1. CIS 321 Data Communications & Networking
Chapter 11 – Data Link Control and Protocols

2. Introduction
Protocol – set of rules governing communication specific to one or more layers of the OSI model
Data link protocols define the rules devices use to implement data link layer functions
Contain rules for line discipline, flow control, and error control
University of South Alabama
Computer and Information Sciences

3. Data Link Layer Functions
University of South Alabama
Computer and Information Sciences

4. Flow Control
Coordinates amount of data sent before receiving acknowledgement
Purpose: prevent overwhelming receiver
Buffer overflow
University of South Alabama
Computer and Information Sciences

5. Error Control Error detection and error correction
Receiver informs sender of any frames lost or damaged and coordinates retransmission of those frames by the sender
Usually handled via automatic repeat request (ARQ)
University of South Alabama
Computer and Information Sciences

6. Flow and Error Control Mechanisms
Stop-and-Wait ARQ
Go-back-N ARQ
Selective-Repeat ARQ
University of South Alabama
Computer and Information Sciences

7. 11.2 Stop-and-Wait ARQ
Sender keeps copy of last frame sent and waits for ACK for that frame
Next frame cannot be sent until ACK has been received
Frames are numbered alternately 0 and 1
Damaged or lost frames are resent
Repeats until EOT is sent
University of South Alabama
Computer and Information Sciences

8. Stop-and-Wait Normal Operation
University of South Alabama
Computer and Information Sciences

9. Lost or Damaged Frame University of South Alabama
Computer and Information Sciences

10. Lost Acknowledgement University of South Alabama
Computer and Information Sciences

11. Delayed Acknowledgement
University of South Alabama
Computer and Information Sciences

12. Stop-and-Wait
Advantage: simplicity; each frame is checked and ACK’d before next frame is sent
Numbering of frames prevents duplication
Disadvantage: inefficiency; slow
Frame and ACKs use entire bandwidth
If distance is long between devices, time spent waiting can be significant
University of South Alabama
Computer and Information Sciences

13. Bidirectional Transmission
Possible if two parties have separate channels for full-duplex transmission or share same channel for half-duplex transmission
University of South Alabama
Computer and Information Sciences

14. Piggybacking Method of combining data frame and acknowledgement
Saves bandwidth due to less overhead from separate data frame and ACK frame into one frame
University of South Alabama
Computer and Information Sciences

15. Piggybacking University of South Alabama
Computer and Information Sciences

16. Sliding Window
Sender may transmit several frames before needing an ACK
Much more efficient; receiver may use a single ACK to confirm multiple frames
Sliding window refers to upper and lower limit on number of frames that may be transmitted before ACK is required
Frames must be numbered to allow receiver to identify which frame is acknowledged
University of South Alabama
Computer and Information Sciences

17. 11.3 Go-Back-N ARQ
Allows for more efficient transmission – send multiple frames before requiring an ACK
Specify a window or range of sequence numbers of frames that may be received
University of South Alabama
Computer and Information Sciences

18. Sliding Window
Receiver includes number of next frame it expects to receive in ACK
Sender then knows all previous frames through that number have been received
University of South Alabama
Computer and Information Sciences

19. Sender Window Window contains 0 to 2m -1 frames
Window shrinks as frames are sent out
Once ACK arrives, window expands equal to number of frames acknowledged by ACK
University of South Alabama
Computer and Information Sciences

20. Receiver Window Size of window is always 1
Receiver expects the next ordered frame (must always be in order)
Any frame arriving out of order is discarded
University of South Alabama
Computer and Information Sciences

21. Control Variables
Sender : S – sequence number of recently sent frame; SF - sequence number of first frame in window; SL – sequence number of last frame in window
Window size is W = SL - SF + 1
Receiver : R – sequence number of next frame expected
University of South Alabama
Computer and Information Sciences

22. Go-Back-N Operation
Timers: sender sets a timer for each frame (none for receiver)
Acknowledgement – receiver sends positive ACK; silent on damaged or out of order frames
Resending Frames – sender sends set of frames from damaged up to last one sent and ACK’d
University of South Alabama
Computer and Information Sciences

