1. Chapter 7: Data Link Control Protocols
WK 13
COE 341: Data & Computer Communications (T061) Dr. Radwan E. Abdel-Aal
Chapter 7:
Data Link Control Protocols

2. Where are we: Chapter 7: Data Link: Flow and Error control Data Link
Chapter 8: Improved utilization: Multiplexing
Physical Layer
Chapter 6: Data Communication: Synchronization, Error detection and correction
Chapter 4: Transmission Media
Transmission Medium
Chapter 5: Encoding: From data to signals
Chapter 3: Signals and their transmission over media, Impairments

3. Contents Flow Control Stop-and-Wait flow control
Sliding-Window flow control
Error Control
Stop-and-Wait ARQ
Sliding-Window ARQ
- Go-Back-N ARQ
- Selective-Reject ARQ
High-Level Data Link (HDLC) Protocol
Basic Characteristics
Frame Structure
Operation

4. What is Data Link Control?
The logic or procedures used to convert the raw stream of bits handled by the physical layer into a “reliable” data link
Performed by the Data Link Control Protocol (Layer)
Requirements and Objectives:
Frame-level synchronization: Recognize frame start and end
Flow control: Regulate sending of frames to match the ability of RX to absorb them
Error control: Retransmission of damaged or unacknowledged frames
Addressing: Identify stations on a multipoint link
Allow control information to go with data on same link
Link management: To initiate, maintain, and terminate data exchange

5. Flow Control
Required to avoid the TX overwhelming the RX by the flow of data it sends
RX does not ‘absorb’ the received data instantly!
It buffers (temporarily stores) the data it receives in a finite-size buffer to do some processing before sending it upward to higher layers
Without flow control, the RX buffer may overflow and data gets lost…

6. Flow Control over a link (assume no error)
For now, assume:
No frames lost (loss over a single link is a kind of error in recognizing the frame start)
No frames arrive in error
Frames arrive in the same order they were sent, after a propagation delay

7. Model for Frame Transmission over a link
Frame lost:
(error in start flag)
Do you allow only one
Or multiple frames to
travel on the link
at any given time
Frame damaged:
(error in data)

8. Main Flow Control Protocols

9. Stop and Wait Frames sent and acknowledged one at a time:
Source transmits frame and waits for ACK
Destination receives frame and replies with an acknowledgement ACK
When source gets ACK, it sends next frame
Disadvantages:
Destination can stop the flow by not sending ACK
(but we can use timeout to overcome this)
Not efficient - wastes time in waiting until short data frames arrive on long links (frame does not ‘fill’ the link)
i.e. I would like to make frames long in time (large in size for a given data rate)
Tf = LTb = L/R

10. Data Fragmentation (smaller frames!)
However, large blocks of data are often split into several smaller frames- Why?
Limited frame buffer size at RX
To reduce frame error rate: remember? FER = 1-(1-BER)F
Errors are detected sooner (when frame arrives)
On error, we need to retransmit a smaller amount of data
On a shared medium, e.g. a LAN, this ensures that a transmitting station does not occupy the medium for a long time
 “Stop and wait” is inadequate in such situations where frames are “short”
Link utilization depends on the frame length in time relative to the link propagation time.

11. Stop and Wait Link Utilization: The ‘a’ ratio
Total number of bits in a frame = L bits, Tb = bit duration
R = Data rate, bps
Frame transmission time, tf sec: Time taken by the TX to emit all the frame bits into medium tf is large for large frames and low data rates
d = Link physical length, m
V = Velocity of propagation over link, m/s
Link Propagation time, tp sec: Time for a bit to traverse the link (link length in time)
Link length in bits, B bits/link: The number of bits that ‘fill’ the link if data is transmitted continuously
a = Propagation time / Transmission time = tp/tf
Smaller ‘a’ means better link utilization (with large frames)
If tf = 1, a = Propagation time tp

12. Stop and Wait Link Utilization (Efficiency)
Let us define link utilization, U, as:
Will demonstrate on next slide that U is given by:
Where ‘a’ is
Utilization is high for small a: U approaches 1, i.e.  100% efficiency
Shorter links, Higher propagation velocities
Larger frames sizes, Lower data rates
Efficiency is poor for large a:
Longer links, Lower propagation velocities
Smaller frames sizes, Higher data rates

