One of the most important protocols that operate in the data link layer is the High-Level Data Link Control (HDLC) protocol. It helps in activating the link, exchanges the data and deactivates the link between two successive nodes.
Basic Features of High-level Data Link Control Protocol:
(i) Primary Station: It is responsible for controlling the operation of the link. Frames are issued by the primary station that is called Commands.
(ii) Secondary Station: It operates under the control of the primary station. Frames are issued by a secondary station that is called response. The primary station maintains a separate logical link with each secondary station on the line.
(iii) Secondary Station: It combines the feature of primary and secondary stations. A combined station may issue both command and response.
HDLC Frame Format :
HDLC uses asynchronous transmission. All the transmissions are in the form of frames. HDLC Frame Format figure is given below.
The flag, address and control fields which precede the information field constitute the header part. The FCS and flag field following the data field constitutes the trailer part.
Flag: Flag fields delimit the frame at both ends with unique pattern 01111110. A single flag may be used for closing a frame as well as for opening the next frame. Bit stuffing is used to avoid the pattern to appear inside the data part of the frame. With the use of bit stuffing, arbitrary bit patterns can be inserted into the data field of the frame. This property is known as Data Transparency.
Address: It identifies the secondary that transmitted or is to receive the frame. This field isn’t required for the point-to-point link but it always used for the sake of maintaining uniformity. This field is usually 8 bit long.
HDLC Data Transfer Modes:
There are three types of Data Transfer Mode in HDLC.
1. Normal Response Mode (NRM)
2. Asynchronous Balanced Mode (ABM)
3. Asynchronous Response Mode (ARM)
Normal Response Mode (NRM):
It is used with an unbalanced configuration. The primary may initiate data transfer to a secondary, but a secondary may only transmit data in response to a command from the primary.
Asynchronous Balanced Mode (ABM):
It is used with balanced configuration. Either of the combined stations may initiate transmission without receiving permission from the other combined station. ABM is most widely used in the world. It makes more efficient full Duplex point-to-point link as there is no polling overhead.
Asynchronous Response Mode (ARM):
It is used with an unbalanced configuration. The secondary may initiate transmission without explicit permission from the primary. The primary still retains the responsibility for the line, including initialization, error recovery and logical disconnection. ARM is rarely used, It applies to some special situations in which a secondary may need to initiate transmission.