HDLC stands for High level data link control. HDLC is among the most important protocol in the data link layer. As we all know protocols are said to as set of rules. HDLC is a bit oriented data link protocol. That means it uses bit stuffing to achieve data transparency. We already know that data link layer has data combined with control signals. The separation of data from the control signals is called as transparency. HDLC is the most important protocol because of two reasons :
- It is widely used ISO standard
- HDLC is also the basis of other important data link protocols that uses same format and same mechanisms similar to HDLC.
- It supports both full duplex and half duplex mode of communications.
Since HDLC satisfy variety of applications, HDLC defines :
- Three types of stations
- Two link configurations
- Three data transfer modes of operations.
I. The three station types are as follows :
PRIMARY STATION: This station is responsible for controlling the operation of the links. The frames which are issued by the primary stations are called as commands.
SECONDARY STATION: This station operates under the control of primary station. The frames which are issued by the secondary station are called as responses. Each primary station maintains a separate logical link with the secondary station.
COMBINED STATION : As the name says it is the combination of both the primary station as well as the secondary station. The only difference is that a combined station can issue both a command or a response.
II. The two link configurations are as under :
UNBALANCED CONFIGURATION: this consists of one primary station and more than one secondary station. and it supports both mode of transmission i.e. full duplex and half duplex.
BALANCED CONFIGURATION: This configuration consists of two combined stations and it supports both full duplex and half duplex mode of transmission.
III. Three data transfer modes of operation are as follows :
NORMAL RESPONSE MODE (NRM): This is only used with an unbalanced configuration. Here in this primary station may initiate data transfer to a secondary station but a secondary station may only transmit data in response to the command from the primary station.
ASYNCHRONOUS BALANCED MODE (ABM): It is only used with the balanced configuration. In this the secondary station can initiate a transmission without the explicit permission of the primary station.
ASYNCHRONOUS RESPONSE MODE (ARM): IT is only used with unbalanced configuration . The secondary station can start transmission without the permission of the primary while primary station still has some responsibilities on it like initialization, error recovery and logical disconnections.
There are three classes of frames used in HDLC
- Unnumbered Frames
- Supervisory Frames
- Information Frames
1. UNNUMBERED FRAMES: These are used in functions such as link setup and disconnections. The name itself tells us that they do not contain any type of acknowledgement information, i.e. in turn contained in the sequence number.
2. INFORMATION FRAMES: These frames carry the actual data. They are normally referred to as I-frames. They can be used to piggyback acknowledgement information related to the I-frame flow.
3. SUPERVISORY FRAMES: These are basically used for error control and flow control. And therefore they contain send and receive sequence numbers.
Related Questions Answers
- WRITE SHORT NOTE ON HDLC.
- EXPLAIN HDLC.
- HOW MANY TYPES OF STATIONS ARE THERE IN HDLC. ALSO TELL ABOUT ITS CONFIGURATIONS AND DATA TRANSFER MODES.
- DISCUSS ABOUT THE DIFFERENT HDLC FRAMES.
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