If your application needs to transfer data over Internet using some mobile device connected to GSM network, then using JSON or other text serializer is not a good solution. It is better to use a binary serializer.
Searching for alternatives I found MessagePack: http://msgpack.org
There are other alternatives like Google’s BufferProtocol, but it is not so efficient as MessagePack.
You can find a listing with many alternatives in the wikipedia:
http://en.wikipedia.org/wiki/Comparison_of_data_serialization_formats