explain the reaction mechanism $\ce{ROH + NaBr + H2SO4 -&gt; RBr + NaHSO4 + H2O}$

What is exactly the reaction mechanism.

Is it $\ce{NaBr + H2SO4 -&gt; NaHSO4 + HBr}$ and then 

$\ce{HBr}$ reacts with $\ce{ROH}$?

If yes the why in the reaction of $\ce{NaBr}$ and $\ce{H2SO4}$ does $\ce{H2SO4}$ oxidise $\ce{NaBr}$?
HBr reacts with ROH?


Exactly. It's just a nucleophilic substitution.