It's HTTP, at 115200 bps with no handshaking at the moment. I also use port B for communication. I guess to fix my problem I need to use (XON/XOFF) software handshaking to control my receiving data, but I'm not sure how to do it.
The data I need to receive is about a few thousands of characters. I'm using a char array to store the read data, but when I make my buffer to be quite large, it still cannot store more than 128 data.