How does your iPhone detect when the person texting you is actively typing?

If you're an iPhone user, this situation is likely familiar: You send a message, see it's delivered and read, then watch those three dots float in the conversation bubble as you anxiously wait for a reply. But how does your iPhone know when the other person is typing?

You may have noticed that the signature ellipsis only appears when using iMessage; that is, when texting with another user on an iPhone, iPad, or iPod Touch. Apple launched iMessage in 2011 as part of iOS 5 [source: Apple]. iMessage, integrated into the iPhone Messages app, mimics texting but functions more like an instant messaging service. When you text another iMessage user, your phone identifies their Apple ID and automatically sends the message through iMessage [source: Apple].

Unlike SMS text messages, which are transmitted through your mobile carrier’s cellular network, iMessage texts are routed through Apple’s servers via a technology called Apple Push Notification service (APNs) [sources: Apple, Dilger, iAnswerGuy]. APNs utilizes eXtensible Messaging and Presence Protocol (XMPP), the same protocol used by Google Talk and Apple’s iChat instant messaging service [sources: Apple, Dilger]. XMPP enables servers to communicate with one another, allowing notifications from one server to be received by a user on a different server. When connected to a network, your iPhone checks with the APNs servers to see if they have anything new for you, including texts, FaceTime calls, or app notifications [sources: Apple, Dilger]. Since the iPhone at the other end of the conversation is also linked to APNs servers, they can detect activity, associate it with your Apple ID, and notify you that a text is on its way.

If your WiFi or cellular connection is unavailable, or if you're texting someone on a different mobile platform, your iPhone will automatically switch from iMessage to standard SMS, leaving you unaware of whether your message has been read or if a response is on the way.