13. Stop and Wait Link Utilization: 2 cases:
Link is longer than frame: tp > tf: a > 1
Link is shorter than frame: tp < tf: a < 1
tf = 1
tp = a
Let frame transmission time tf = 1  a = Propagation time = tp
Link ‘full’ most of the time
 Better utilized
Link ‘empty’ most of the time !
 Underutilized
a < 1
a > 1
+ Overhead of an ACK frame in both cases !
tp > tf
Useful TX time
tp < tf
Elapsed time

14. Stop and Wait Efficiency: Example
Compare the efficiency of stop-and-wait flow control for two links using the parameter ‘a’:
Fame size, L = 1000 characters of 8 bits each, = 8000 bits
Link is longer than frame: a > 1
Link is shorter than frame: a < 1
Satellite link between 2 ground stations
d = 2 x 36,000 km, Data rate, R = 1 Mbps
Typical wave velocity, V = 3 x 108 m/s
Frame TX time, tf = L/R = 8000/(1x106) = 8 ms
Propagation time, tp = d/V
= 2x36x106/(3x108) = 240 ms
End of first frame reaches RX after = 248 ms from start
ACK takes 240 ms more to reach TX, i.e. it starts sending 2nd frame after 488 ms
Utilization = 8/488 = 1.6% (=1/(2a+1))
200-m optical fiber link
Data rate, R = 1 Gbps
Typical wave velocity, V = 2 x 108 m/s
Frame TX time, tf = L/R = 8000/(1x109) = 8 ms
Propagation time, tp = d/V
= 200/(2x108) = 1 ms
End of first frame reaches RX after 8+1 = 9 ms from start
ACK takes 1 ms more to reach TX, i.e. it starts sending 2nd frame after 10 ms
Utilization = 8/10 = 80% (=1/(2a+1))

15. Sliding Windows Flow Control
Avoids the low efficiency of Stop-and-wait when a > 1
Allows multiple frames to be “in transit” simultaneously on the link
RX keeps a buffer store (in memory) for W frames
So, TX can send up to W frames without waiting for ACK
Each frame carries a sequence number
ACK from RX shows the number of next expected frame
TX keeps a list of frames it can send
RX keeps a list of frames it expects to receive
These lists form sliding windows at TX and RX that shrink/expand as frames are sent, and ACKs are sent/received
Hence, Sliding Windows Flow Control

16. Frame Sequence Numbering
Frame sequence number is limited by the size of a corresponding field in the frame, e.g. k bits
Frames are numbered modulo 2k
e.g. for k = 3, frame sequence # is modulo 23 = 8, i.e.= 0,1,..., 7, 0,1,…
Window size (W) is limited to a maximum of 2k-1
i.e. Wmax = 7 in the above example
Frame Sequence Number
k = 3 bits
FCS
HDLC Frame

17. Sliding Window Send/Receive Cycle
Window covers
frames to be received
Window covers
frames to be sent TX RX
Flexible-
(Not rigid) Windows
When you want to ACK some frames:
1. Delete ACKed frames from buffer
2. Expand Window to receive more
3. Send Acknowledgement
1. Delete ACKed frames from buffer
2. Expand Window to send more
ACKs
When you want to send more:
Send frames
Shrink window past received frames
Frames
Receive Frames
Shrink window past sent frames
Acknowledging frames is a separate issue from receiving them

18. Sliding-Window Diagram
FCS
Deletion
Marker
k = 3 bits, W = 7
At TX
Window covers
frames to be sent
Expand
Delete
W = 7
Max # of frames TXed without ACKed
Shrink
Send/Receive Frames
Receive/Send ACKs
Remove ACKed frames from buffer
At RX
Window covers
frames to be received

19. Example Sliding Window
Max window size of 7 RX TX
Shrink
RR = Receiver Ready
Delete
Expand
Received up to 2
Ready to receive 7 frames starting with 3
Received up to 3
Ready to receive 7 frames starting with 4

20. Sliding Window Enhancements
RR n: Positive receive ACK that asks for more (received up to frame n-1 and ready for n)
Receiver can also acknowledge receiving frames without permitting further transmission:
(Receive Not Ready RNR)
Example RNR 5: “Received frames up to 4, but not ready for 5 and beyond yet”
When it becomes ready, RX must send a normal acknowledge (RR 5) later for TX to resume sending frames

