Configuring Point to Point WANs HDLC and PPP

HDLC or High-Level Data Link Control

High-Level Data Link Control, also know as HDLC, is a bit oriented, switched and non-switched protocol. It is a data link control protocol, and falls within layer 2, the Data Link Layer, of the Open Systems Interface(OSI) model.

HDLC is a protocol developed by the International Organization for Standardization(ISO). It falls under the ISO standards ISO 3309 and ISO 4335. It has found itself being used throughout the world. 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. The procedures outlined in HDLC are designed to permit synchronous, code-transparent data transmission. Other benefits of HDLC are that the control information is always in the same position, and specific bit patterns used for control differ dramatically from those in representing data, which reduces the chance of errors.

It has also led to many subsets. Two subsets widely in use are Synchronous Data Link Control(SDLC) and Link Access Procedure-Balanced(LAP-B).

This technical overview will be concerned with the following aspects of HDLC:

  • Stations and Configurations
  • Operational Modes
  • Non-Operational Modes
  • Frame Structure
  • Commands and Responses
  • HDLC Subsets(SDLC and LAPB)


HDLC specifies the following three types of stations for data link control:

  • Primary Station
  • Secondary Station
  • Combined Station


Within a network using HDLC as it's data link protocol, if a configuration is used in which there is a primary station, it is used as the controlling station on the link. It has the responsibility of controlling all other stations on the link(usually secondary stations). Despite this important aspect of being on the link, the primary station is also responsible for the organization of data flow on the link. It also takes care of error recovery at the data link level(layer 2 of the OSI model).


If the data link protocol being used is HDLC, and a primary station is present, a secondary station must also be present on the data link. The secondary station is under the control of the primary station. It has no ability, or direct responsibility for controlling the link. It is only activated when requested by the primary station. It only responds to the primary station. The secondary station's frames are called responses. It can only send response frames when requested by the primary station.


A combined station is a combination of a primary and secondary station. On the link, all combined stations are able to send and receive commands and responses without any permission from any other stations on the link. Each combined station is in full control of itself, and does not rely on any other stations on the link. No other stations can control any combined station.

HDLC also defines three types of configurations for the three types of stations:

  • Unbalanced Configuration
  • Balanced Configuration
  • Symmetrical Configuration


The unbalanced configuration in an HDLC link consists of a primary station and one or more secondary stations. The unbalanced occurs because one stations controls the other stations. In a unbalanced configurations, any of the following can be used:

  • Full - Duplex or Half - Duplex operation
  • Point to Point or Multi-point networks


The balanced configuration in an HDLC link consists of two or more combined stations. Each of the stations have equal and complimentary responsibility compared to each other. Balanced configurations can used only the following:

  • Full -