The Grid Explicit Congestion Notification (GECN) protocol is designed for quasi-real-time seamless control of distributed energy resources in active distribution networks, such as smart loads, or energy storage systems. GECN can be used for voltage control and uses the inertia in the system to compensate for the volatility of renewable generation. On each network bus a low bit-rate GECN signal is periodically broadcast. This signal is a number between -1 and 1: the closer it is to -1, the more it encourages consumption, and the closer it is to 1 the more it inhibits consumption. GECN-enabled resources (e.g., smart appliances, super-capacitors) interpret this signal and react to it based on their own internal state. For example, a smart fridge can choose not to react to a signal telling it to switch off if its internal temperature is too high, but if enough fridges are able to follow the signal, the desired behaviour can be achieved. The per-bus GECN signals are computed by a centralized controller based on the most recent estimation of the network state. Specifically, the centralized controller computes desired active and reactive power adjustments in each bus for optimal network operation (e.g., voltage control) and maps them to GECN signals. This mapping is adaptive: if the resources under- or overreact, then the GECN signal is adjusted accordingly. The feedback is thus implicit via the state estimation process.