Link Access Procedure Balanced

LAPB is a data link-layer protocol that manages communication and packet framing between DTE and DCE devices and is best known for its presence in the X.25 protocol stack.  LAPB shares the same frame format, frame types, and field functions as SDLC and HDLC. Unlike either of these, however, LAPB is restricted to the ABM transfer mode and is appropriate only for combined stations.  Also, LAPB circuits can be established by either the data terminal equipment (DTE) or the data circuit-terminating equipment (DCE).  The station initiating the call is determined to be the primary, and the responding station is the secondary.  Finally, LAPB use of the P/F bit is somewhat different from that of the other protocols.

This bit-oriented protocol, derived from HDLC, makes sure that frames are error free and properly sequenced.

• I-Frames (Information frames): Carries upper-layer information and some control information. I-frame functions include sequencing, flow control, and error detection and recovery.  I-frames carry send and receive sequence numbers.
• S-Frames (Supervisory Frames): Carries control information.  S-frame functions include requesting and suspending transmissions, reporting on status, and acknowledging the receipt of I-frames.  S-frames carry only receive sequence numbers.
• U-Frames (Unnumbered Frames): carries control information.  U-frame functions include link setup and disconnection, as well as error reporting.  U-frames carry no sequence numbers.