1. EEC4113 Data Communication & Multimedia System Chapter 4: Flow Control by Muhazam Mustapha, August 2010

2. Learning Outcome
By the end of this chapter, students are expected to be able to explain link level flow control of transmission involving various protocols and techniques

3. Chapter Content Stop-and-Wait ARQ Go-Back-N ARQ Selective Reject ARQ
Line Utilization
High-level Data Link Control

4. Automatic Repeat Request (ARQ)

5. Stop-and-Wait ARQ Data and acknowledgement (ACK) frames are numbered.
For each correctly received data frame, there will be an ACK frame.
Only on receiving an ACK, sender sends the next frame.

6. Stop-and-Wait ARQ If there is a damaged data frame
Receiver discards the frame and NAK frame is returned.
On receiving NAK, sender re-transmits the frame.
If ACK or NAK is damaged or never reach sender
Sender re-transmits on wait time-out.

7. Stop-and-Wait ARQ Normal operation diagram: . . . Sender Receiver
Frame1
Wait
ACK1
Frame2
Wait
ACK2
Frame3
Wait
Time
ACK3
Frame4
Wait
ACK4
. . .

8. Stop-and-Wait ARQ Event of damage frame: . . . × Sender Receiver
ACK1 ×
Frame2
Error occur
Wait
NAK2
Retransmit
Frame2
Wait
Time
ACK2
Frame3
Wait
ACK3
. . .

9. Stop-and-Wait ARQ Event of damage ACK: . . . × Sender Receiver
Frame1
Wait
ACK1
Frame2
ACK2 ×
Error occur
Wait
Time
Time out
Frame2
. . .
Retransmit
Wait
ACK2

10. Stop-and-Wait ARQ Event of frame lost: . . . × Sender Receiver
ACK1 ×
Frame2
Frame lost
Wait
Time
Time out
Frame2
Retransmit
ACK2
. . .

11. Stop-and-Wait ARQ Event of lost ACK: . . . × Sender Receiver ACK Lost
Frame1
Wait
ACK1
Frame2
ACK2 ×
ACK Lost
Wait
Time
Time out
Frame2
Retransmit
Wait
ACK2
. . .

12. Go-Back-N ARQ A pipelined protocol.
Inefficiency of Stop-and-Wait: Sender has to wait for ACK for each and every data frame sent.
In Go-Back-N, sender doesn’t wait for ACK.
Instead, sender keeps sending but not too rapidly – sliding window constrained.
Sender will respond to ACK as it comes.

13. Go-Back-N ARQ
At receiver, all good, in-sequence frames must be ACK as normal.
All out-of-sequence frames will be discarded even if they are good.
On receiving bad frame, receiver sends NAK and requests for re-transmission of that frame.

14. Go-Back-N ARQ
At sender, on receiving an NAK, it re-transmits that frame, and all subsequence frames.
If sender finishes sending frames within its window, but lagging behind in receiving ACK, it re-transmits in-sequence starting from earliest frame not ACK-ed.

15. Go-Back-N ARQ Normal operation diagram: . . . Sender Receiver Time
Frame1
Frame2
ACK1
Frame3
ACK2
ACK3
Frame4
Frame5
ACK4
ACK5
Frame6
ACK6
Time
. . .

16. Go-Back-N ARQ Event of damage frame: . . . Error occur Receiver ×
Time
Receiver
Frame1
Frame2
Frame3
Frame4
Frame5
Frame6
Frame3
Frame4
Frame5 ×
ACK1
ACK2
NAK3
ACK3
ACK4
ACK5
Sender
Retransmit

17. Go-Back-N ARQ Event of lost frame: (sender window = 6 frames) . . .
Retransmit after time-out
Frame lost
Time
Receiver
Frame1
Frame2
Frame4
Frame5
Frame6
Frame7
Frame8
Frame3
Frame4
Frame5
Frame6
Frame7
Frame8 ×
Frame3
ACK1
ACK2
ACK3
ACK4
ACK5
Sender
1 frame buffered
2 frames buffered
3 frames buffered
4 frames buffered
4 frames buffered
4 frames buffered
5 frames buffered
6 frames buffered
1 frame buffered
2 frames buffered
3 frames buffered
4 frames buffered
4 frames buffered
4 frames buffered

