iOS Periodic Sync with Web Service

Background: I have already written an Android application that periodically sy开发者_JS百科ncs a database with a web service (using the abstract threaded sync adapter in the framework).

Question: I've read lots of discussions on how an iPhone application can fetch data from a web service but I haven't found a good answer as to how an iPhone application can periodically fetch data from a web service.

I've read about NSTimer but I don't see how I can start this when the phone starts (i.e. before my app is started).

Yes, I know I should probably use Apple Push Notification but my app (and the associated web service) may well be deployed inside a Corporate firewall and have no access to the outside world) so I need to use "pull" mechanisms.

I'm a complete n00b when it comes to iOS development so please be gentle with me.

Can someone please point me in the right direction?