linux/Documentation/isdn/m_isdn.rst

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mISDN Driver
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mISDN is a new modular ISDN driver, in the long term it should replace
the old I4L driver architecture for passive ISDN cards.
It was designed to allow a broad range of applications and interfaces
but only have the basic function in kernel, the interface to the user
space is based on sockets with a own address family AF_ISDN.