“Matt Henderson has written a HiSax US ISDN Driver for Linux
that allows the use of low cost passive ISDN cards.“
“Based on the European version, this version includes the US
National ISDN-1 (NI-1) protocol used throughout the USA and Canada.
Combined with a low cost ISDN card, HiSax can turn a Linux machine
into a powerful ISDN router and firewall at 64kbps or 128kbps.
HiSax NI-1 drivers are available for 2.2 and 2.0 kernels.”