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Frame Relay


WAN Technology


HDLC stands for High-Level Data Link Control. It is a bit oriented, switched and non-switched protocol. It is a data link layer protocol and falls within layer 2. HDLC is a protocol developed by the International Organization for Standardization(ISO). It has been so widely implemented because it supports both half duplex and full duplex communication lines, point to point(peer to peer) and multi-point networks, and switched or non-switched channels.

Two subsets widely in use are:

1. Synchronous Data Link Control(SDLC) and

2. Link Access Procedure-Balanced(LAP-B).

HDLC have three stations, The three stations are :

Primary station:

  • Has the responsibility of controlling the operation of data flow the link.
  • Handles error recovery
  • Frames issued by the primary station are called commands.

Secondary station:

  • Operates under the control of the primary station.
  • Frames issued by a secondary station are called responses.
  • The primary station maintains a separate logical link with each secondary station.

Combined station:

  • Acts as both as primary and secondary station.
  • Does not rely on other for sending data

HDLC Frame Structure

The HDLC frame contains a beginning flag, a single byte address field, a single byte control field. The control field is then followed by a variable length information field and a Frame Check Sequence (FCS) that is two bytes (16 bits) long.

  • Flag
  • Address
  • Control
  • Information Field
  • Frame Check Sequence


The three modes of data transfer operations are:

Normal Response Mode (NRM)

  • Mainly used in terminal-mainframe networks. In this case,
  • Secondaries (terminals) can only transmit when specifically instructed by the primary station in response to a polling
  • Unbalanced configuration, good for multi-point links

Asynchronous Response Mode (ARM)

  • Same as NRM except that the secondaries can initiate transmissions without direct polling from the primary station
  • Reduces overhead as no frames need to be sent to allow secondary nodes to transmit
  • Transmission proceeds when channel is detected idle , used mostly in point-to-point-links

Asynchronous Balanced Mode (ABM)

  • Mainly used in point-to-point links, for communication between combined stations

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