The Z85x30 family synchronous/asynchronous controller chips are used on a larg number of cheap network interface cards. The kernel provides a core interface layer that is designed to make it easy to provide WAN services using this chip.
The current driver only support synchronous operation. Merging the asynchronous driver support into this code to allow any Z85x30 device to be used as both a tty interface and as a synchronous controller is a project for Linux post the 2.4 release
The support code handles most common card configurations and supports running both Cisco HDLC and Synchronous PPP. With extra glue the frame relay and X.25 protocols can also be used with this driver.