Today, these have been replaced with optical glass fibres. Light is sent into one end of a glass or plastic cable at an angle large enough so that it will not be able to pass out of the cable into the surrounding air. The message is sent by light pulses and is more efficient and reliable. Since fiber can't transmit power, however, power has to be provided by the consumer instead of the telephone company. This leaves customers vulnerable to telephone outages during power outages.