The fifth model of Apple's smartphone, introduced in October 2011. Housed in an almost-identical case as the iPhone 4, the 4S sports numerous internal changes, including a fast dual-core A5 CPU, dual antennas and an eight-megapixel camera with improved optics. The iPhone 4S debuted with the iOS 5 operating system, which provides cloud storage for a user's content (see iOS versions and iCloud). Models with 16, 32 and 64GB of storage are available. As of 2012 in the U.S., AT&T, Verizon and Sprint offer the iPhone 4S. See iPhone, iPhone 5 and iPhone 5S. The Siri Intelligent Assistant A highly touted feature of the 4S is Siri, a voice-based command system that answers questions and takes dictation. For more details, see Siri.