LAPB
LAPB is the data link layer protocol of X.25. It manages communication and packet framing between DTE and DCE.
From the perspective of layering, the link layer is like a bridge, across which the packet layers of DTE and DCE exchange packets continuously.
Use the link layer functions to perform the following tasks:
Transmit data effectively between DTE and DCE.
Ensure the synchronization of information between receiver and sender.
Detect and correct errors during transmission.
Identify and report procedural errors to the higher layer protocols.
Inform the packet layer of the link layer state.
LAPB adopts the framing structure of HDLC and is a subset of HDLC. LAPB uses the SABM command to initiate link setup. Two devices establish a two-way link after one of them sends an SABM command and the other replies with an UA response.
Although initially defined for X.25, LAPB, as an independent link layer protocol, can directly carry non-X.25 upper layer protocols. You can set the link layer protocol of a serial interface as LAPB for local data transmission.