Discussion on HDLC Frame Format of HDLC & Transfer Mode

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Discussion on HDLC: Frame Format of HDLC & Transfer Mode (Computer Science)


  • Introduction on HDLC
  • Primary and Secondary Stations
  • HDLC Frame Format
  • Transfer Modes:
  • Asynchronous Response Mode (ARM) : Normal Response Mode (NRM)
  • Asynchronous Balanced Mode (ABM)


  • HDLC stands for High Level Data Link Control
  • It is a bit oriented transmission protocol
  • It works at a data link layer which is second layer (layer 2) of OSI seven layered architecture model for data communication
  • It embeds information in a data frame that allows devices to control data flow and correct errors
  • HDLC is proposed by IBM in 1970s from SDLC i.e.. Synchronous Data Link Control Standard
  • Sessions are created for communication purpose in it.

Primary & Secondary Stations

  • Primary Station: The one who sends commands. Data Link Management and Connection/Disconnection are the basic responsibilities of primary station. Frames issued by primary station are called as commands.
  • Secondary Station: These are the stations which responses and frames issued by secondary stations are called as responses.
  • Combined Station: It acts both like primary and secondary stations. They have both commands and responses.

Frame Format of HDLC

HDLC Frame Format
  • I-Frame: Information Frame-0 bit in control field
  • S-Frame: Supervisory Frame-10 bit in control field
  • U-Frame: Un-numbered Frame-11 bit in control field

Transfer Mode

Transfer Modes:

  1. Normal Response Mode (NRM) : Point to Point and Point to Multipoint
  2. Asynchronous Balanced Mode (ABM) : Point to Point

Normal Response Mode (NRM)

Normal Response Mode (NRM)

Asynchronous Balanced Mode (ABM)

  • Point to Point:
Asynchronous Balanced Mode (ABM)
  • Workstation acts as both primary and secondary stations depending upon the requirement.


Q1. HDLC stands for:

  1. High Data Logical Control
  2. High Level Data Link Control
  3. High Destination Logical Control
  4. High Definition Logical Control

Answer: B

Q2. HDLC is:

  1. Bit oriented
  2. Code transparent
  3. Both (a) and (b)
  4. None of these

Answer: C

#Introduction on HDLC

#Primary and Secondary Stations

#HDLC Frame Format

#Transfer Modes:

#Asynchronous Response Mode (ARM)

#Normal Response Mode (NRM)

#Asynchronous Balanced Mode (ABM)