An AT&T repair crew works on a cable in Newark, N.J., where earlier crews had pulled an old cable from underground, inadvertently breaking splices in a nearby fiber-optic line. The broken cable disrupted communications in the New York area, halting financial trading and delaying hundreds of air flights all day. The cable, no thicker than a thumb, carries more than 100,000 calls at once over hairlike glass fibers, which transmit the calls in pulses of laser light.