Also, the V.34 modem standard has some forward error checking. Used for point to point full duplex transmissions. Xmodem Ymodem Zmodem Kermit Synchronous Transmission protocols. It uses a SYNchronization character to start things off rather than stop bits. Throughput can be controlled by the packet size and the protocol.
Some protocols are better for some things.
Example, xmodem is slow, ymodem has bigger blocks and less error correction data, so its faster.