The latest iOS update is aimed at fixing mobile performance issues with the iPhone 4S.
Since the release of iOS 6.1, some iPhone users have experienced problems with 3G cellular performance. Voice, data and SMS operation can all be affected.
Some European carriers advised their customers to avoid the update from iOS 6.0 to 6.1.
iOS 6.1.1 has now been released, which Apple says "fixes an issue that could impact cellular performance and reliability for iPhone 4S."
The update appears to be specifically for the iPhone 4S, and it appears that it does not any other issues that have been reported with iOS 6.1 such as reduced battery life or a problem with excessively large log files associated with Exchange synchronisation.
A few users have reported a complete loss of cellular connectivity after updating to iOS 6.1.1, but it is too early to tell whether there is a systematic problem or if this is a more or less random occurrence of the kind that can happen with any device update.
The update can be downloaded directly onto the iPhone 4S (Settings>General>Software Update) or by connecting the phone to a computer and checking for updates with iTunes.