Comment by dspillett

The asymmetry of 56k standards was 2:1, so if you got a 56k6 link (the best you could get in theory IIRC) your upload rate would be ~28k3. In my expereience the best you would get in real world use was ~48k (so 48kbpd down, 24kbps up), and 42k (so 21k up) was the most I could guarantee would be stable (baring in mind “unstable” meant the link might completely drop randomly, not that there would be a blip here-or-there and all would be well again PDQ afterwards) for a significant length of time.

To get 33k6 up (or even just 28k8 - some ISPs had banks of modems that supported one the 56k6 standards but would not support more than 28k8 symmetric) you needed to force your modem to connect using the older symmetric standards.