Subspace radio

Subspace radio is a long-range communications system used by most civilizations in Star Trek. Transmissions through subspace travel faster than light, allowing communications to occur over distances of hundreds or thousands of light-years.

Limitations

Subspace radio transmissions do not travel fast enough to allow real-time communications over distances of more than a few light-years; it is not unusual for a starship or outpost on the outskirts of the Federation to have to wait hours or even days to receive a reply to a transmission that it sent.

Subspace radio is subject to many kinds of interference, both natural and artificial.