HDLC(High level Data Link Control)

High-Level Data Link Control (HDLC)
 HDLC (High-level Data Link Control) is a group of protocols or rules for
transmitting data between network points (sometimes called nodes).
 It is a bit oriented protocol.
 HDLC was defined by ISO for use on both point-to-point and multipoint data
links.
 It supports full-duplex communication
 Data is organized into a unit (called a frame) and sent across a network to a
destination that verifies its successful arrival.

HDLC defines three type of stations

HDLC stations
1. Primary station
 Has the responsibility of controlling the operation of data flow .
 Frames issued by the primary station are called commands.
2. Secondary Station
 Operates under the control of the primary station.
 Frames issued by a secondary station are called responses.
 The primary station maintains a separate logical link with each secondary station.
3. Combined Station
 Acts as both as primary and secondary station.

1. Normal Response Mode (NRM)
 Secondary station can send ONLY when the primary station instruct it to do so
 Two common configurations
- Point-to-Point link (one primary station and one secondary station)
- Multipoint link (the primary station maintain different sessions with different
secondary stations)
2. Asynchronous Response Mode (ARM)
 More independent secondary station
 Can send data or control information without explicit permission to do so (note that it
can not send commands)
3. Asynchronous Balanced Mode (ABM)
 Mainly used in point-to-point links, for communication between combined stations
 Either stations can send data, control information and commands

 Flag: 01111110- indicates start and ending of frames
 Address field:
• When a primary station is sending a frame, the address field contains the receiver
identity
• If a secondary station is sending the frame, the address field contains the sender
identity
 FCS: 16-bit CRC
 Control flag: It is used for flow and error control.
 Information flag: It contains the user’s data or control information.

Types of HDLC frames
 Unnumbered frames, used in link setup and disconnection, and hence do not
contain ACK.
 Information frames, which carry actual information. Such frames can piggyback
ACK in case of ABM
 Supervisory frames, which are used for error and flow control purposes and hence
contain send and receive sequence numbers

Different supervisory frames
 SS=00, Receiver Ready (RR), and N(R) ACKs all frames received up to and
including the one with sequence number N(R) - 1
 SS=10, Receiver Not Ready (RNR), and N(R) has the same meaning as above
 SS=01, Reject; all frames with sequence number N(R) or higher are rejected,
which in turns ACKs frames with sequence number N(R) -1 or lower.
 SS=11, Selective Reject; the receive rejects the frame with sequence number
N(R)

Information Frames
 I-frames carry user's data and control information about user's data.
 I-frame carries user data in the information field.
 The I-frame format is shown in diagram.

Unnumbered frames
 U-frames are reserved for system management and information carried by them is
used for managing the link
 U-frames are used to exchange session management and control information
between the two connected devices.
 Information field in U-frame does not carry user information rather, it carries
system management information.

Example of connection and disconnection

Example of piggybacking without error

Example of piggybacking with error

HDLC(High level Data Link Control)

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