Abstract: In a fiber-optical WDM-transmission system channel switching is used to achieve a high transmission capability and/or quality in spite of the polarization mode dispersion experienced by pulses propagating along an optical fiber. The WDM-channels having best transmission properties among the available WDM-channels are used to transmit high-priority traffic, the remaining channels being used for e.g. low-priority traffic. The high- and low-priority information arrives on input lines and is switched in a cross-connect element coupling the high-priority information to the best channels and the other information to the other channels, the channels being combined in a multiplexer to be injected in the transmission fiber.