In the US reaching 56k was not possible. Sending at 56k required transmit levels above those allowed by the FCC. The maximum in the US over a perfect line was 5

In the US reaching 56k was not possible. Sending at 56k required transmit levels above those allowed by the FCC. The maximum in the US over a perfect line was 53.3k. During that era I had a dedicated dual channel 128k ISDN connect at home provided by my work but while traveling I would occasionally get a 53.3k connection but 48k was more normal.