channels occupy frequencies of 5 to 42 MHz. While the price of generating capacity is high, energy demand is variable, making it often cheaper to import needed power than to generate it locally. Electric power transmission is the bulk movement of electrical energy from a generating site, such as a power plant, to an electrical substation. This restricted the distance between generating plant and loads. This is an everyday occurrence in AC systems, but one that can become disrupted when AC system components fail and place unexpected loads on the grid. Due to widespread cable theft in earlier analog systems, the signals are typically encrypted on modern digital cable systems, and the set-top box must be activated by an activation code sent by the cable company before it will function, which is only sent after the subscriber signs up. For large conductors (more than a few centimetres in diameter), much of the current flow is concentrated near the surface due to the skin effect.

Thus, generators were sited near their loads, a practice that later became known as distributed generation using large numbers of small generators. The transmission system usually does not have a large buffering capability to match loads with generation. Different classes of loads (for example, lighting, fixed motors, and traction/railway systems) required different voltages, and so used different generators and circuits. Lower voltages, such as 66 kV and 33 kV, are usually considered subtransmission voltages, but are occasionally used on long lines with light loads. As power systems evolved, voltages formerly used for transmission were used for subtransmission, and subtransmission voltages became distribution voltages. At the optical node, the optical signal is translated back into an electrical signal and carried by coaxial cable distribution lines on utility poles, from which cables branch out to a series of signal amplifiers and line extenders. Transmission companies determine the maximum reliable capacity of each line (ordinarily less than its physical or thermal limit) to ensure that spare capacity is available in the event of a failure in another part of the network.

Oscillatory motion of the physical line is termed conductor gallop or flutter depending on the frequency and amplitude of oscillation. By giving each channel a different frequency slot on the cable, the separate television signals do not interfere with each other. Historically, transmission and distribution lines were often owned by the same company, but starting in the 1990s, what is electric cable many countries liberalized the regulation of the electricity market in ways that led to separate companies handling transmission and distribution. HVDC links stabilize power distribution networks where sudden new loads, or blackouts, in one part of a network might otherwise result in synchronization problems and cascading failures. High-voltage direct current (HVDC) technology is also used in submarine power cables (typically longer than 30 miles (50 km)), and in the interchange of power between grids that are not mutually synchronized. The center of the conductor carries little current but contributes weight and cost. Underground power transmission has a significantly higher installation cost and greater operational limitations, but lowers maintenance costs. Thus, reducing the current by a factor of two lowers the energy lost to conductor resistance by a factor of four for any given size of conductor.

Transmission lines use either alternating current (AC) or direct current (DC). The system proved the feasibility of AC electric power transmission over long distances. DC technology is used for greater efficiency over longer distances, typically hundreds of miles. Voltage was stepped down to 100 volts using the Stanley transformer to power incandescent lamps at 23 businesses over 4,000 feet (1,200 m). It was powered by a 2 kV, 130 Hz Siemens & Halske alternator and featured several Gaulard transformers with primary windings connected in series, which fed incandescent lamps. It was powered by two Siemens & Halske alternators rated 30 hp (22 kW), 2 kV at 120 Hz and used 19 km of cables and 200 parallel-connected 2 kV to 20 V step-down transformers provided with a closed magnetic circuit, one for each lamp. The voltage level is changed with transformers. In 1882, DC voltage could not easily be increased for long-distance transmission. Costs can be prohibitive for transmission lines, but high capacity, long distance super grid transmission network costs could be recovered with modest usage fees. Efficient long-distance transmission of electric power requires high voltages.