Space is a huge place and even light takes quite a while to get around in it. This means that all radio based communications systems will take over four years to send a message to Alpha Centauri and a further four to get the answer back.
Quite a few stories accept this limit on communications even if they have faster than light starships. In this case either their societies are splintered, with many societies in a relatively small area or there is a heavily centralised society. Rather interestingly, the society in Ursula Le Guin's 'Hainish' series has sub-light starships and a faster than light communications device - the ansible device. Another faster than light device was James Blish's Dirac receiver which had the ability to act as a receiver for all other Dirac devices whenever they were used (and yes, that is correct!).
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