Vintage video game cartridges with built-in modems

They formed a company called Catapult Entertainment and began work on the XBAND, an inexpensive modem built into a pass-through video game cartridge, along with an online service for it to communicate with. The XBAND service would let gamers play a selection of ordinary, off-the-shelf video game cartridges against other players, regardless of their physical location. Sega Channel consisted of a special cartridge with a coaxial input on the back, paired with an online service that offered a rotating set of downloadable games each month.

Source: writing.markchristian.org