Wormhole relay station in action

subspace relay

A subspace relay was a subspace device designed to receive and re-transmit subspace communications signals to other relays in a larger subspace communications network. Subspace relays might be installed in starships, on planetary surfaces, or on dedicated Subspace relay stations. (TNG: "Ensign Ro"; DS9: "The Search, Part I")

Organizations and species including the the Cardassians, the Dominion and the Federation used subspace relays as part of networks for communication purposes. (DS9: "Return to Grace", "The Search, Part I",TNG: "Conspiracy", okudagram)

Signals sent in the subspace realm traveled faster-than-light (FTL). Over vast distances, such signals eventually emerged into normal space, dropping to very slow light speed, which made interstellar communications very nearly impossible. Subspace relays solved this problem by receiving FTL subspace communications before they emerged into normal space and retransmitted them in subspace at higher energies, allowing the signal to travel further in subspace, and to arrive at its final destination much faster, than it could without the relay.(citation needededit)

Federation starships, themselves equipped with subspace transmitters, relied on a network of subspace relays to boost inter-starship communication over the vast distances of interstellar space. (TNG: "Legacy")

In 2369, the Bajoran engineer Neela activated a short range subspace relay she previously placed in the security office aboard Deep Space 9. Combined with her creation, the program ANA, this was designed to help her escape after she assassinated Vedek Bareil Antos, after deactivating the promenade's weapon detectors. (DS9: "In the Hands of the Prophets")

In the 2370s, the Dominion had an unmanned subspace relay on Callinon VII which the Karemma were told by the Vorta to send messages to. The USS Defiant transported Jadzia Dax and Miles O'Brien to the subspace relay in 2371 to see where the messages were relayed to. They were detected and a shield was activated around the relay. (DS9: "The Search, Part I")