21. Sliding Window in a Duplex System
In a Duplex System, destination also transmits data back to source
Piggybacking: Utilizing data frames from destination to carry ACK signals back to source to improve channel utilization
Additional field in the data frame for use only by +ive (RR) ACK
If you have no data to send now or your ACK is not RR, use a normal (dedicated) ACK frame (e.g. RR or RNR)
If data is to be sent but no acknowledgement needed, insert the last acknowledgement number to prevent RX from using the number existing in the ACK field of the data frame.
(When RX station receives a duplicate ACK, it ignores it)

22. Sliding Window Protocol: Efficiency
Much more efficient than Stop and wait for a>1
Treats link as a pipeline to be filled with several frames in transit simultaneously- not just one by one
With window size W and assuming no error, link utilization, U, is given by (Appendix 7A)
where a = Propagation time/Frame transmission time = tp/tf
i.e. Sliding window protocol can achieve 100% utilization for W  (2a + 1).
The smaller the W needed for this the better! (Why?). This requires a small a (so small a is still advantageous!)

23. Sliding Window Efficiency: Example
Shorter links are better (small a)
Compare the efficiency of Sliding Window flow control for two links using the parameter ‘a’:
Fame size, L = 1000 characters of 8 bits each, = 8000 bits
Satellite link between 2 ground stations
d = 2 x 36,000 km, Data rate, R = 1 Mbps
Typical wave velocity, V = 3 x 108 m/s
Frame TX time, tf = L/R = 8 ms
Propagation time, tp = d/V = 240 ms
a = tp / tf = 30
100 % link utilization is achieved with window size W:
W  (2 a+1)  (2 x 30 +1)  61
W = 61, k = 6 bit (Large window, large buffers at TX, RX)
For k = 3 bits, W = 7:
Utilization U = W/(2a+1) = 7/(61) = 11.5% > 1.6% for Stop and wait.
200-m optical fiber link
Data rate, R = 1 Gbps
Typical wave velocity, V = 2 x 108 m/s
Frame TX time, tf = L/R = 8 ms
Propagation time, tp = d/V = 1 ms
a = tp / tf = 0.125
100 % link utilization is achieved with window size W:
W  (2 a+1)  (2 x )  1.25
i.e. W = 2 (A window of just 2 frames!)
- easily achieved in practice!

24. Error Control
WK 14
Use of retransmission to handle errors detected in frames (Backward Error Handling)
This process is called Automatic Repeat Request (ARQ)
Types of Problems:
Damaged frames
(Frame arrives at RX but in error)
Lost frames (Noise burst damages frame header beyond recognition- so not recognized by RX)
For connections across a network, frames arriving too late e.g. due to network congestion- Will be ignored or dropped by time-out.

25. Error Control Techniques:
Apply error check mechanism (chapter 6)
Send Positive acknowledgment: (for one or more frames)
From RX for Error-free frames, e.g. RR i
Send Negative acknowledgement requesting retransmission of a lost or damaged frame:
RX sends negative ACK for damaged or lost frames, requesting retransmission, e.g. REJ i
How does RX detect a “lost” frame? Through receiving the next frame “out of sequence” – Unexpected (frames are numbered!)
Retransmission after timeout:
TX automatically retransmits a frame that has not been acknowledged following a predetermined time-out interval

26. Categories of Error Control Mechanisms
Main types of ARQ-based
standard error control mechanisms

27. Stop and Wait ARQ: Possible Scenarios Scenario for Damaged/Lost Frame
TX transmits a single frame (keeping a copy)
Then waits for ACK from RX
If frame reaches RX damaged (in error):
RX discovers this through error detection
It then discards the frame, and does not send ACK
TX “times out” on waiting for ACK
… and then retransmits the frame again automatically
RX thus receives only one correct copy of the frame
From the RX side, this is identical to a lost frame scenario

