SEAlink

SEAlink is a file transfer protocol that is backward compatible with XMODEM but features a sliding window system for improved throughput.

Although the time needed to send the ACK or NAK is almost zero, the propagation delay of the phone system itself introduces a latency.

However, at 9600 bit/s the packet only takes about 0.1 seconds to send, meaning that at least half of the potential throughput is lost while the sender waits for the ACK or NAK to arrive.

SEAlink made one minor modification to this "standard", adding a byte in position 40 of the packet to indicate whether or not to use "overdrive".

When turned on, by placing any non-zero number in byte 40, the receiver no longer expected ACK messages, but only NAKs.

In this case, the receiver always checks to see if an identical filename exists locally, and if so, sends a SYN character instead of the normal ACK or NAK.