1. William Stallings Data and Computer Communications 7th Edition
Chapter 7
Data Link Control Protocols

2. Data Link Control Protocols
need layer of logic above Physical
to manage exchange of data over a link
frame synchronization
flow control
error control
addressing
control and data
link management
In this chapter, we shift our emphasis to that of sending data over a data communications link. To achieve the necessary control, a layer of logic is added above the physical layer, referred to as data link control or a data link control protocol. Some of the requirements and objectives for effective data communication between two directly connected transmitting-receiving stations:
• Frame synchronization: Data are sent in blocks called frames. The beginning and end of each frame must be recognizable.
• Flow control: The sending station must not send frames at a rate faster than the receiving station can absorb them.
• Error control: Bit errors introduced by the transmission system should be corrected.
• Addressing: On a shared link, such as a local area network (LAN), the identity of the two stations involved in a transmission must be specified.
• Control and data on same link: the receiver must be able to distinguish control information from the data being transmitted.
• Link management: Procedures for the management of initiation, maintenance, and termination of a sustained data exchange over a link.

3. Effective D.C. Requirements
Frame synchronization
Beginning and end of each frame must be recognizable
Flow control
Sender must not send frames at a rate faster than the receiver.
Error control
Bit errors introduced by the transmission system should be corrected.
Addressing
On a multi point line, the identity of the two stations involved must be specified.

4. Effective D.C. Requirements
Control and data on same link
The receiver must be able to distinguish control information from the data being transmitted.
Link management
Initiation, maintenance, and termination of a sustained data exchange require a fair amount of coordination and cooperation among stations

5. Flow Control
Ensuring the sending entity does not overwhelm the receiving entity
Preventing buffer overflow
influenced by:
transmission time
time taken to emit all bits into medium
propagation time
time for a bit to traverse the link
assume here no errors but varying delays
Options
Stop-and-Wait
Sliding Window

6. Model of Frame Transmission

7. Stop-and-Wait Flow Control
How does it work?
Source transmits frame
Destination receives frame and replies with acknowledgement
Source waits for ACK before sending next frame
Destination can stop flow by not sending ACK
Works well for a few large frames
Not good when fragmentation is used
Fragmentation  Large block of data may be split into smaller frames
Used when:
Limited buffer size
Errors detected sooner (when whole frame received)
On error, retransmission of smaller frames is needed
Prevents one station occupying medium for long periods

8. Fragmentation Large block of data may be split into small frames
Limited buffer size
Errors detected sooner (when whole frame received)
On error, retransmission of smaller frames is needed
Prevents one station occupying medium for long periods
Stop and wait becomes inadequate

9. Performance Issues B = length of the link in bits
R = data rate of the link, in bps
d = length, or distance, of the link in meters
V = velocity of propagation, in m/s
L = the number of bits in the frame

10. Performance Issues Where d  Link distance (m)
V  Velocity of propagation (m/s)
L  Length of frame in bits
R  Data Rate (bps)
a =
Propagation Time
Transmission Time
d/V
L/R Rd VL =

11. Stop and Wait Link Utilization
Tx Time = 1, Propagation time = a

12. Example 7.1

13. Example 7.1- cont.

14. Sliding Windows Flow Control
Allow multiple frames to be in transit
Receiver has buffer W long
Transmitter can send up to W frames without ACK
Each frame is numbered
ACK includes number of next frame expected
Sequence number bounded by size of field (k)
Frames are numbered modulo 2k

15. Sliding Window Diagram

16. Sliding Window Enhancements
Receiver can acknowledge frames without permitting further transmission
RNR  Receive Not Ready
Must send a normal acknowledge to resume
If duplex, use piggybacking
If no data to send, use acknowledgement frame
If data but no acknowledgement to send, send last acknowledgement number again, or have ACK valid flag (TCP)

