A brief hint for those using Linux: If you have an old computer, and Linux chooses to disable ACPI, leave it at that. That is, do not use
acpi=force. I just had a case of a computer that appeared to work fine with it. Only that the ACPI option would make the serial port (apparently) lose interrupts, and thus put "breaks" or "pauses" into any longer download. The data transfer would stop, and then, with the next LCP Echo Request on the PPP link, resume. The data transfer was pretty "bursty", not nearly exploiting the full (stable) transfer rate on ISDN of roughly 8 kilobytes/second.