Here’s an Underlay tunnel interface card (don’t call it a UTI card😆)

The word Ansible refers to a chassis that contains one or more of these cards, serving to power them as well as bridge the underlay tunnel with a conventional realspace network. The cards, specifically the magenta wafer in the middle, are what do the actual FTL communicating, and the other end of the tunnel has to have a wafer of tailstone shaved from the same monocrystal.

Claravian womb ships, like the Dewfall that discovers Earth, have one ansible chassis with at least two cards, one in production and another serving as a hot spare. These cards have counterparts at a mission control station that monitors the ships systems while the crew is in metabolic suspension.

Both the womb ship and mission control have raw pieces of the same monocrystal used in the cards in case both cards fail and they need to manufacture new ones. These crystals are the single most precious objects to the mission. If either mission control or the womb ship loses their crystal, the ship is flying blind while the crew is in metabolic suspension.

Much of the mystery surrounding Firefly the Apostate is thanks to just such a communication failure. The mission control team was attacked, their ansible was destroyed, and the monocrystal was stolen. When contact was re-established two of the three missionaries had died

I describe ansibles further in this story