Using a PC as a Modem

I recently watched a TekSyndicate video on using an old PC as a router. I thought it was interesting, and a fun thing to do.

Now, I'm curious about taking it a step further, and using said PC as a modem as well. Would any of you have any experience or knowledge on doing this? Especially if I can use a VPN with it.

I'm willing to learn whatever I need to to pull this off.

Is that possible? I've never heard of that one.

Neither have I. I know the ISPs allow using your own modem.

I'm just imagining having a PC be the modern, router, wireless access point, and have VPN set up. I think it'd be fun to try.

I know that modems are basically tiny computers. I know that phones can access the web and act as an access point. So, I've decided I'm going to try figuring this out. It'd just be nice if someone else has experience pulling this off or failing before.
this is also a basic tutorial on how to do it

For cable, it wouldn't be possible. You would need to possess a QAM decoder, which is only available in the cable modem itself. Then you have to figure out what kind of backend hardware they're using (Alcatel, Motorola, Cisco). Then you have to know if it's DOCSIS 1.1, 2.0, or 3.x. Last but not least, you have to figure out how to decode the upstream and downstream channels.

For DSL, you can acquire a ADSL2+ / VDSL2 card that you can use inside the PC. But you would have to figure out which DSL profile you're using and if the chipset will talk back to the DSLAM down the road. That can be quite a futile experience. Some systems are only designed to register modems that are assigned to the ISP.

If you hate the modem/gateway that you have now, you're better off researching a different brand that you can purchase.

That's the video I was talking about that set me down this road of thought.

Thanks, Squirrel, for the info.