systems like Telebit models which have 19 kB speed in one direction and 75 bits/s in the return channel. SEAlink One of the first third-party mailers for the FidoNet system was SEAdog, written by the same author as the then-popular .arc data compression format. SEAdog included a wide variety of improvements, including SEAlink, an improved transfer protocol based on the same sliding window concept as WXmodem. It differed from WXmodem mostly in details. One difference is that SEAlink supported the "zero packet" introduced by TeLink, which is needed in order to operate as a drop-in replacement for TeLink in FidoNet systems where the header was expected. ACKs and NAKs were extended to three-byte "packets", starting with the ACK or NAK, then the packet number, then the complement of the packet number, in the same fashion as the original XMODEM packet header. The window size was normally set to six packets. SEAlink was not expected to operate over X.25 or similar links, and thus did not perform escaping. This was also needed so the zero packet would work properly, as this standard used the SYN character that WXmodem had re-purposed. On top of these changes, it added an "Overdrive" mode for half duplex links. This suppressed ACKs for packets that were successfully transferred, in effect making the window of infinite size. This mode was indicated by a flag in the zero block. SEAlink later added a number of other improvements and was a useful general-purpose protocol. However, it remained rare outside the FidoNet world, and was rarely seen in user-facing software. XMODEM-1K Another way to solve the throughput problem is to increase the packet size. Although the fundamental problem of latency remains, the speed at which it becomes a problem is higher. XMODEM-1K with 1024-byte packets was the most popular such solution. In this case, the throughput at 9600 bit/s is 81%, given the same assumptions as above. XMODEM-1K was an expanded version of XMODEM-CRC, which indicated the longer block size in the sender by starting a packet with the <STX> character instead of <SOH>. Like other backward-compatible XMODEM extensions, it was intended that a -1K transfer could be started with any implementation of XMODEM on the other end, backing off features as required. XMODEM-1K was originally one of the many improvements to XMODEM introduced by Chuck Forsberg in his YMODEM protocol. Forsberg suggested that the various improvements were optional, expecting