An extended protocol variant of Link Access Procedure D (LAP D) developed for signaling in frame relay is called Link Access Procedure Frame
Relay (LAP F). LAP-F is standardized in by the International Telecommunication Union (ITU) under Q.922 and supports congestion controls
in frame relay. LAP-F transmits at the user level data units for the bearer service between the Data Link Service users via the subscriber network interface (UNI) on the B, D and H channel ofISDN.