PhD Thesis based on published articles on different topics I am in 3rd year of PhD and I was suppose to graduate after 1 year, but suddenly due to some unknown reason my adviser gave me 2 months to write my PhD dissertation. He said I have published 7 SCI journals (as a first author) and what I need to do is just compile all articles into single document. 
I know it is common to publish article based on PhD dissertation.  Is it also common to compile a PhD thesis based on published SCI journal Articles? More tricky part is that out of 7 only 2 articles are written on same topic rest of work is totally different. But he insist I can create a link between all articles, which in my opinion doesn't make any sense. It something like saying apple and orange are same fruit.Let say I have no other option except compile my dissertation within 2 months. Is it possible that the PhD dissertation chapters may cover unrelated topics? Let say one part is related to Operating System and other part is related to Future Internet?
First of all, writing a PhD dissertation in 2 months is one hell of a task. You should probably talk to your supervisor about more time.

I believe it is not uncommon, as Thomas stated in the comments, that people assemble a PhD disseration from the publications they had during their time. Many people work of different topics at the same time, but usually there is something that combines them. The title of your dissertation can be very vague and cover a wide topic. In your dissertation you can split it into different chapters of sections according to your covered topics. Of course, it is good if you find a common thread and can build upon that. Even if you don't, I still think it is okay. After all, that seems to be what your supervisor wants.

If you are really unsure, there is no way around talking to your supervisor about it. Ask for his expectations and tell him about your plans. Then, you will be able to meet somewhere in the middle.