The Hirogen sensor network or relay network was an ancient network of relay stations that covered an area spanning from the Delta Quadrant to the outer edges of the Alpha Quadrant. It was claimed by the Hirogen, who were assumed to use it for communication.
The station explored by Voyager was powered by a small quantum singularity, estimated to be about a centimeter across, but outputting almost four terawatts of energy and extending a gravimetric field two light years out from the station, and gravimetric eddies closer in. It put out as much energy in a minute as a typical star did in a year. The singularity was believed to have been harvested, and was kept in check by a containment field, without which the station would implode. However, this station appeared to be malfunctioning, as its containment field was unstable. (VOY: "Hunters")
The sensor network was first encountered by the USS Voyager in 2374. It promptly managed to use it to locate the USS Prometheus, a Starfleet vessel on the edge of the Alpha Quadrant over 60,000 light years away, and attempted to communicate with it. However, the message quickly became degraded, and bounced back to Voyager. It was determined that a holographic data stream would not suffer degradation to the same extent, and so, The Doctor, Voyager's EMH, was transferred to the Prometheus to make contact.
When the Hirogen, operators and controllers of the network, discovered Voyager's use of their technology, they attempted to cut off the ship's connection with the network. When negotiating failed, Seven of Nine sent a feedback surge along the communications link, incapacitating the Hirogen commander.
Some time later, Starfleet attempted to use the network to contact Voyager directly. However, the same problems with communications caused a large portion of the data transfer, containing messages and letters from the crew's families, to become lodged in a relay station near to Voyager's position. While attempting to retrieve the data, Voyager came under attack from a Hirogen hunting party, and was forced to use an antithoron burst against the relay station, causing the quantum singularity within to be exposed, destroying the Hirogen vessel, along with the station and the remaining messages. The energy released in the process spread to every relay in the network, disabling them. (VOY: "Hunters")