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Oct 27

Importance of The HDLC Protocol

HDLC Protocol

High-level link Control or HDLC is a group of communication protocols of the data link layer for transmitting data between network points. HDLC is among the foremost important protocol within the link layer. In HDLC, data is organized into frames. A frame is transmitted through the network to the destination and then it verifies its successful arrival. It is applicable for both point – to – point and multipoint communications.

HDLC provides encapsulation method for serial link. Serial link and Ethernet link both use different encapsulation methods for data transmission. Ethernet encapsulation method and protocols are basically laid out in LAN technology. Serial protocols and encapsulation methods are primarily described in WAN technology. Router is used to connect two different technologies. HDLC is an encapsulation method for serial link.

HDLC Protocol Structure:

  • Flag: Flag value is usually (0x7E)
  • Address field: Defines the address of the secondary station which is sending the frame from the primary station. It contains Service Access Point, a Command bit to point whether the frame relates to information frames that was sent or received by the node, and an address extension bit.
  • Extended address: HDLC provides another form of extension to the fundamental format. The address field is also extended to over one byte by agreement between the involved parties
  • Control field: Serves to spot the kind of the frame. In addition, it includes sequence numbers, control features and error tracking in line with the frame type
  • Frame Check Sequence: This enables a high level of physical error control by allowing the integrity of the transmitted frame data to be checked.

HDLC adds error correction and flow control, the three modes of HDLC are:

  • Normal Response Mode: Secondary nodes can transmit when given permission by the first
  • Asynchronous Response Mode (ARM): Secondary nodes may initiate communication with the first
  • Asynchronous Balanced Mode (ABM): Combined mode where nodes may act as primary or secondary, initiating transmissions without receiving permission.

Types of HDLC Frames
There are three types of frame in HDLC

  • Information frame: Information frames carry user data from the network layer. They also include flow and error control information which is backed on user data. The first bit of control field of Information frame is 0
  • Supervisory frame: Supervisory frames do not contain information field. They are used for flow and error control when piggybacking is not required. The first two bits of control field of Supervisory frame is 10
  • Unnumbered frame: Unnumbered frames are used for miscellaneous functions, like link management. It may contain an information field, if required. The first two bits of control field of unnumbered frame is 11.

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