  We study the duality between IIB string theory on a pp-wave background,
arising as a Penrose limit of the AdS_3 times S^3 times M, where M is T^4 (or
K3), and the 2D CFT which is given by the N=(4,4) orbifold (M)^N/S_N, resolved
by a blowing-up mode. After analizying the action of the supercharges on both
sides, we establish a correspondence between the states of the two theories. In
particular and for the T^4 case, we identify both massive and massless
oscillators on the pp-wave, with certain classes of excited states in the
resolved CFT carrying large R-charge n. For the former, the excited states
involve fractional modes of the generators of the N=4 chiral algebra acting on
the Z_n ground states. For the latter, they involve, fractional modes of the
U(1)^4_L times U(1)^4_R super-current algebra acting on the Z_n ground states.
By using conformal perturbation theory we compute the leading order correction
to the conformal dimensions of the first class of states, due to the presence
of the blowing up mode. We find agreement, to this order, with the
corresponding spectrum of massive oscillators on the pp-wave. We also discuss
the issue of higher order corrections.
