There are only two alternatives to C2DM, as shown below.
a survey . The application itself periodically polled your servers to check for new messages. You will need to implement everything from queuing messages to writing polling code. Warnings are not appropriate if they are delayed due to a low polling period, but the more often you do the polling, the more the battery will die.
SMS Android can intercept SMS messages, and you can enable the payload to tell the application what to do. But why not just use SMS first?
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