23. 11.4 Selective Repeat ARQ
Go-Back-N less efficient since all out of order or damaged frames must be resent in order
Selective Repeat is a more efficient method, yet required more processing
University of South Alabama
Computer and Information Sciences

24. Sender and Receiver Windows
Sender window and control variables are same as Go-Back-N
Receiver window is same size; looks for range of sequence numbers
Requires two control variables to define window boundaries: RF and RL
Also defines a negative acknowledgement (NAK) to report sequence number of damaged frame
University of South Alabama
Computer and Information Sciences

25. Selective Repeat ARQ University of South Alabama
Computer and Information Sciences

26. Selective Repeat ARQ, Lost Frame
University of South Alabama
Computer and Information Sciences

27. Comparisons between Go-back-n and Selective-Reject
Sending only specific damaged or lost frames requires complexity of sorting and more storage is required in select-reject
Go-back-n is typically used due to simplicity
University of South Alabama
Computer and Information Sciences

28. 11.5 HDLC
High-level Data Link Control – protocol supporting half-duplex and full-duplex communication over point-to-point and multipoint links
University of South Alabama
Computer and Information Sciences

29. HDLC Modes of Communication
Relationship between two devices involved in an exchange
Defines who controls the link
Two modes:
Normal response mode (NRM)
Asynchronous balanced mode (ABM)
University of South Alabama
Computer and Information Sciences

30. Normal Response Mode
Refers to standard primary-secondary relationships
Used for all exchanges in unbalanced configurations
Primary can issues commands
Secondary must have permission from primary before responding or sending data
University of South Alabama
Computer and Information Sciences

31. NRM
University of South Alabama
Computer and Information Sciences

32. Asynchronous Balanced Mode
All stations are equal
Stations in point-to-point configurations act as both primary and secondary
University of South Alabama
Computer and Information Sciences

33. HDLC Frames
Three types; each functions as an envelope to transmit a specific type of message
Information frames (I-frames) – transports user data and control info relating to user data
Supervisory frames (S-frames) – used to transport control info for data link layer flow and error controls
Unnumbered frames (U-frames) – used for system mgmt and link mgmt
University of South Alabama
Computer and Information Sciences

34. HDLC Frame University of South Alabama
Computer and Information Sciences

35. Flag Field
Marks the beginning and end of frame and provides synchronization with
University of South Alabama
Computer and Information Sciences

36. Address Field
Contains address of secondary station that is originator or destination of the frame
If created by primary, contains a to address
If created by secondary, contains a from address
May be one byte or several bytes long
University of South Alabama
Computer and Information Sciences

37. Control Field One- or two-byte segment for flow management
Fields differ depending on frame type
First or first and second bits identify type of frame
All three frame types contain a poll/final (P/F) bit; used to identify whether frame was sent by primary to a secondary or from secondary to primary
University of South Alabama
Computer and Information Sciences

38. Information Field
Contains user’s data in an I-frame; network management in a U-frame; not included in an S- frame
Often used in a data frame to acknowledge receipt of another separate frame – called piggybacking
University of South Alabama
Computer and Information Sciences

39. FCS Field Error detection field Stores either two- or four-byte CRC
University of South Alabama
Computer and Information Sciences

40. Different Control Frames
I-frame – used for user information and transport and piggyback acknowledgements
S-frame – used for acknowledgement, flow control, and error control when piggybacking is not appropriate
U-frame – used to exchange session management and control information between devices
University of South Alabama
Computer and Information Sciences

41. Data Transparency
To address the possibility that a bit pattern may match flag field indicator and be misinterpreted, bit stuffing may be used
Anytime 5 consecutive 1s are encountered, a redundant 0 is inserted
Identifies that the current sequence is not a flag
University of South Alabama
Computer and Information Sciences

42. Bit Stuffing and Removal
University of South Alabama
Computer and Information Sciences

43. Coming Up… Ch 12 Point-to-Point Access: PPP (brief)
University of South Alabama
Computer and Information Sciences

44. Credits
All figures obtained from publisher-provided instructor downloads
Data Communications and Networking, 3rd edition by Behrouz A. Forouzan.  McGraw Hill Publishing, 2004
University of South Alabama
Computer and Information Sciences