17. Example Sliding Window

18. Error Control Detection and correction of errors
Lost frames
Damaged frames
techniques for error control are based on some or all of the following ingredients :
Error detection (ch. 6)
Positive acknowledgment
Retransmission after timeout
Negative acknowledgement and retransmission
Collectively, these mechanisms are all referred to as automatic repeat request (ARQ)

19. Automatic Repeat Request (ARQ)
Stop and wait
Go back N
Selective reject
selective retransmission

20. Stop and Wait Source transmits single frame Wait for ACK
If received frame damaged, discard it
Transmitter has timeout
If no ACK within timeout, retransmit
If ACK damaged, transmitter will not recognize it
Transmitter will retransmit
Receive gets two copies of frame
use alternate numbering and ACK0 / ACK1

21. Stop and Wait - Diagram see example with both types of errors
pros and cons
simple
Inefficient
Referred to as Idle RQ

22. Go Back N Based on sliding window
If no error, ACK as usual with next frame expected
Use window to control number of outstanding frames
If error, reply with rejection
Discard that frame and all future frames until error frame received correctly
Transmitter must go back and retransmit that frame and all subsequent frames

23. Go Back N - Handling Damaged Frame Lost Frame
error in frame i so receiver rejects frame i
transmitter retransmits frames from i
Lost Frame
frame i lost and either
transmitter sends i+1 and receiver gets frame i+1 out of seq and rejects frame i
or transmitter times out and send ACK with P bit set which receiver responds to with ACK i
transmitter then retransmits frames from i
Suppose that station A is sending frames to station B. After each transmission, A sets an acknowledgment timer for the frame just transmitted. Suppose that B has previously successfully received frame (i – 1) and A has just transmitted frame i. The go-back-N technique takes into account the following contingencies:
1. Damaged frame.
2. Lost Frame
3. Damaged Ack
4. Damaged Reject

24. Go Back N - Handling Damaged Acknowledgement Damaged Rejection
receiver gets frame i, sends ack (i+1) which is lost
acks are cumulative, so next ack (i+n) may arrive before transmitter times out on frame i
if transmitter times out, it sends ack with P bit set
can be repeated a number of times before a reset procedure is initiated
Damaged Rejection
reject for damaged frame is lost
handled as for lost frame when transmitter times out

25. Sequence # vs Window Size
Example:
seq #’s: 0, 1, 2, 3, 4, 5, 6, 7
window size = 8
Frame 0 is sent and get back RR1
Then send frames 1, 2, 3, 4, 5, 6, 7, 0, and gets another RR1
This could mean all frames are received correctly or all frames were damaged.

26. Go Back N - Diagram

27. Selective Reject Also called selective retransmission
Only rejected frames are retransmitted
Subsequent frames are accepted by the receiver and buffered
Minimizes retransmission
Receiver must maintain large enough buffer
More complex logic in transmitter
hence less widely used
useful for satellite links with long propagation delays

28. Selective Reject - Diagram

29. Selective repeat: dilemma
Station A sends frames 0 through 6 to station B.
Station B receives all seven frames and cumulatively acknowledges with RR 7.
Because of a noise burst, the RR 7 is lost.
A times out and retransmits frame 0.
B has already advanced its receive window to accept frames 7, 0, 1, 2, 3, 4, and 5. Thus it assumes that frame 7 has been lost and that this is a new frame 0, which it accepts.
Q: what relationship between seq # size and window size?

30. High Level Data Link Control
HDLC
ISO 33009, ISO 4335

31. HDLC Station Types Primary station Secondary station Combined station
Controls operation of link
Frames issued are called commands
Maintains separate logical link to each secondary station
Secondary station
Under control of primary station
Frames issued called responses
Combined station
May issue commands and responses

32. HDLC Link Configurations
Unbalanced
One primary and one or more secondary stations
Supports full duplex and half duplex
Balanced
Two combined stations

