Explain about HDLC?


12.Explain about HDLC?

HDLC stands for High-Level Data Link Control, and it is a widely used data link layer protocol for point-to-point and multipoint communication over various networks. HDLC was originally defined by the International Organization for Standardization (ISO) in 1979 and is based on the older Synchronous Data Link Control (SDLC) protocol developed by IBM.

HDLC provides a reliable and efficient data link layer service, offering features such as error detection, error correction, flow control, and link management. It is used in various applications, including telecommunications, wide area networks (WANs), and satellite communications. HDLC has also served as the basis for other data link protocols like LAPB (Link Access Procedure, Balanced) used in X.25 networks and PPP (Point-to-Point Protocol) used in dial-up and broadband connections.

HDLC operates by framing data into packets and adding control information to these frames. The protocol defines three different types of stations:


  •     Primary Station: The primary station is responsible for initiating communication and controlling the data link connection. There can be only one primary station in an HDLC link.
  •     Secondary Station: Secondary stations are passive and respond to the primary station's commands. Multiple secondary stations can be connected to a primary station.
  •     Combined Station: A combined station can function both as a primary and secondary station.

HDLC frames consist of the following components:

  1.     Flag: A special bit pattern (01111110) used to indicate the start and end of an HDLC frame.
  2.     Address: An 8-bit field that identifies the destination station (secondary address) or indicates that it is a command from the primary station (primary address).
  3.     Control: A 8-bit field that contains various control information, such as sequence numbers for managing frames, supervisory frames for flow control, and unnumbered frames for link management.
  4.     Data: The data field carries the information from higher-layer protocols, such as network layer packets or user data.
  5.     Frame Check Sequence (FCS): A 16- or 32-bit field used for error detection. HDLC uses a cyclic redundancy check (CRC) for error detection.

The HDLC communication process involves the following stages:

  1.     Initialization: The primary station initiates communication by sending an initialization frame to the secondary station.
  2.     Information Transfer: Data frames are exchanged between the primary and secondary stations.
  3.     Flow Control: The primary station can send supervisory frames (e.g., ACK and NAK) to control the flow of data.
  4.     Termination: Communication is terminated when the primary station sends a termination frame to the secondary station.

HDLC is a versatile and efficient protocol with widespread use in various networking environments. Its reliability and simplicity have made it a foundation for other data link protocols and communication systems.

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