The only reason I can think of is perhaps you are someone like me, someone who absolutely loves and adores retro-telecom technologies and old telco gear of all kinds, and wishes to keep the real Siemens BSC and its TRAU as a matter of philosophical principle. If this situation indeed holds, please let me know! I have to explicitly ask this question because motivations like mine are extremely rare in this community, most other participants around here seem to approach from a commercial PoV, and their mindset tends to be totally different from mine.
Yes, you are right, this is the motivation. I used to work with this kind of equipment and I would like to see it working again. Since we have here at the university Mobile Communications courses, I am also interested in the didactic aspect of this project.
In any case, I am still interested in hearing what is your motivation for getting the Siemens BSC (with TRAU) working with Osmocom CNI, and not just the BTS part - if your motivation is akin to mine (philosophically principled retronetworking), then maybe we can be friends!
It seems that we have the same motivation, so indeed we can be friends and work on this nice project!
Sincerely, Dan.