28. Stop and Wait ARQ: Scenario for Lost ACK
ACK from RX for a correct frame is lost (reaches TX damaged beyond recognition):
TX will timeout and retransmit the same frame again!
RX gets two good copies of that frame!
Without numbering the frames, RX will consider both copies as two different valid frames
(but data duplication is not “reliable” data transport!)
To avoid this, TX labels frames alternately as 0, (enough for Stop and Wait)  duplication detected at RX
RX uses ACK0 & ACK1, Similar to sliding window RRn:
ACK0: Received 1 and ready for 0 (better named RR 0)
ACK1: Received 0 and ready for 1 (better named RR 1)

29. Stop and Wait ARQ Lost Frame Scenario Lost ACK Scenario = RR 1 = RR 0
Same scenario if F0 was
received damaged (in error)
but RX kept quiet about it!
Lost ACK
Scenario
RX gets two good copies of F1.
Labeling frames allows RX to
detect this and discard
one of them.

30. Stop and Wait - Pros and Cons
Simple
Inefficient (As seen with flow control)
For improved efficiency, we use sliding-window based ARQ (Continuous ARQ)

31. Sliding Window ARQ
Improves line utilization by sending up to W frames before worrying about ACK
A form of Pipelining (several tasks started before 1st task is finished)
TX uses a window to mark frames to be transmitted until they are sent and acknowledged
The window size W should be ≤ 2k – 1, k is the size of the frame sequence field in the frame header,
Frame are given sequence numbers modulo 2k, i.e. for k = 3:
0,1,2,3,4,5,6,7,0,1,2,…..
W is fixed in this protocol, but may be variable in others
Each time a proper ACK is received for a number of frames the TX window slides past them, hence the name Sliding Window. This:
Releases those frames for deletion from TX buffer memory
Introduces new frames for transmission

32. TX Sliding Window
- Window now has a fixed width (W) and slides rigidly as one piece
upon receiving ACKs.
- Within the window, frame sending is handled using a send (S) pointer
Frames already sent
but not yet ACKed
Frames that can be sent
S Pointer to next frame to be sent
F0, F1
Positively ACKed
Slide the window as a whole
(Important: Pointer is not pushed with window)

33. RX Sliding Window Error Control
Size of the RX window for error control is always 1
The RX window contains the sequence number for the frame expected to be received next
If a different frame arrives (i.e. out of sequence arrival), it is immediately discarded and the window does not slide
Once the expected frame arrives correctly, the window slides one step to point to the next expected frame

34. RX Sliding Window F0 now expected F0 Received Correctly

35. Sliding Window ARQ: Summary
Uses sliding windows (now rigid- moves as a whole) at TX and RX to track frame movement
TX uses timeout on waiting for ACK
If no error: RX acknowledges with RR i, where i is number of the next frame expected
As you receive
expected frame
without error k
As you receive ACKs
Frames received correctly
and have been or
will soon be acknowledged
Frames received correctly.
Have or will soon be
acknowledged
Frames waiting
to be received
As you transmit
Expected next frame to be received
Next frame to be sent
Size = 1
Size ≤ 2k-1

36. Sliding Window ARQ: Two main standard approaches: Go Back N
Selective Reject

37. Sliding Window ARQ: Go Back N
Error Scenarios:
Will consider the following error scenarios
Damaged Frame
Lost Frame
Lost ACK
Lost Positive ACK (RR)
Lost Negative ACK (REJ)

38. Go Back N ARQ: Error Scenarios
Damaged Frame:
RX received frame i damaged
RX discards frame i and all subsequent frames until frame i is received correctly
RX either:
Scenario 1.A: Sends a negative ACK (REJ i )
TX must go back (hence the name go back N) and retransmit that frame and all subsequent frames that were transmitted in the mean time
Scenario 1.B: Does not send REJ (relies on TX time out) Handled as a lost frame (next)
(and also as a lost negative ACK)

39. Go Back N ARQ: Error Scenarios
Lost Frame:
RX expects frame i but does not get it,
So TX does not get any ACK for it
Scenario 2.A: TX can send more frames
TX carries on sending subsequent frames, i+1, …
RX gets frame (i+1) out of sequence (as it did not get frame i) This allows RX to detect the problem
RX then either:
Sends REJ i (Note i, not i+1) to TX
Or: Takes no further action (relies on TX time out)
In both cases, TX goes back and retransmit frame i and all subsequent frames transmitted in the mean time

