HDLC PCI
ISO 3309, ISO 4335, ISO 7809.
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HDLC is used by machines already connected via layer-1 (physical) protocol. It was standardized by ISO and is similar to IBM's SDLC (HDLC was derived from SDLC). HDLC was specified before the OSI reference model. HDLC and LLC use the same control field codes.
2 - LLC - Logical Link Control
HDLC operation consists of the exchange of I-frames, S-frames, and U-frames between two stations. The operation of HDLC involves three phases. First, on side initializes the data link so that frames may be exchanged in an orderly fashion. During this phase, the options that are to be used are agreed upon (3-bit or 7-bit sequence number). After initialization, the two sides exchange user data and the control information to exercise flow and error control. Finally, one of the two sides signals the termination of the operation.
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