Ancient network of relay stations spanning the Delta Quadrant, used by the Hirogen, through which Voyager made its first contact with Starfleet in years.
The Hirogen communications network was a chain of relay stations stretching across a large part of the galaxy, built long ago and later used by the Hirogen. Its stations drew power from quantum singularities and could carry data across thousands of light-years.
In Message in a Bottle, Voyager discovered that the network reached the edge of the Alpha Quadrant and sent the Doctor along it as a data stream. He arrived aboard the USS Prometheus, helped retake the stolen prototype from the Romulans, and returned with the news that Starfleet now knew Voyager had survived.
In Hunters, Starfleet sent letters from home through the network, but the Hirogen objected to Voyager using their relays and captured Tuvok and Seven of Nine. Janeway freed them, and the relay station was destroyed in the fighting, cutting off contact once more until Starfleet found other means.