i’m not familiar with the USD window, and the terms you use. But maybe what i’m going to tell you will answer your question :
when a comm is located on a certain AterMux, it doen’t mean the signalling will go through the SS7 which is n the same Ater Mux.
Even more complicated : Most of the time, for one communication, the signalling BSC-MSC is on one SS7 TS, and the signalling MSC-BSC is on another S7 TS 🙂
It’s totally dynamic and “random”.