Adopted by the DoD as the U.S. military's primary protocol for imagery and signals intelligence communications, Common Data Link (CDL) has been the primary secure communications protocol for the U.S. military since 1991. The complexities of securing these communications, however, resulted in early generations of CDL radios being very large and heavy – around 300 pounds – which was fine for a large aircraft, but had a negative SWaP impact on lighter aircraft, including being unworkable on most unmanned aerial vehicles (UAVs). One result of this has been that a lot of CDL engineering in recent years has focused on shrinking the size and weight of the radios, even as its capabilities grew.