Pronounced lap bee. A bit-oriented protocol that is part of the X.25 protocol suite, LAPB runs at Layer 2, the Data Link Layer, of the OSI Reference Model.The LAPB frame comprises a header and trailer that encapsulate the packet formed by the X.25 packet layer protocol (PLP) at the Network Layer. The LAPB frame provides a mechanism for transporting packets across a link, ensuring that frames of data are ordered correctly and are free from error. LAPB is a balanced protocol that operates in asynchronous balanced mode (ABM), referring to the fact that the devices have a balanced, rather than a master/slave, relationship. LAPB is a derivative of the High-level Data Link Control (HDLC) protocol, which, in turn, is based on the IBM Synchronous Data Link Control (SDLC) frame. Figure L-1 illustrates the LAPB frame and its component fields. The fields in the LAPB frame are as follows.