18. Go-Back-N ARQ Event of lost ACK: (sender window = 6 frames) . . .
ACK damaged or lost
Time
Receiver
Frame5
Frame10
Frame1
Frame2
Frame3
Frame4
Frame6
Frame7
Frame8
Frame9
Frame11
Frame12
Frame13
Frame14 × ×
ACK2
ACK6
ACK1
ACK3
ACK4
ACK5
ACK7
ACK8
ACK9
ACK10
ACK11
Sender
1 frame buffered
2 frames buffered
3 frames buffered
4 frames buffered
4 frames buffered
5 frames buffered
4 frames buffered
4 frames buffered
4 frames buffered
5 frames buffered
4 frames buffered
4 frames buffered
4 frames buffered
4 frames buffered

19. Selective Reject ARQ
An extension of Go-Back-N but now the sender is asked to re-transmit only the damaged or lost frames.
Now the receiver is required to have buffer too (same size as the sender), and to respond to any subsequence frames after the error within its buffer limit.
The receiver will respond with a SREJ (or SREQ) upon a damaged or lost frame.

20. Selective Reject ARQ
The receiver will buffer the subsequence frames after the damaged or lost frames.
Once the missing one is obtained, the receiver buffer is flushed up to the next missing frame (or all if no more missing).
If the receiver run out of buffer before receiving the missing frame, it will stop responding, and this reduces the whole protocol to Go-Back-N ARQ.

21. Selective Reject ARQ
At sender side, sender is required to respond to SREJ immediately, then continue to normal frame sequence.
If it reaches to a condition where all its buffered frames are not ACK-ed, it is too will reduce to Go-Back-N ARQ and re-transmit them all in sequence.

22. Selective Reject ARQ Event of lost frame: . . . Frame damaged
Frame lost 1 3
4 3
5 4 3 7 7
8 7
9 8 7 10 11
Receiver
Frame5
Frame8
Frame1
Frame2
Frame3
Frame4
Frame2
Frame7
Frame9
Frame6
Frame10
Frame11 × ×
Frame6
Time
ACK1
SREJ2
ACK3
ACK4
ACK5
ACK2
ACK7
SREP6
ACK8
ACK9
ACK6 1
2 1
3 2 1
* 2
* 3* 2
7 6 5* 4* 3* 2
8 7 6
9 8 7* 6
* 6
* 7* 6
* 8* 7* 6
11 10
Sender

23. Windowed & Pipelined Processing
Go-Back-N and Selective Reject are pipelined and windowed ARQ.
Pipelined means they don’t wait for the transmitted frames to be ACK-ed, instead continue with transmission of next frame in buffer.
In Go-Back-N ARQ, only sender needs buffers, receiver doesn’t.

24. Windowed & Pipelined Processing
In Selective Reject ARQ, both sender and receiver need buffers.
For both sides, the buffer (window) size must be the same so that flushing can be properly timed.
And the window size <= half of possible frame number = 2n-1, n is the no. bits available to encode the frames – to avoid ambiguity in the frame numbers.

25. Detailed Window Processing
Sender sends frames to receiver
Sender’s windows shrinks according to the no. frames sent
Sent frames would still be kept in sender’s buffer
Receiver receives frames from sender
Receiver’s windows shrinks according to the no. frames received

26. Detailed Window Processing
Receiver sends ACK to sender
Receiver’s windows expands according to the no. ACK sent
Receiver’s windows restores to the original size
Sender receives ACK-s from receiver
Sender’s windows expands according to the no. ACK-s received
Sender’s windows restores to the original size

27. Detailed Window Processing
Sender Buffer
Initial 1 2 3 4 5 6 7 1 2 3 4 5 6 7
Receiver Buffer
After 3 frames sent and received 1 2 3 4 5 6 7 1 2 3 4 5 6 7
Sender Buffer 1 2 3 4 5 6 7 1 2 3 4 5 6 7
Receiver Buffer
After 3 ACK received 1 2 3 4 5 6 7 1 2 3 4 5 6 7
Sender Buffer 1 2 3 4 5 6 7 1 2 3 4 5 6
Receiver Buffer 1 2 3 4 5 6 7 1 2 3 4 5 6 7

