The primary restriction on applications using the LayGO Hardware Interface is that the data link layer and other higher layer protocols may not be used. Only the physical layer capabilities of the communication hardware can be accessed. In most cases, this means that only raw HDLC framing and signal detection are provided. In the case of custom bisynchronous or monosynchronous LayGO drivers, only the driver implemented functionality is accessible.

As a consequence, the following LayGO API functions will always fail:

In all other respects, API functions behave as if there were a protocol stack which contained only the X.21 bis physical layer. No other protocol services are available.