40. Go Back N ARQ: Error Scenarios
Lost Frame, i:
TX kept sending, RX does nothing, TX times out
LHS of TX window:
Frames sent but not yet
ACKed
RX frame pointer
does not leave ‘2’
until F2 is correctly
received
RR 2
Slide W to
Uncover
F0, F1
F3 Arriving out
of sequence
(Not the next
expected frame ),
RX ignored it, but
did nothing
Go back 1
On F2
S points to next frame to
be TXed
R points to next frame to
be RXed

41. Go Back N ARQ: Error Scenarios
Lost Frame, Contd.
Scenario 2.B:
TX does not send further frames after i, and RX does not send any RR or REJ (RX could not detect the problem!. It does not know that a frame was sent and got lost)
TX times out waiting for RX response
Scenario B-1:
TX sends a polling command (RR with P bit = 1) to force RX to report its receiving status by sending RR i
When TX receives the RR i response, it retransmits frame I and all subsequently sent frames
Scenario B-2:
TX retransmits frame i after timeout

42. Go Back N ARQ: Error Scenarios
Lost Positive ACK (RR) from RX:
RX gets frame i OK and sends a positive ACK: RR (i+1)
This RR frame is lost on its way to TX
Two Scenarios:
Scenario 3.A:
A later Acknowledgement from RX, e.g. RR (i+n) manages to reach TX before TX times out. This solves the problem, since ACKs are cumulative
Scenario 3.B:
TX times out before receiving any subsequent RR acks:
TX sends a polling command (RR with P bit = 1) to force RX to report its receiving status by sending RR i.
TX Repeats step above a few times, if no success it initiates a Reset procedure

43. Go Back N ARQ: Error Scenarios
Lost Negative ACK (REJ) from RX:
Similar to RX not sending REJ
i.e. Scenario 1.B for Damaged Frame.

44. Go Back N Example Transmitter Receiver TX does not wait Got 0, 1
Ready for 2
Frame 4
lost on
the way
Got 3
Ready for 4 7 1 2 3 4 5 6 7 1 2
Got 5 not 4
 REJ 4 and beyond
Go back in window until
you meet the rejected
Frame #, and resume
transmission from there
Go back 3
(4, 5, 6, 7) and
Retransmit
them
RR 7 (+ ive ACK)
lost on the way.
TX times out
Before getting
any subsequent
RRs after lost Ack
TX polls RX
To send its
Receiving status
This is a
command
F bit = 1
Any corrective action
needed here at TX?

45. Example:
Two neighboring nodes (A and B) use a sliding-window go-back-N for error control with a 3-bit sequence number. The window size is 4. Assuming A is TX and B is RX, show the window positions at A for the following situations:
Before A sends any frames
After A sends frames 0, 1, 2 and receives RR 2 from B
Later, after A sends frames 3, 4, and 5 and receives RR 5 from B
At TX
Send Next

46. Window Size limit for Go-back-N ARQ
By mistake
RX gets 2
copies of F0
Window Size limit for Go-back-N ARQ
W  2k – 1
Size of the window must be W  2k – 1 i.e. W < 2k where “k” is the number of bits reserved (in the control field) for the sequence number
Let k = 2,
i.e. W should be < 4
By comparing the figures opposite for W = 3 and W = 4, justify the need for this limit on window size in Go Back N
ACK
ACK
on F0
TX goes back
to send F0 F0
on F0 F0 F0
TX goes back
to send F0 x F0
All happened before timeout F0
F0 sent twice in both cases. Mistake is detected only on the left (for the correct W size)

47. Selective Reject (Selective Repeat ARQ)
Also called selective retransmission
RX requests retransmission of only the rejected frame using SREJ i
Subsequent frames received after the rejected frame are buffered at RX (not thrown away as in Go Back N ARQ)
TX retransmits only the frame that was specifically rejected and those that timed out
 Less retransmission traffic than Go Back N

48. Selective Reject: Example
- Waiting for is lost
5 received out of sequence, Problem detected, SREJ 4 sent
5, 6
Only rejected frame (4) retransmitted
(not 4,5,6)
Normal transmission resumes where left (7)
TX sends:
5,6,4,7,0
(complex sequencing)
Acknowledge up to 6 (5 & 6 were kept!)
+ ive ACK lost on its way!
TX times out before getting subsequent RRs to the lost ones
So it polls RX for its receiving status
F bit = 1
Things turned out OK

