An HDLC frame is structured as follows:

FLAG ADDRESS CONTROL INFORMATION FCS FLAG
8 bits 8 bits 8 / 16 bits variable 8 8 bits

FRAME FIELDS

FLAG
The 8 bit pattern "01111110" which signals the beginning and end of an HDLC frame. If a piece of data within the frame to be transmitted contains a series of 5 or more 1's, the transmitting station must insert a 0 to distinguish this set of 1's in the data from the flags at the beginning and end of the frame. This technique of inserting bits is called bit-stuffing. These bits are detected and removed upon receipt. If a pattern of five 1's is followed by a 1 and then a 0, it marks the end of the frame. If a pattern of five ones is followed by two more 1's, it is a signal from the transmitting station to abort.
ADDRESS
The address field always contains the address of a secondary station. When the primary station transmits, it is the receiving secondary station's address, if it is a secondary responding, it is it's own address. This field is only populated for Unbalanced connections, it is otherwise empty for point-to-point (Balanced) links.
CONTROL
The format of the control field varies with the data it contains. There are three categories of HDLC frames:
  • Information (I-frame) - Carries data
  • Supervisory (S-frame) - Carries commands and responses
  • Unnumbered (U-frame) - Carries additional command sequences

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