使用NSURLSessionConfigurations在ios7中下载多个后台

I want to know about how to download a bunch of files one after the other. It is like if I have 5 files to download, I should automatically download all the 5 files in a sequence 1 complete then 2 , 2 then 3 this way all five should be completed automatically. Its should all done in the background of my app . Thanks in advanceI want to know about how to download a bunch of