Hellmuth Michaelis, developper of isdn4bsd in 1998 refrained from implementing support of the Stac compression standard into FreeBSD, thus making it difficult to communicate between FreeBSD systems and most commercial operating systems, whose owners pay license fees for being able to decompress Stac data. Upon inquiry, he explained:

The software you are referring to is the PPP Stac compression which is patented by Stac/HiFn. Although it is described with the exact details required to implement it, it seems to be impossible to actually code it (or better to release that code as source or binary) because of the patent rights Stac/HiFn holds.

Another leading ISDN developper explains

The damage is

1. Most ISDN switches are aware of no other data compression than Stac. But nobody is allowed to offer Stac-capable software without paying patent royalties. The result is that proper ISDN communication is not possible outside of Microsoft operating systems.
2. Most data compression schemes are trivial. Usually, they are based either on Storer or on LZW. Since the basic algorithms have all been patented, there is not much point in trying to develop a patent-free alternative to Stac. This damage is even greater than the damage caused by the fact that Stac is a de-facto standard.