XMODEM packets) along with the normal ACK or NAK character. On reception of the very first ACK or NAK, the SEAlink sender would "notice" the packet number and continue to use windows. If the packet number was missing, it was assumed it was sending to a non-SEAlink XMODEM, and set the window size to one. Other features SEAlink also included a number of features included in other XMODEM derivatives. Notable was the use of a "zero packet", which was introduced in the FidoNet-related TeLink protocol in order to send file information and support batch file transfers. 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. This was intended to reduce the bandwidth required from the receiver back to the sender, which was an important consideration on many of the early high-speed modems, which featured a very slow backchannel (75 bit/s in the Telebit modems, for instance). A later addition was "RESYNC", allowing for file transfers to be restarted from where they left off. RESYNC added another indicator in the zero packet, byte 42, to indicate that the sender supports the feature. 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. The SYN is followed by the packet number at which to restart. References SEAlink 1.20 program and source code BBS file transfer protocols