The High Level Data Link Control protocol is a group of rules for transmitting data between network points. It is one of the most commonly used protocols which is used in the Data Link layer i.e. Layer 2 of the seven layer OSI (Open Systems Interconnection) model. Layer 1 is the physical layer that is involved in generating and receiving the electronic signals. The network layer is Layer 3, the higher level that contains information where to forward or send data. While sending, layer 3 creates a frame that contains the source and destination address of the network. The HDLC in layer 2 encapsulates the network layer frame, adding datalink control information to a new and larger frame. The HDLC is the default encapsulation method used between point-to-point connections, dedicated links, and circuit switched connections when the link uses two Cisco devices.
For the HDLC protocol, there are three types of stations which is defined below.
- Primary Station: This station takes care of data link management and frames sent by the primary station is called commands.
- Secondary Station: It operates under the primary station and frames sent from this station is called responses.
- Combined Station: This station can behave as both primary and secondary stations, thus it can send both commands and responses.
In the HDLC protocol, there are three types of operating modes used for data transfer. They are discussed below.
- Normal Response Mode (NRM): It is a synchronous mode of communication where a primary station sends the command and a secondary station can only respond. Here you have only one primary and multiple secondary stations which make it suitable for point-to-point or point-to-multipoint configurations.
- Asynchronous Response Mode (ARM): It is an asynchronous mode of communication where the primary and secondary stations communicate with each other.
- Asynchronous Balanced Mode (ABM): In this mode, the communication occurs between tow combined stations and the link is point-to-point. Here the configuration is balanced and they are capable of link management functions.
The advantages of the HDLC protocol are:
- In HDLC protocol, multiple frames can be acknowledged at once.
- They are code independent and have a standard frame format to utilize.
- They got an effective error checking process which covers all data between flags
- The HDLC protocol supports both half-duplex and full-duplex
- The bit stuffing is used to overcome the lack of data transparency when the data field of the HDLC frame carries text and non-text information.
To sum up, the HDLC protocol is flexible, adaptable, reliable, and efficient in its operation.
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