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The mixed media network including the terrestrial and the satellite communication links is considered, aiming at the realization of a highly reliable information network. This paper proposes a method of physical level switching control in the high- speed multiplexer between the terrestrial and the satellite links. The following three aspects of the physical level switching control method are described. The high-speed digital network management system instructs the link switching to the high-speed multiplexer according to detected faults. The high-speed multiplexer has a function to handle equally the high-speed digital link and the satellite link. The Earth station has the interface function which enables the high-speed multiplexer to switch the master clock. This paper outlines the switching control method at the physical level using the high-speed multiplexer in the transmission network. It discusses also the technical issues as well as their solutions in the implementation, such as the loss of paths for managing information, management of intermittent faults and switching of the master clock. The link switching experiments actually are made between the earth and the satellite in the "pilot study for use of satellite," underwritten by the Mm. Post & Telecomm. The time from the fault detection to the complete link switching in the proposed network management system is within 30 s, indicating the effectiveness of the proposed switching control method. [ABSTRACT FROM AUTHOR]