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HDLC Protocol in Computer Networks
HDLC protocol in computer networks is a mechanism which is used in data-link layer. HDLC is a bit oriented protocol for making communication over point to point or multipoint links. Full form of HDLC is high level data link control protocol.
Bit oriented protocol means, here the data collected from the network layer are actually tackling in the form of sequence of bits in the frame.
Frequently Asked Questions
Some frequently asked questions on HDLC are as follow —
- What is the use of HDLC protocol?
- How does HDLC work?
- What is PPP session ?
- Is HDLC full duplex ?
- What is HDLC and SDLC ?
- Why do we need HDLC ?
After reading this HDLC Protocol Tutorial students can answer the all above questions.
The Synchronous Data Link Control Protocol (SDLC) protocol was first developed by IBM and later standardized by ISO (International Organization for Standardization) as High-Level Data Link Control protocol.
HDLC protocol in computer networks has mainly three stations defined.
Primary Station
The station that provides or transmits data is the primary station. It also controls the data flow.
Secondary Station
Secondary station works as a receiver. This station depends on the data flow of primary station.
Combined Station
This station can transmit and receive the data and can send response too. This station can work both as primary and secondary station.
HDLC Modes of Operation
Following three modes of operations are possible in HDLC Protocol.
Normal Response Mode (NRM)
This mode is suitable for both point-to-point and point-to-multipoint configuration.
Asynchronous Response Mode (ARM)
This mode is used for communication between primary and secondary stations.
Asynchronous balanced Mode (ABM)
This mode is used for point-to-point communication between combined stations.
Frame Structure in HDLC Protocol in Computer Networks
HDLC protocol in computer networks uses 0111 1110 as the beginning and end sequence, i.e. flag fields. They provide information about the beginning and ending of the frame.
These flag fields are sent so that the sender and the receiver can keep their clocks synchronized when the link is idle.
The address and control fields together are called as Header field. The body field is also called as payload which is of variable size.
The information from the network layer will be sent in this body part. CRC means cyclic redundancy check which is used in error detection.
HDLC protocol in Computer Networks frame format is the basis for other frame formats in the data link layer.
Types of HDLC Frames
The type of frame is determined by the control field of HDLC frame.
I-Frame (Information Frame)
If the first bit of control field is 0 then the frame is an I-frame and is going to carry the information.
S-Frame (Supervisory Frame)
If the first two bits of control field are 10 then it is an S-frame. Supervisory frame plays a vital role in error control and flow control.
U-Frame (Un-numbered Frame)
If first two bits of the control field are 11 then it is a U-frame. U-frame is mainly used for some miscellaneous activities, e.g. link management.
Conclusion and Summary
In this tutorial we have discussed the various aspects of HDLC protocol in computer networks including the HDLC full form. Various modes of operations and station of HDLC are discussed here in this tutorial.
I hope that various aspects of HDLC protocol in computer networks are discussed in this tutorial will be helpful to computer networks.
