Link Access Process Balance (LAPB is a data link layer protocol), responsible for managing the organization of communication and packets between DTE devices and DCE devices. LAPB, is a bits-oriented protocol source from HDLC, which is actually HDLC in a BAC (balanced asynchronous mode category) mode. LAPB ensures error release and correct sorting of frames.
LAPB, SDLC, and HDLC share the same frame format, frame type, and field functions, but it is different from the above two, and the LAPB is limited by the equilibrium operation asynchronous mode category, only for the combination station. The LAPB circuit can be established via DTE or DCE. Start the call to the station is called a base station, and the other station corresponding to the station is called the secondary station. In addition, LAPB uses P / F bits, which is different from other protocols.
In LAPB, since there is no master-slave relationship, the sender uses Poll bits to insist on responding immediately. In response frame, the final bits of the receiving end also use Poll Bit. The receiving end always uses the final bits to respond to passwords set from the sending end POLL bit. Since the response may be lost, each terminal does not ensure the correct sorting of the frame, then it is necessary to rebuild a reference point. In this case, we usually recommend the use of Poll Bit.
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