A method for decreasing upward-joining-noise in an upward-signal-inputting
position of a center device includes the steps of: measuring a noise level
of each of a plurality of upward signals supplied from a plurality of
transmission lines; determining whether the measured noise level of each
upward signal is low enough or not; and based on a determination result,
controlling and adjusting a level of each upward signal. Therefore, the
level of an upward signal, for which the noise level of the upward signal
is determined to be low enough, is adjusted higher but still kept at a
level low enough not to interfere with upward signals from other
transmission lines.
