What are common bandwidth control mechanisms on L2 and L3?

I know of TCP sliding window, but I'm sure there's some other mechanisms to avoid packet loss on a congested network.

I just had a ADSL modem completely drop everything just because one client is using up the upload bandwidth. Other internet connections/routers are still fine even when someone uploads something.

We implemented a workaround by enabling traffic shaping on the firewall before the ISP router, limiting it to just below the maximum bandwidth. I'm trying to understand where this problem could be coming from. What are some good search terms so I can read up on that?

Also the ISP told us it is a common problem with ADSL, and moving SDSL should fix it, but costs a lot more. right...