  We investigate the scattering and localization properties of edge and bulk
states in a disordered two-dimensional topological insulator when they coexist
at the same fermi energy. Due to edge-bulk backscattering (which is not
prohibited \emph{a priori} by topology or symmetry), Anderson disorder makes
the edge and bulk states localized indistinguishably. Two methods are proposed
to effectively decouple them and to restore robust transport. The first kind of
decouple is from long range disorder, since edge and bulk states are well
separated in $k$ space. The second one is from an edge gating, owing to the
edge nature of edge states in real space. The latter can be used to
electrically tune a system between an Anderson insulator and a topologically
robust conductor, i.e., a realization of a topological transistor.
