High level Data link control , HDLC , Point to point and multi point link ,

1. HDLC
(High-level data link control)
Prof. Chintan Patel
Chintan.patel@marwadieducation.edu.in
Prof. Chintan Patel

2. Point to Point Link and Multipoint Link
Prof. Chintan Patel

3. Introduction
• HDLC is a bit oriented data link layer protocol used for Point
to point and Multi point Links.
• ISO standard ISO 3309 and ISO 3335
• Supports both Half-Duplex and Full duplex Links
• Two subsets :
– SDLC (Synchronous data link control)
– LAP-B (Link access procedure-Balanced)
Prof. Chintan Patel

4. HDLC stations
• Primary station
– Used as a controlling station on the link
– Issues a command to secondary stations.
– Responsible for organization of data flow
– Responsible for Error recovery at data link layer
• Secondary station
– Works under control of primary station
– No ability of controlling links
– It can only send response frame when it requested by Primary station.
• Combined station
– Can send command as well as send response without any permission from
any other station on link.
Prof. Chintan Patel

5. HDLC Station Configuration
• Unbalanced Configuration
– Consists of one primary stations and One or more secondary stations.
– Its called unbalanced because one station controls all other
– We can used it for Full duplex as well as Half duple links
– We can use it in Point to point and Multi point Links.
Prof. Chintan Patel

6. • Balanced Configuration
– Consists of two or more combined stations.
– Full duplex and Half duplex links
– Point to point networks
Prof. Chintan Patel

7. • Symmetric Configuration
– Not widely used
– Two unbalanced point-to-point links
Prof. Chintan Patel

8. HDLC operational Mode
• A mode in HDLC is the relationship between two devices involved
in an exchange
• The mode describes who controls the link.
• Exchanges over unbalanced configurations are always conducted
in Normal Response Mode(NRM).
• Operational Modes :
– Normal Response Mode(NRM)
– Asynchronous Response Mode(ARM)
– Asynchronous Balanced Mode(ABM)
Prof. Chintan Patel

9. • 1. Normal Response Mode
– Primary station initiate the transfer to secondary station
– After receiving permission from the primary station, the secondary
station initiates its transmission
– Normal Response Mode is only used within an unbalanced
configuration.
Prof. Chintan Patel

10. • 2. Asynchronous Balanced Mode
– Unbalanced Configuration.
– Primary station does not initiate transmission unlike NRM.
– Secondary station does not need permission of primary station for
transmission unlike NRM.
– Secondary station must wail for Ideal channel before frame
transmission.
– Primary station still responsible for Error recovery , link setup and Link
disconnect.
Prof. Chintan Patel

11. • 3. Asynchronous Balanced Mode
– Used in case of Combined station
– combined stations do not require any sort of instructions to perform
any task on the link
– no need for permission on the part of any station
Prof. Chintan Patel

12. HDLC Frame structure
• Information frame (I-frame) = used to transport Data + Control info
• Supervisory frame (S-Frame) = used to transport only control info
• Unnumbered frame (U-Frame) = Used for managing the link
Prof. Chintan Patel

13. • Flag Field( F ) : 8 bits
• Address Field( A ): 8 bits
• Control Field( C ): 8 or 16 bits
• Information Field( I ) OR Data Variable; Not used in some
frames
• Frame Check Sequence( FCS ) : 16 or 32 bits
• Closing Flag Field( F ) : 8 bits
Prof. Chintan Patel

14. • Flag Field :
– Each frame must start and end with FLAG field
– Flag sequence is : 01111110
• Two other bit sequences are used in HDLC as signals for the stations on the
link. These two bit sequences are:
– Seven 1's, but less than 15 signal an abort signal. The stations on the link know
there is a problem on the link.
– 15 or more 1's indicate that the channel is in an idle state.
• HDLC is not concerned with any specific bit code inside the data stream. It is
only concerned with keeping flags unique.
Prof. Chintan Patel

15. • Address Field :
– The address field (A) identifies the primary or secondary stations
involvement in the frame transmission or reception
– In an unbalanced configuration, the A field in both commands and
responses refer to the secondary station
– balanced configuration, the command frame contains the destination
station address and the response frame has the sending station's
address.
Prof. Chintan Patel

16. • The control Field :
– control field is a 1- or 2-byte segment of the frame used for flow and error control.
• 1. Information Transfer Format:
– The frame is used to transmit end-user data between two devices.
• 2. Supervisory Format:
– The control field performs control functions such as acknowledgment of frames, requests
for re-transmission, and requests for temporary suspension of frames being transmitted.
• 3. Unnumbered Format:
– This control field format is also used for control purposes.
– It is used to perform link initialization, link disconnection and other link control functions.
Prof. Chintan Patel

17. Prof. Chintan Patel

18. • If first bit is 0 , its I Frame(Information frame)
• If first 2 bits are 10 , its S Frame (Supervisory frame)
• If first 2 bits are 11 , its U Frame(Unnumbered frame)
Prof. Chintan Patel

19. • Supervisory Frame Format Command and Response
• Receive Ready (RR) is used by the primary or secondary station to indicate that it is ready to
receive an information frame and/or acknowledge previously received frames. Indicated
using 00 as a supervisory code
• Receive Not Ready (RNR) is used to indicate that the primary or secondary station is not ready
to receive any information frames or acknowledgments. Indicated using 10 as a supervisory
code. works as a congestion control mechanism to slow down the sender.
• Reject (REJ) is used to request the retransmission of frames. Indicated using 01 bits. Used in
Go Back-N protocol as a Negative ACK.
• Selective Reject (SREJ) is used by a station to request retransmission of specific frames. An
SREJ must be transmitted for each erroneous frame; each frame is treated as a separate
error. Only one SREJ can remain outstanding on the link at any one time. Indicated using 11
bits. Used in Selective repeat protocol as a Negative ACK
Prof. Chintan Patel

20. The Poll/Final Bit (P/F)
• The 5th bit position in the control field is called the poll/final bit, or P/F bit.
• It can only be recognized when it is set to 1.
• If it is set to 0, it is ignored.
• The poll/final bit is used to provide dialogue between the primary station
and secondary station.
• The primary station uses P=1 to acquire a status response from the
secondary station. The P bit signifies a poll.
• The secondary station responds to the P bit by transmitting a data or status
frame to the primary station with the P/F bit set to F=1. The F bit can also
be used to signal the end of a transmission from the secondary station
under Normal Response Mode. Prof. Chintan Patel

21. • The Frame check Sequence field
– This field contains a 16-bit, or 32-bit cyclic redundancy check bits. It is
used for error detection
Prof. Chintan Patel

22. Reference
• IIT Kharagpur , NPTEL.
Prof. Chintan Patel

23. Prof. Chintan Patel