XBAND
The following is a compilation of source code and files obtained over the years that made the xband modem for the SNES and Sega Genesis what it was.
The xband hardware is a 2400 baud Rockwell chipset modem with a custom chip on board called "Fred" which has patch vectors (10 Approximately). It worked in a very similar fashion to game genie in terms of patching.
Basically you would dial up to a central server which would then handshake and your modem would puke its guts so to speak to the server. It would grab mail, get player profiles, and any system patches (if there were any). It would also report to the server how much free space in SRAM it had. Sram was 64kb in size and held game patches and your profile and all your stats and custom player icons and such and news and mail.
For matchmaking, if you chose to challenge a player or get a random match, it would dial the central server, and spent a minute or two downloading a 10kb or smaller game patch which would be used with the FRED chip to patch in its own network game code into the existing game on the fly and interact with the modem.
The game networking was quite an impressive feat. It would send controller data back and fourth between consoles and also sync vblanks between machines. It would switch between interlaced and non interlaced video modes to slow down the faster consoles to they would be running in lock-stop as much as possible or at most, a few frames apart (up to about 6-8 max).
However, due to latency, this device is sensitive. Copper phone lines produce stable latency that was much lower and stable, however today over the internet its much less-predictable and latency varies quite a bit. In some of the source code docs you'll find reference to about 35-40ms of latency being the absolute max it can handle (has to be stable). Ive tested this over voip many a time and this absolutely holds true that it cannot handle anything over that, otherwise you repeatdly trip the resync code in the game patch and it either lags out or plays horribly.
The xband network protocol is a modified early version of ADSP Appletalk. Most if not all packets end with 10 03 (In hex) which equates to DLE/ETX characters. The first packet received from xband when it initially connects is 26 bytes in length Eesentially you need to puke the same packet back to the xband but with modifications to initiate the handshake.
See xbsega.go (sample handshake in go) for an example of how to do this along with functions to see how it parses whats called the "puke" packet which comes after the handshake completes after initial connect. Sending packets back to the modem isnt implemented yet unfortunately but shouldnt be much of an issue to do so.
Beyond this, most of the xband protocol once handshakes has to layer the adsp packet header on top of the actual payload packet you are sending. The XBAND actual core packet format uses opcodes (byte) that determines the action you want to do. There are actual server side opcodes and client side opcodes. These can all be found within the source code provided in the catapult.tar.gz file listed here but may require some digging. Ensure that when you use/send these values, that you find the sizes of the data you are going to send as well to the xband. If the packet length and sizing of the data isnt exactly right, you can spend a bunch of time waiting for the xband to timeout or crash entirely and do nothing, Its very much a trial and error thing in some cases. Also the snes is little endian and Sega is big endian format. Keep this in mind when crafting packets.
See packets.txt file for samples on crafted packets that you have to attach/append the adsp header packet on top of and also adjust the crc value in the adsp header to include the full length of the payload.
Xband game patches are hard to come by obviously and without them the gaming side of xband is a lost cause. Fortunately for sega there is mk2, mk, nba jam, madden95 and nhl94/95 patches in the source provided. Some patches also have source code as well. For snes, there is only a binary copy of super mario kart with no source and same goes for a super street fighter 2 which i recovered from a japanese xband and dumped,tested and confirmed working between xbands for gaming.
Be aware of sending game patches to the modem that contain 10 03 (dle/etx) characters on the sega side as it will absolutely think you are terminating a packet. Not entirely sure how to get around this one just yet but if you get fairly far in uploading patches to modem sram you'll run into this sooner or later.