Bulk data transfer using GSM Cell modem

Hi ... I am new to GSM Modem communications. I have some issues given below. Please help
I need to transfer large number of data from a MCU( Host) to remote server using GSM Cell modem. Cell Modem has 2 interfaces. 1 UART and 1 USB. UART interface is used for configuration of the modem using AT Commands and USB interface is for bulk data transfer to and from the modem. I am new to modem programming. My doubts are

1) Which protocol need to be used for data transfer in modem?( GPRS, PPP ....etc?)

2) Can i keep on writing to modem USB endpoint? Any memory overflow can happen inside the modem? Any handshake will happen between modem and host( MCU)?

3) I need to receive the data from server to MCU through modem USB interface. In this case how can MCU get the trigger for data is ready for reading?( Any interrupt will generate when a new set of data available for reading?)

Please give some explanation to these queries. I need to know the basic operation of the cell modem for bulk data transfer.

Thanks
KK

Please give some explanation to these queries. I need to know the basic operation of the cell modem for bulk data transfer.

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Explanation to these queries should be present in the modem's manual or datasheet. We don't know which modem you are using.