early 20th century military photograph, need help

[Dave]

Hello,

I have attempted to find out what I could about this picture on my own, with extremely limited success. This is a scan of a picture of my Grandfather (pictured sitting on the left side in the white tunic). All I am sure of is he was in the Tsar's army, and this picture was taken just prior to the Bolshevik revolution I am told. Who can tell me anything about what is in this picture? What are the ranks of those pictured, type of military unit, etc...

Any insight would be greatly appreciated.