This isn't entirely hard science but bear with me.
(Prologue) In the near future, a series of increasing heavy elements are synthesized building on the theory of islands of stability, incredibly strong compounds are made and all of the engineering benefits that come with them. All science progresses in turn, with various advancements made, allowing for fast, 0.8c-ish interstellar travel and so forth. This is then supplemented with the largest advancement made so far, atoms of which where thought to be impossible are made, elements with more neutrons and protons than 174 each are synthesized. A strange phenomenon occurs, where quantum waveforms of each atom result in waveforms which stretch out in such a way that they result in FTL transmission of information. Time progresses from then on, and it turns out that that's the only way to send information FTL, even with thousands of years of scientific advancement during which FTL transmission of information is possible, and so all FTL transmission lines have to be physical. Of course if you could lay these lines then interstellar civilization would be truly possible. Which begs the question, how do you lay light years worth of cable?
TLDR: FTL transmissions have to be over fantasy "fiber optic" (actually metal / elements with atom number > 174) cables, and so even with relativistic spacecraft, I'm not entirely certain how you could haul a cable that extends multiple light years. Even if you stagger it with segments and transmitters, that's still a lot of cable over a very long distance.
The cables have a radius of half a millimeter, and weight a tenth of a kg for every kilometer of distance covered. The cable has a relatively hight tensile strength, being made of ultra dense materials, let's say about 1300 gigapascals. Beyond this, the cable snaps, that being said, it can't take an extended beating from a relativistic medium, it is able to survive for a few minutes at 0.8 c, and conversely proportional for the relative energy levels of the velocity in turn, after which is ensuingly useless.
The spaceships in concern use exotic particle interactions to propel spaceships up to 0.8 c, using the production a generic dark matter particle at high energies for reaction mass so the exhaust wont toast the cable. The cable for example is being laid between two stars 3 light years apart.
If more technical information is needed I'll be happy to add it to the question.
The cables, within the solar system would be 100 to 200 au out, as to minimize gravitational forces experienced.
