A clock transmission apparatus for network synchronization between systems wherein a communication system having a main unit and a remote unit, the main unit generates an even-second pulse and transmits it to the remote unit and the remote unit uses the received PP2S to generate a system clock and a 10 MHz clock, so that network synchronization is realized between two systems. The method includes determining a current data transmission rate capacity of a wireless channel; and controlling a data transmission rate of a wireless transmission device based on the current data transmission rate capacity. The changeover valve is normally kept in a closing position and will be shifted to an opening position at the time of simultaneous activation of a travel selection switch and a boom release switch. The protrusions of the engaging arms are disengaged from the engaging groove in the electrical terminal by moving the free ends of the engaging arms toward the inner surface of the sleeve body using a tool.