Method and apparatus for improving data transmission in a digital subscriber line system. The laser light is generated from an oscillator, passes through an attenuator that includes a filter, and passes through an optical system after passing through the attenuator. The lens is mounted on the base and the bottom surface of the LED chip electrically and mechanically connects with the electrode assembly. When the diaphragm driving does not occur in the second operation mode, the control unit of a stepping motor shuts off the current in the coil at the state where the rotor of the motor and the magnetic poles of the stator face to each other. Threshold values and further factors such as proportionality factors for the controller, for example, are determined depending upon significant characteristics. A selected request is released from a command queue, and further release of requests from the queue is interrupted when a total number of subsequently completed requests reaches a predetermined threshold.