  Aims: The aim of this work is to study the structure of the protoplanetary
disk surrounding the Herbig Ae star HD 163296. Methods: We have used
high-resolution and high-sensitivity ALMA observations of the CO(3-2) emission
line and the continuum at 850 microns, as well as the 3- dimensional radiative
transfer code MCFOST to model the data presented in this work. Results: The
CO(3-2) emission unveils for the first time at sub-millimeter frequencies the
vertical structure details of a gaseous disk in Keplerian rotation, showing the
back- and the front-side of a flared disk. Continuum emission at 850 microns
reveals a compact dust disk with a 240 AU outer radius and a surface brightness
profile that shows a very steep decline at radius larger than 125 AU. The
gaseous disk is more than two times larger than the dust disk, with a similar
critical radius but with a shallower radial profile. Radiative transfer models
of the continuum data confirms the need for a sharp outer edge to the dust
disk. The models for the CO(3-2) channel map require the disk to be slightly
more geometrically thick than previous models suggested, and that the
temperature at which CO gas becomes depleted (frozen-out) from the outer
regions of the disk midplane is T < 20 K, in agreement with previous studies.
