High Level Data Link Control (HDLC)
The High Level Data Link Control protocol was developed by the International Organization for Standardization (ISO). It is used for switched and non-switched networks and is a bit oriented architecture. The High Level Data Link Control has been accepted and used widely because it supports almost all kinds of functionalities like half duplex, full duplex, point to point and multi-point networks. The bit patterns are standard in HDLC and therefore the information exchanged follows the same pattern. This minimizes the chances of any errors.
The main features of HDLC are divided into various aspects
• The modes for operation
• Frames and Structures
• The subsets of HDLC
HDLC Stations and Configurations
The HDLC has three levels of stations, the primary station, secondary station and the combined station.
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.
The secondary station is controlled by the primary station and is activated when the primary station sends a request.
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.
These stations are further dependant on the configuration types and basically follow three different types of configuration.
Unbalanced Configuration: In an unbalanced configuration there is one primary station and the remaining stations are all secondary stations that are controlled by the primary station. Unbalanced configuration supports many types of operations like half duplex, full duplex, point to point configuration and multi point configuration.
Balanced Configuration: In a balanced configuration there are two or more combined configurations however there are limitations when it comes to the balanced configurations since it can use only specific configurations like the half and full duplex operations or point to point network configurations.
Symmetrical Configuration: The symmetrical configuration is a combination of balanced and unbalanced configurations and is rarely used in current day technology. In this type of configuration each station in the network has two kinds of status; one is primary and the secondary status.
There are several modes of operation for the above configurations in the HDLC.
Normal Response Mode (NRM): The Normal Response Mode is used only with the unbalanced configurations. The primary station usually requests information from the secondary station and only then the secondary station initiates the data transfer. The response is transferred after the primary station authorizes the transaction.
This procedure is followed for each and every frame that is being transferred. After the last frame is sent or transferred then the procedure sets back itself to zero where the entire gamut of taking permission is repeated for the next set of frames.
Asynchronous Response Mode (ARM): The ARM can be used with balanced or combined configuration as it does not need any authorization from the primary station to start transferring the frames from the secondary station. It reduced the time overheads while transferring the frames and since the permissions are not required.
Asynchronous Balanced Mode (ABM): The asynchronous balanced mode is not very different from the asynchronous response mode. Even this mode does not require permissions from the primary station in order to transfer data or frames from the secondary station. The ABM mode is better for multi point nodes whereas the ARM is better for the point to point nodes. However this mode is not being widely used in modern day technology.