  Zelevinsky's classification theory of discrete series of $p$-adic general
linear groups has been well known. M{\oe}glin and Tadic gave the same kind of
theory for $p$-adic classical groups, which is more complicated due to the
occurrence of nontrivial structure of L-packets. Nonetheless, their work is
independent of the endoscopic classification theory of Arthur (also Mok in the
unitary case), which concerns the structure of L-packets in these cases. So our
goal in this paper is to make more explicit the connection between these two
very different types of theories. To do so, we reprove the results of
M{\oe}glin and Tadic in the case of quasisplit symplectic groups and orthogonal
groups by using Arthur's theory.