49. Selective Reject: Pros and Cons
Minimizes retransmission  Better link utilization
(Useful where link utilization is poor- sending a frame is an inefficient process) e.g. short frames on long (e.g. satellite) links
But more complex: (so, less common than Go back N)
Receiver:
Must maintain large enough buffer to save post-SREJ frames until missing frame arrives
Needs logic for inserting requested frame in place when it arrives later
Transmitter:
Needs logic to allow sending the requested frame out of normal sending sequence
Also, more restricted window size, W:
With k bits, max window size is 2(k-1) (vs 2k-1 for Go Back N),
e.g. for k = 3: Wmax = 4 for SREJ, Wmax = 7 for GO Back N

50. Runs in the Data Link Layer (Layer 2 in OSI) Main Functions:
How are such flow and error mechanisms implemented: High-Level Data Link Control Protocol (HDLC)
HDLC is the most common data link control protocol and forms the basis for many others
Runs in the Data Link Layer (Layer 2 in OSI)
Main Functions:
Flow Control: Data is transmitted by TX- only as fast as RX can absorb it.
Error Control: Objective: Pass data up to higher layer exactly as transmitted, i.e.:
Without error,
Without loss,
Without duplication,
and in the correct order

51. High-Level Data Link Control Protocol (HDLC)
To satisfy various applications, HDLC defines:
Three types of communicating stations
Two types of link configurations, and
Three data transfer modes of operation.

52. High-Level Data Link Control Protocol (HDLC)
Station types:
Primary Station (PS): (e.g. computer)
Responsible for controlling link operation
Control frames issued by the PS are called commands
Secondary Station (SS): (e.g. terminal, sensor)
Operates under the control of a primary station
Control frames issued by the SS are called responses
Combined Station (CS):
Issues both commands and responses

53. High-Level Data Link Control Protocol (HDLC)
Link configurations:
Determined by the types of stations on the link
Unbalanced (different status): e.g. One primary station plus one or more secondary stations
Balanced (same status): e.g. Two combined stations

54. High-Level Data Link Control Protocol (HDLC)
Data Transfer modes: (Who can send?, what? and when?)
For unbalanced link configuration: (Response Modes)
Normal Response Mode (NRM)
Secondary may send data only in response to a command from primary
e.g. Computer (PS) connected to a number of terminals (SS) over a multi drop line. Computer polls terminals for data
Asynchronous Response Mode (ARM) Secondary may send data without explicit permission from primary. Primary still retains link control (initialization, error recovery, logical disconnection, …)
(Rarely used)

55. High-Level Data Link Control Protocol (HDLC)
Data Transfer modes: (Who can send?, what? and when?)
For balanced link configuration
Asynchronous Balanced Mode (ABM) Either combined stations may send data without obtaining permission from the other station
Most widely used (no polling involved)
e.g. full duplex point-to-point

56. HDLC – Frame Structure
HDLC Uses Synchronous Transmission, i.e. large frames
Frame consists of the following ‘fields’:
Flag fields at start and end: for frame-level sync
Address field: for addressing in multi-point links
Control field: for Flow and Error control
Information field: (payload data or link management data)
FCS field: for error detection

57. HDLC – Frame Structure Frame Address Field Control Field k = 3 bits

58. HDLC Frame Format Flag: Address: Control:
Size: 1 Byte
Special pattern used as frame begin/end and synch.
Used for Header and trailer
Address:
Size: 1 Byte (or extendible to more bytes for larger networks)
If primary station created the frame, the address is that of the destination secondary station
If secondary station created the frame, the address is that of the source secondary station
Networks not using “primary/secondary” (e.g. Ethernet) use 2-Byte address (source/destination)
Control:
Size: 1 or 2 Bytes
Identifies frame type (I, S, U)
Used for error & flow control functions
Information: Payload of user data (I Frames) or link management data (U)
Size: Varies (as multiple bytes) from network to network. Always fixed within a network
I frames contains user data received from the higher layer (Network layer)
FCS:
Size: 2 or 4 Bytes (depending on the divisor P used)
Implements CRC error detection

