Confusion in AT Command to serial port using WriteFile()

unsigned char dat[3]={'A','T'};

//tried char,int..

DWORD dwNumBytesWritten=sizeof(dat);
    check=WriteFile( hPort,              // Port is getting correct 
               &dat,               // Pointer to the data to write
               2,                  // Number of bytes to write n tried[1.2.3...] 
               &dwNumBytesWritten, // Pointer to the number of bytes written
               NULL                // Must be NULL for Windows CE
            );

bResult=ReadFile( hPort,                // Port handle
                  &cp.Byte,                // out Pointer to the buffer that receives the data read from the file.
                  1,                    // Number of bytes to read
                  &cp.dwBytesTransferred,  // out Pointer to number of bytes read
                  NULL                  // Must be NULL
                );

cp.CommTimeouts.ReadIntervalTimeout = 100;    //MAXDWORD
cp.CommTimeouts.ReadTotalTimeoutMultiplier = 0;  //10
cp.CommTimeouts.ReadTotalTimeoutConstant = 10;    //13
cp.CommTimeouts.WriteTotalTimeoutMultiplier = 10;  //10
cp.CommTimeouts.WriteTotalTimeoutConstant = 10;   //1000


cp._dcb.BaudRate = 115200;                   // Current baud
cp._dcb.fBinary = TRUE;                      // Binary mode; no EOF check
cp._dcb.fParity = FALSE;                         // Enable parity checking
cp._dcb.fOutxCtsFlow = TRUE;                 // No CTS output flow control
cp._dcb.fOutxDsrFlow = TRUE;                 // No DSR output flow control
cp._dcb.fDtrControl = DTR_CONTROL_ENABLE;    // DTR flow control type
cp._dcb.fDsrSensitivity = FALSE;             // DSR sensitivity
cp._dcb.fTXContinueOnXoff = TRUE;            // XOFF continues Tx
cp._dcb.fOutX = FALSE;                       // No XON/XOFF out flow control
cp._dcb.fInX = FALSE;                        // No XON/XOFF in flow control
cp._dcb.fErrorChar = FALSE;                  // Disable error replacement
cp._dcb.fNull = FALSE;                       // Disable null stripping
cp._dcb.fRtsControl = RTS_CONTROL_ENABLE;    // RTS flow control
cp._dcb.fAbortOnError = FALSE;               // Do not abort reads/writes on error
cp._dcb.ByteSize = 8;                        // Number of bits/byte, 4-8
cp._dcb.Parity = NOPARITY;                   // 0-4=no,odd,even,mark,space
cp._dcb.StopBits = ONESTOPBIT;               // 0,1,2 = 1, 1.5, 2

int main(int argc, char* argv[])
{    
   initPort();
   settimeDCB();
   TransxPortGSM();  //sending AT
   ReadxPortGSM();   // getting AT but expected OK
   closePort();
}