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.
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.
- Kirk: Captain to bridge.
- Uhura: This is Uhura.
- Kirk: What's our position?
- Uhura: Course unchanged. Estimating treaty boundary in 21 minutes.
- Kirk: Are you broadcasting tactical reports?
- Uhura: Affirmative.
- Kirk: At this distance?
- Uhura: Approximately three hours before receiving a reply to our first message.
- -- TOS "Balance of Terror"
Subspace radio is subject to many kinds of interference, both natural and artificial.