  The nonlinear Schr\"odinger (NLS) equation and the Whitham modulation
equations both describe slowly varying, locally periodic nonlinear wavetrains,
albeit in differing amplitude-frequency domains. In this paper, we take
advantage of the overlapping asymptotic regime that applies to both the NLS and
Whitham modulation descriptions in order to develop a universal analytical
description of dispersive shock waves (DSWs) generated in Riemann problems for
a broad class of integrable and non-integrable nonlinear dispersive equations.
The proposed method extends DSW fitting theory that prescribes the motion of a
DSW's edges into the DSW's interior, i.e., this work reveals the DSW structure.
Our approach also provides a natural framework in which to analyze DSW
stability. We consider several representative, physically relevant examples
that illustrate the efficacy of the developed general theory. Comparisons with
direct numerical simulations show that inclusion of higher order terms in the
NLS equation enables a remarkably accurate description of the DSW structure in
a broad region that extends from the harmonic, small amplitude edge.
