In the early days of protective relaying, it was recognized that communications between substations could improve relaying performance. Schemes such as power line telephony (1920s) and pilot wire relaying (1930s) evolved into digital time division multiplexers (TDMs) (invented in 1960s and used for power system communications in 1970s). TDM allowed audio teleprotection, audio-based relaying, SCADA, and telephony to share a single link …