28. Piggyback Transmission
There are times where the receiver too has to transmit data to the sender besides the acknowledgement data.
E.g. AJAX transaction, HTML page with many images (request for next image is made before the currently sending finished)
To improve efficiency of such bi-directional transmission, piggyback technique is used
Data frame with additional field to carry ACK

29. Piggyback Transmission
If the receiver wants to send both data and ACK to sender, it will send both fields.
If the receiver wants to send only ACK, a separate ACK frame is sent – not a piggybacked data.
If the receiver wants to send only data, the data is sent with the previous ACK which is ignored by the sender side due to duplication.

30. Line Utilization

31. Line Utilization
A measure (a) of ratio between no. bits present on a physical link (B) over no. bits per frame (L)
a = B / L
a is to be made as close to 1 as possible for the line to be utilized optimally.
This can be achieved by adjusting L.
Value of L needs to be optimized for various protocols so that a is close enough to 1.

32. Line Utilization B = R × d/V
B can be computed from data rate (R in bps), physical distance (d in meters) and velocity of propagation (V in ms-1):
B = R × d/V

33. Line Utilization
Consider 1Mbps (R) link between 2 ground stations communicating through satellite relay.
Geo-satellite: altitude of 36000km (d)
Propagation velocity: 3×108 ms-1 (V)
B = 240,000 bits
For a frame length of 8000 bits, a = 30

34. Line Utilization
8000bits
8000bits
240,000 bits
Sender finishes sending long before first bit physically arrived at receiver.
This is a serious inefficiency in link utilization.

35. Line Utilization
Consider 200 m (d) optical fiber operating at 1 Gbps (R).
Propagation velocity: 2×108 ms-1 (V)
B = 1000 bits
For a frame length of 8000 bits, a = 0.125

36. Line Utilization
8000bits
8000bits
1000 bits
Sender doesn’t finish sending long after first bit physically arrived at receiver.
This is also an inefficiency because the rest of the circuitry might need wait too long for this operation to finish.

37. High-level Data Link Control

38. HDLC High-level Data Link Control
A set of control commands made by OSI to control data link communication.
To accomplish the data link control task, HDLC defines the following:
3 types of stations
2 link configurations
3 data transfer modes

39. HDLC Station Types Primary station Secondary station Combined station
Controls operation of link
Frames sent-out are called commands
Maintains separate logical link to each secondary station
Secondary station
Under control of primary station
Frames sent out are called responses
Combined station
May send out commands and responses

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

41. HDLC Transfer Modes Three HDLC transfer modes
Normal Response Mode (NRM)
Asynchronous Balanced Mode (ABM)
Asynchronous Response Mode (ARM)

42. Normal Response Mode (NRM)
Used with unbalanced configuration
Primary initiates transfer to secondary
Secondary may only transmit data in response to command from primary
Host computer as primary
Terminals as secondary

43. Normal Response Mode (NRM)
Primary
Secondary
Command
Response
Unicast / Point-to-point
Secondary
Secondary
Primary
Command
Response
Response
Multicast

44. Asynchronous Balanced Mode (ABM)
Used with balanced configuration
Either station may initiate transmission without receiving permission
Most widely used
No polling overhead
Combined
Combined
Command/Response
Command/Response

45. HDLC Frame Structure HDLC uses synchronous transmission
All transmissions in frames
Single frame for all data and control exchanges
Flag
Address
Control
Information
FCS
Flag
8 bits
8 bits extendable
8 or 16 bits
variable
16 or 32 bits
8 bits

46. Flag Fields Delimit a frame at both ends
Address
Control
Information
FCS
Flag
8 bits
8 bits
Delimit a frame at both ends
An 8 bit sequence with a pattern
Each frame has 1 opening and 1 closing flag
Receiver hunts for flag sequence to synchronize
Bit stuffing used to avoid confusion with data containing

47. Bit Stuffing
Bit stuffing used to avoid confusion with data containing
Insert a 0 after every five consecutive 1-s
If receiver reads five 1-s, it checks the next bit:
If it is 0, it is ignored
If it is 1, and the next is 0, then it is a flag
If it is 1, and the next is 1, then it is abort flag

