Computer Network Unit 1 q what are the topologies in computer n/w ?

HDLC Features

The main features of HDLC are divided into various aspects
• The modes for operation
• Stations
• Configuration
• Frames and Structures
• The subsets of HDLC

HDLC Stations

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  The HDLC has three levels of stations, the primary station, secondary station and the combined station.
  
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  The primary station is responsible for controlling all the other secondary stations for a network that uses the HDLC protocol. The primary station also takes care of the error control aspect and organizes the data flow on the links.
  
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  The secondary station is controlled by the primary station and is activated when the primary station sends a request.
  
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  The combined station controls the links and overlooks the primary and the secondary stations functions. The combined stations have complete control over the links and do not need the authorization of any other station.
  
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  These stations are further dependant on the configuration types and basically follow three different types of configuration.
  

HDLC has three operational modes:
Normal Response Mode (NRM) - Normal Response Mode is used in unbalanced configurations. In this mode, slave stations (or secondary) can only transmit when specially instructed by the master (primary station). The link may be point-to-point or multipoint. In the latter case only one primary station is allowed.
Asynchronous Response Mode (ARM) - Asynchronous Response Mode: This mode is used in unbalanced configurations. [unbalanced configurations]. It allows a secondary station to initiate a transmission without receiving permission from the primary station . This mode is normally used with point-to-point configurations and full duplex links and allows the secondary station to send frames asynchronously with respect to the primary station)
Asynchronous Balanced Mode (ABM) - The Asynchronous Balanced Mode (ABM), is used mainly on full duplex point-to-point links for computer to computer communications and for connections between a computer and a packed switched data network, in this case each station has an equal status and performs the role of both primary and secondary functions. This mode is used in the protocol set known as X.25.