And then https://en.wikipedia.org/wiki/SMODEM

SModem supported simultaneous upload/download and chatting.

Fair play -- I didn't know about that one. It does seem to be less of a protocol and more of a specialised DOS client app, though...

Smodem was a game changer. So was HS/Link. Being able to chat while still doing a download was awesome. Realizing that you can upload and download at the same time was awesome. When those protocols came out they blew my mind... and before that the first time I got the added speed from Zmodem over Xmodem I was amazed as well.

I sure miss those days. First modem was 2400 baud, so I was kind of a late comer.