Echo 2 being deployed

A subspace amplifier or buoy was a communications array designed to allow real-time subspace communication.

A series of subspace amplifiers were deployed by the Earth starship Enterprise NX-01 during its initial exploratory mission. The amplifiers were deployed to allow better communication with Earth and United Earth Starfleet Command. The amplifiers were designated by letter-number combinations. The first such device, Echo 1, was deployed in August of 2151, with Echo 2 being deployed one month later. (ENT: "Fortunate Son", "Silent Enemy")

In September of 2151, an unknown alien vessel destroyed both Echo 1 and 2 during a sustained attack on Enterprise. Following the successful elimination of the alien threat, a third amplifier was dropped, to replace the lost two. (ENT: "Silent Enemy")

To distract the automated repair station after it made repairs to Enterprise, Commander Trip Tucker claimed that the subspace amplifiers aboard Enterprise only picked up distortion on all the high-band frequencies following the repair work. (ENT: "Dead Stop")

Starfleet Command Council held contact via subspace buoys with Enterprise while the ship was on its mission in the Delphic Expanse. (ENT: "Twilight")

Although referenced in multiple episodes, a subspace amplifier appears only in "Silent Enemy".
In the first draft script of "Fortunate Son", the device was termed a "subspace relay" instead. In the script for "Silent Enemy", the amplifier was described thus; "It looks like a 22nd century version of a communications satellite."
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