SEAlink

SEAlink is a file transfer protocol that is backward compatible with XMODEM but features a sliding window system for improved throughput. SEAlink was written in 1986 as a part of the SEAdog FidoNet mailer written by System Enhancement Associates, creators of the famous ARC program. It was licensed with a simple "give credit" requirement, but nevertheless was not very widely used except in FidoNet mailers. SEAlink, and most other XMODEM enhancements, were quickly displaced following the introduction of ZMODEM. Description XMODEM breaks down the file to be sent into a numbered series of packets of 128 bytes each. Packets are sent one at a time to the receiver, who either acknowledges the proper reception by sending a ACK character or requests it to be resent by sending a NAK instead. The transfer then continues with another packet. Although the time needed to send the ACK or NAK is almost zero, the propagation delay of the phone system itself introduces a latency. A latency of 0.1 seconds is not at all uncommon and can be much longer on overseas links. That means that the maximum transfer rate of XMODEM is a function of the phone system more than the speed of the modems themselves. At 300 bit/s this is not a serious concern, as the packet will take about 3.5 seconds to send, the 0.1 delay being insignificant. 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 addresses this through the use of a "sliding window". In this case the sender does not wait for the ACK or NAK, and instead simply sends out its packets in a continuous stream. The receiver still has to ACK or NAK the packets, but does so by attaching a packet number to each one, allowing the sender to identify which packets worked or failed. The "window" refers to the number of packets that can be sent before an ACK or NAK is required. In the case of SEAlink, the window was six packets, allowing up to 768 bytes of data to be sent at once.