  Let $G$ be a symplectic group over a nonarchimedean local field of
characteristic zero and odd residual characteristic. Given an irreducible
cuspidal representation of G, we determine its Langlands parameter
(equivalently, its Jordan blocks in the language of Moeglin) in terms of the
local data from which the representation is explicitly constructed, up to a
possible unramified twist in each block of the parameter. We deduce a
Ramification Theorem for $G$, giving a bijection between the set of
endo-parameters for $G$ and the set of restrictions to wild inertia of discrete
Langlands parameters for $G$, compatible with the local Langlands
correspondence. The main tool consists in analysing the intertwining Hecke
algebra of a good cover, in the sense of Bushnell--Kutzko, for parabolic
induction from a cuspidal representation of $G\times\mathrm{GL}_n$, seen as a
maximal Levi subgroup of a bigger symplectic group, in order to determine its
(ir)reducibility; a criterion of Moeglin then relates this to Langlands
parameters.
