说明：  4G模块N720驱动程序，非阻塞式，底层驱动已封装好，留出了借口，按照说明初始化相关变量，调用函数即可使用。
(4G module N720 driver, non-blocking, the underlying driver has been packaged, leaving an excuse, according to the instructions to initialize the relevant variables, call the function can be used.)