Cranial transceiver
The cranial transceiver was an interplexing beacon implanted within the braincase of every Borg drone. Each transceiver contained a unique translink frequency that allowed the drone to stay in contact with the collective. ()

In an alternate version of the year 2390 in which  crashed after a quantum slipstream flight, The Doctor was able to extract Seven of Nine's cranial transceiver and determine its translink frequency. This allowed Harry Kim and Chakotay to contact Seven in the past and avert Voyagers destruction. Seven's translink frequency was 108.44236000. ()

Borg components