The slide consist of short information about the HDLC.

1. HDLC
Presented by: Sonang Rai

2. 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.

3. HDLC defines three type of stations

4. 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.

6. HDLC data transfer mode

7. 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

8. HDLC frames

9. HDLC frame structure

10.  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.

11. 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

12. 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)

13. 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.

14. 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.

16. Example of connection and disconnection

17. Example of piggybacking without error

18. Example of piggybacking with error