48. Bit Stuffing Before bit stuffing 111111111111011111101111110
After bit stuffing
Inserted bits

49. Address Field Identifies secondary station that sent or receive frame
Flag
Address
Control
Information
FCS
Flag
8 bits extendable
Identifies secondary station that sent or receive frame
Usually 8 bits long with binary 1 in LSB
LSB indicates if the byte is the last byte (binary 1) or not (binary 0)
MSB binary 0: unicast
MSB binary 1: multicast / broadcast

50. Control Field Control field defines the three types:
Flag
Address
Control
Information
FCS
Flag
8 or 16 bits
Control field defines the three types:
Information Frames (I-frames)
Supervisory Frames (S-frames)
Unnumbered Frames (U-frames)
First 1 or 2 bits of control field identify the frame type

51. Control Field – I-frames
Flag
Address
Control
Information
FCS
Flag
Carry user data
Flow and error control are piggybacked on this frame

52. Control Field – S-frames
Flag
Address
Control
FCS
Flag
Provide ARQ mechanism when piggyback is not used

53. Control Field – U-frames
Flag
Address
Control
Management Information
FCS
Flag
Provide supplemental link control functions

54. Control Field – 8 bits format
I-frame
N(S)
P/F
N(R)
S-frame 1 S
P/F
N(R)
U-frame 1 1 M
P/F M
N(S): Send sequence number N(R): Receive sequence number S: Supervisory function bits M: Unnumbered function bits P/F: Poll/Final bit

55. Control Field – 16 bits format2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
I-frame
N(S)
P/F
N(R)
S-frame 1 S
P/F
N(R)
N(S): Send sequence number N(R): Receive sequence number S: Supervisory function bits P/F: Poll/Final bit

56. Poll / Final Bit
If it is a command frame (from primary to secondary), then,
The bit is called P (poll) bit
If the bit is 1, it means the command is polling (soliciting) response from the peer
If it is a response frame (from secondary to primary), then,
The bit is called F (final) bit
If the bit is 1, it means the response is responding to a soliciting (polling) command

57. Information Field Appears only in I-frames or some U-frames
Flag
Address
Control
Information
FCS
Flag
variable
Appears only in I-frames or some U-frames
It is variable length, but must be in integral number of bytes

58. FCS Field
Flag
Address
Control
Information
FCS
Flag
16 or 32 bits
Contains the Frame Check Sequence for error detection purposes
Usually 16 bit CRC (if 16 FCS) or may be 32 bit CRC (if 32 FCS)

59. HDLC Commands & Responses
Name
Command/Response
Information (I)
C/R
Supervisory (S)
Receive ready (RR / ACK)
Receive not ready (RNR)
Reject (REJ)
Selective Reject (SREJ)
Unnumbered (U)
Set normal response mode (SNRM) C
Set asynchronous response mode (SARM)
Set asynchronous balanced mode (SABM)
Set initialization mode (SIM)

60. HDLC Commands & Responses
Name
Command/Response
Unnumbered (U) continued
Disconnect (DISC) C
Unnumbered acknowledge (UA) R
Disconnected mode (DM)
Request disconnect (RD)
Request initialization mode (RIM)
Unnumbered information (UI)
C/R
Unnumbered poll (UP)
Reset (RSET)
Exchange identification (XID)
Test (TEST)
Frame reject (FRMR)

61. HDLC Operation
Three phases:
Initialization
Data transfer
Disconnect

62. Example Operation (1) . . . A B Setup and disconnect SABM SABM UA DISC
Time out
SABM UA
. . .
DISC UA

63. Example Operation (2) N(S) N(R) A B Two way data exchange I,0,0 I,0,1
RR,4

64. Example Operation (3) A B Busy condition I,3,0 RNR,4 RR,0,P RNR,4,F
RR,4,F
I,4,0

65. Example Operation (4) A B Reject recovery I,3,0 I,4,0 I,5,0 REJ,4

66. Example Operation (5) A B Timeout recovery I,2,0 RR,3 I,3,0 RR,0,P
RR,3,F
I,3,0
RR,4