An out of band data link for performance monitoring information using existing telecommunications transmission facilities. The Bell telephone network includes a variety of central offices and network interface units that should communicate with each other regarding performance. The data link allows the network interface unit to advise the telephone company's central office of difficulties. A first modem is interconnected between a pair of simplex leads in the central office telephone company's central office repeater transformers, and a second modem is interconnected between a pair of simplex leads in the network interface unit's transformers. The voltage between each pair of leads is used to effect communication between the telephone company's central office and the network interface unit. Accordingly existing cables can continued to be dedicated to the transmission of payload, rather than requiring the capacity of the cables to be reduced so that the network interface unit can transmit performance data to the central office.