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.

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.