Fast Dormancy is a relatively recent technology that is aimed at saving battery life on your phone as well as reducing congestion on mobile networks. Essentially when your phone stops submitting data, it waits for a message from the mobile network telling it to close the connection. On some phones you might notice this is when the H or H+ symbol becomes a 3 or 3G symbol. Essentially its a fast way of making the mobile data connection become dormant, thus saving battery. Many recent devices, including the Samsung Galaxy SIII support Fast Dormancy and in general that is a good thing.*
Why on earth would I want to disable it then?
Even though Fast Dormancy sounds like a brilliant idea (and it is) some mobile networks choose not to implement it citing reasons such as cost and network congestion (I call bs on this one though). Unfortunately if Fast Dormancy is not supported by your network and it is enabled on your phone it can cause problems.
