merge timestamps to intervals

Hi!

I have a table where I repeatingly log the status of some service, which
looks something like this:

< timestamp, status >

Now, everytime my service is up I save the timestamp and a status of
"up", if it's down I save the timestamp with "down", eg:
10:13 up
10:14 up
10:15 up
10:16 down
10:17 up
10:18 up

I'd like to merge this information to intervals where the service was up
or down.

< intervall, status >
10:13-10:15 up
10:16-10:16 down
10:17-1018 up

I've no clue how to approach this problem.

Any ideas/hints?
Also suggestions on a feasible better schema are welcome. :-)

Thanks
Patrick