298、Computer networks also use flexible glass fibers to transmit data.Known as an optical fiber, the medium uses light to transport data, The miniature glass fiber is encased in a plastic jacket which allows the fiber to bend without breaking.A transmitter at one end of a fiber uses a light emitting diode ( ) or a laser to send pulses of light down the fiber.A receiver at the other end uses a light sensitive transistor to detect the pulses. Optical fibers have four main advantages over wires.First, because they use light, optical fibers neither cause electrical interference in other cables nor are they susceptible to electrical interference.Second, because glass fibers cab be manufactured to reflect most of the light inward, a fiber can carry a pulse of light much farther than a copper wire can carry a signal.Third, because light can encode more information than electrical signals, an optical fiber can carry more information than a wire.Fourth, unlike electricity, which always requires a pair of wires connected into a complete circuit, light can travel from one computer to another over a single fiber. Despite their advantages, optical fibers do have some disadvantages.First, installing a fiber requires special equipment that polishes the ends to allow light to pass through.Second, if a fiber breaks inside the plastic jacket ( ), finding the location of the problem is difficult.Third, repairing a broken fiber is difficult because special equipment is needed to join two fibers so that light can pass through the joint.(5)、What is the central idea of this short essay?