The Silent Line was a series of defensive emplacements located on the surface with the assistance of satellite array to prevent access to the second Layered. Although the defensive line was not formally named, it gained the nickname after all of the (earlier) survey teams sent to these areas lost contact after crossing an unmarked border; thus, the "silent line" moniker was born.
All of the Silent Line's defenses were controlled by the IBIS, a counterpart to the Controller (DOVE). During Silent Line: Armored Core, The Raven breached the defensive line, encountered multiple Big Whites and a weapons satellite. The line is rendered inert with the destruction of the IBIS.
-In the opening movie, the plane that was shot down is most likely the one at the entrance to the IBIS core.