In the PSA model, the prepayment rates are set to be:

Variable according to interest rates

Fixed at $2\%$ annually to year $15$ when it goes to $5\%$ annually

Variable according to the underlying stability of the mortgage pool

First month at $\mathrm{.}2\%,$ increasing monthly by $\mathrm{.}2\%$ until Month $30$ where it plateaus at a constant $6\%$ a year