출처: http://jultech.tistory.com/m/post/2 GCM 이란? http://developer.android.com/google/gcm/gcm.html 중요한 구문 It uses an existing connection for Google services. For pre-3.0 devices, this requires users to set up their Google account on their mobile devices. A Google account is not a requirement on devices running Android 4.0.4 or higher. 즉, OS 버전 3.0 이하(..