Continuous verification of the transmission integrity of a complex broadband network such as the L5 coaxial system requires many repetitive measurements at different locations and considerable processing of the measured data. To minimize the cost and time for these maintenance operations and to provide the best system reliability, a centralized, automated transmission surveillance system has been developed. The measuring and control portion consists of two basic units or facilities. The transmission surveillance center (TSC), located at a large manned main station, originates automatic control operations and accumulates and processes measured data from local and remote locations. The transmission surveillance auxiliary (TSA), located at all other main stations, makes measurements as directed by a controlling TSC and sends the resulting raw data back to the TSC. Typically, a TSC will control up to ten auxiliaries. A four-wire data-transmission system transmits all remote-control commands and remotely measured data in digital form.