Link Access Procedure Balanced
LAPB (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.
LAPB's Frame Types:
- 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.
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