33. HDLC Link Configurations

34. HDLC Data Transfer Modes (1)
Normal Response Mode (NRM)
Unbalanced configuration
Primary initiates transfer to secondary
Secondary may only transmit data in response to command from primary
Used on multi-drop lines
Host computer as primary
Terminals as secondary
The computer polls each terminal for input

35. HDLC Transfer Modes (2) Asynchronous Balanced Mode (ABM)
Balanced configuration
Either station may initiate transmission without receiving permission
Most widely used
No polling overhead

36. HDLC Transfer Modes (3) Asynchronous Response Mode (ARM)
Unbalanced configuration
Secondary may initiate transmission without permission form primary
Primary responsible for line
rarely used

37. Frame Structure Synchronous transmission All transmissions in frames
Single frame format for all data and control exchanges

38. Frame Structure

39. Flag Fields Delimit frame at both ends 01111110
May close one frame and open another
Receiver hunts for flag sequence to synchronize
Bit stuffing used to avoid confusion with data containing
0 inserted after every sequence of five 1s
If receiver detects five 1s it checks next bit
If 0, it is deleted
If 1 and seventh bit is 0, accept as flag
If sixth and seventh bits 1, sender is indicating abort

40. Bit Stuffing
Example with possible errors

41. Address Field
Identifies secondary station that sent or will receive frame
Usually 8 bits long
May be extended to multiples of 7 bits
LSB of each octet indicates that it is the last octet (1) or not (0)
All ones ( ) is broadcast

42. Control Field Different for different frame type
Information - data to be transmitted to user (next layer up)
Flow and error control piggybacked on information frames
Supervisory - ARQ when piggyback not used
Unnumbered - supplementary link control
First one or two bits of control filed identify frame type
Remaining bits explained later

43. Control Field Diagram

44. Poll/Final Bit Use depends on context Command frame Response frame
P bit
1 to solicit (poll) response from peer
Response frame
F bit
1 indicates response to soliciting command

45. Information Field Only in information and some unnumbered frames
Must contain integral number of octets
Variable length

46. Frame Check Sequence Field
FCS
Error detection
16 bit CRC
Optional 32 bit CRC

47. HDLC Operation Initialization Data Transfer
It signals the other side that initialization is requested.
It specifies which of the three modes (NRM,ABM,ARM) is requested.
It specifies whether 3- or 7-bit sequence numbers are to be used.
Data Transfer
After the initialization is accepted, then a logical connection is established
Both sides may begin to send user data in I-frames, starting with sequence number 0.
The N(S) and N(R) fields of the I-frame are sequence numbers that support flow control and error control.
S-frames are also used for flow control and error control

48. HDLC Operation Disconnect
Either HDLC module can initiate a disconnect ((DISC) frame)
some sort of fault, or at the request of its higher-layer user

49. Examples of Operation (1)

50. Examples of Operation (2)

51. PERFORMANCE ISSUES

52. Performance Issues TF  Time to send a frame and receive and ACK
Stop-and-Wait Flow Control (No Errors)
A sends a frame to B
After receiving frame, B sends and ACK to A
Process is repeated n times
Thus, total time to transmit n frames can be expressed as
Where
TF  Time to send a frame and receive and ACK

53. Performance Issues Where tprop  Propagation time from A to B
tframe  Time to transmit a frame
tproc  Processing time at each station
tack  Time to transmit and ACK

54. Performance Issues
Since a = tprop / tframe 

55. Performance Issues
Sliding Window Flow Control (No Errors)

56. Performance Issues Stop-and-Wait ARQ (With Errors)
Where Nr  Expected # of transmissions of a frame

57. Performance Issues Stop-and-Wait ARQ (With Errors)
Where Nr  Expected # of transmissions of a frame
P is a single frame is in error

58. Performance Issues
Selective Reject ARQ (With Errors)

59. Performance Issues
Go-back-N ARQ (With Errors)

60. Problem Assignments Solve all the review questions
Try the following problems:
7.1, 7.4, 7.5, 7.9, 7.13, 7.17, 7.18