Fixed Income Questions

Was revising fixed income and the following 2 questions came to mind: 1. For a PAC tranche, how exactly do you determine the lower and upper limits of an effective collar for a seasoned tranche? 2. For synthetic CDOs, the 2 sections are called “senior” and “junior” sections because only the junior section is financed with debt, and thus notes are issued on the junior section. The junior section thus receives a return from both the CDS premium and the high quality collateral that is invested. My question is, who holds the senior section and are notes issued on them? If so, what is the return earned by the senior section? Thanks!!