The 100a protection switching system is intended to ensure continuity of service in TD Microwave Radio Relay Systems by transferring service from a failed working channel to a standby protection channel. Switching is done at IF on a switching section basis. A switching section may contain up to ten radio repeater stations, with the average section containing three to four repeaters. A fully equipped 100A system provides means to replace any one of ten regular channels with either of two protection channels. Two regular channels may be switched simultaneously to the two protection channels. The circuit interruption due to a fade initiated or a maintenance switch is less than 10 microseconds. The interruption due to an equipment failure is less than 35 milliseconds, a time short enough to prevent false operations in the telephone switching plant. The control information necessary to sequence the operation of the switches at the transmitting and receiving ends of the switching section consists of coded voice-frequency tones. These may be transmitted over any reliable voice-frequency facility having suitable transmission and delay characteristics. All of the active devices in the system are solid state. The system requires sources of both +24 and −24 volt power.