  We present {\it Chandra} HETG observations of SS Cygni in quiescence and
outburst. The spectra are characterized by He-like and H-like Ka emission lines
from O to Fe, as well as L-shell emission lines from Fe. In quiescence, the
spectra are dominated by the H-like Ka lines, whereas in outburst the He-like
lines are as intense as the H-like lines. In outburst, the H-like Ka lines from
O to Si are broad, with widths of 4--14 eV in Gaussian $\sigma$ (1800--2300$
{\rm km s^{-1}}$). The large line widths, together with line profiles, indicate
that the line-emitting plasma is associated with the Keplerian disk and still
retains the azimuthal bulk motion. In quiescence, the emission lines are
narrower, with a Gaussian $\sigma$ of 1--3 eV (420--620$ {\rm km s^{-1}}$). A
slightly larger velocity for lighter elements suggests that the lines in
quiescence are emitted from an ionizing plasma at the entrance of the boundary
layer, where the bulk motion of the optically thick accretion disk is converted
into heat due to friction. Using the line intensity ratio of He-like and H-like
Ka lines for each element, we have also investigated the temperature
distribution in the boundary layer both in quiescence and outburst. The
distribution of SS Cyg is found to be consistent with other dwarf novae
investigated systematically with {\it ASCA} data.
