Sessionizing Web Log Data using MySQL (by simulating window functions)

I have a large MySQL Table containing about 15 Million rows of web log data. Depending on the IP, useragent and site_id (different sections on the website) I want to create user sessions. The session delimiter should be 30 minutes. So if a unique user (same IP and useragent) is inactive for 30 minutes or if he visits a different site area (2, 3, or 4) a new session starts.

I found a very good blog entry, that describes the general procedure: http://ift.tt/1BYqxB3 But I can't implement this 1:1 because MySQL doesn't support window functions and I'm not proficient enough with MySQL to figure it out on my own, e.g. with GROUP_CONCAT().

Here is anonymized example data on sqlfiddle: http://ift.tt/1SsknzR

It would be really great if you could help me.