  This article proposes a software defined space-air-ground integrated network
architecture for supporting diverse vehicular services in a seamless,
efficient, and cost-effective manner. Firstly, the motivations and challenges
for integration of space-air-ground networks are reviewed. Secondly, a software
defined network architecture with a layered structure is presented. To protect
the legacy services in satellite, aerial, and territorial segments, resources
in each segment are sliced through network slicing to achieve service
isolation. Then, available resources are put into a common and dynamic
space-air-ground resource pool, which is managed by hierarchical controllers to
accommodate vehicular services. Finally, a case study is carried out, followed
by discussion on some open research topics.
