An ansible is a variation of an Alcubierre drive, used to accelerate an energy wave to speeds faster than light. It can also ‘receive’ such accelerated waves, making it possible to establish entire FTL communications networks. However, due to their energy requirements, bandwidth in such a network is usually strictly limited.
Since ansibles are subject to the same interference from gravity as Alcubierre drives, they are usually found in starbases or on satellites placed in orbit at a Lagrange point near major inhabited planets. They are also sometimes installed in capital starships, especially those used by prominent war commanders or clan leaders.