59. HDLC – Frame Structure: Flag field
Flag Field: unique pattern at both start and end of frame
Used for frame-level synchronization
This pattern should not exist in any other part of the frame
Two ways to ensure this:
Ensure that higher layers avoid using these pattern in the data they generate (causes lack of data transparency)
TX uses bit stuffing for data (inserts a 0 after each consecutive five 1s) to ensure data transparency at higher layers

60. HDLC Bit Stuffing: At TX: At RX:
Inserts a 0 after each consecutive five 1s of non-flag data
(e.g ….or )
At RX:
After detecting the preamble flag, RX monitors incoming bits – when a pattern of five 1s appears; the 6th and the 7th bit are checked:
If 00 or 01  Bit 6 is a stuffed 0  Remove it
If 10  This is the postamble flag  end of frame
If 11  ABORT (Error- there must have a 0 after every five 1’s)

61. HDLC Bit Stuffing & Removal
Data handed in for transmission
Would this data have
caused any
frame sync problem?
TX ensures that the flag pattern
does not occur in non-data fields

62. HDLC Bit Stuffing
Problems: Single-bit errors could split a frame into 2 or merge two frames into 1.
Frame
Splitting
(Bit stuffed)
“lost” frame
Frame
Merging
(Bit stuffed)

63. HDLC Frame Types HDLC defines three types of frames
Frame type determined by first bits in the control field
User Information frames (I-frames): 8 or 16 bit control field
Used to transport user data
In a duplex system, they can also carry control messages regarding previously received data to be carried along with data TXed (piggybacking)
Supervisory frames (S-frames): 8 or 16 bit control field
Used to transport control information for the data transfer (ACKs) only- Short frame, Carries No Data
Unnumbered frames (U-frames): 8 bit control field
Convey link management functions
Does not contain a frame sequence number (N or S) (unnumbered)
Can carry Management data

64. HDLC I, S & U Frames Format
Arrow indicates that the first transmitted field is the Header flag, then the address, then the control…
RR j
RNR j
REJ j
SREJ j

65. HDLC I-Frames
Designed to carry user data delivered from the Network layer
They can also include limited flow & error control information (piggybacking in duplex links)
Their Control field can be either 8 bits or 16 bits (Defined at link initialization)
An 8-bit Control field consists of:
First bit identifies the frame type: “0”: I-frames “1”: Non I-frames
Next 3-bits, called N(S), indicate the sequence number of the transmitted frame
So frame sequence numbers are 0,1,2,3,4,5,6,7
Next bit is called P/F (Poll or Final)
When a primary station polls other stations, it sets this bit to 1 (P bit)
When a secondary station responds to a poll, sets this bit to 1 (F bit)
i.e. same bit has different meanings depending on source
Next 3-bits, called N(R), define the frame sequence number for a default RR N(R) ACK carried when piggybacking is used (next expected frame)
Only difference in 16-bit Control field: k = 7 bits for N(S) and N(R) instead of bits

66. HDLC I-Frames
Duplex Link
- Source
- Destination
Carries 2 sequence numbers
8-bit Control filed
With Piggybacking
# of Frame sent
(This frame)
# of next Frame expected
Positive ACK: = RR N(R)
16-bit control field: Extends N(S) and N(R) to 7 bits each instead of 3 bits, allowing larger windows to be used
Note: Control field contains no ‘Function’ part So, only the + ive ACK function (RR) can be sent as a default by piggybacking!
Frame sequence
Numbering is modulo ?

67. HDLC Supervisory (S)-Frames (S is also for short!)
The workhorse for sending separate ACKs. Used when:
- No user data to send, or ACK is not RR, i.e. (RNR, REJ, SREJ)
This is the only method to send ACKs that are different from RR
Example: RNR 6 1 1 1
Assume: RR
RNR
REJ
SREJ
First 2 bits:
’10’ identifies frame as an S-Frame
Carries only 1 sequence number 00 01 10 11

68. HDLC U-Frames (no frame sequence number)
Used for Link management operations
Examples of
Link Management
Functions
(up to 32)
As a
As a

69. HDLC U-Frames
Examples of
Network Management
Functions
As a
As a

70. HDLC Frame Structure – Address Field
Extended Address Field
‘1’ marks last octet of extendible address
Address field identifies the secondary destination station transmitting the frame or intended to receive the frame
Not needed for point-to-point links (only one source and one destination) - but included for uniformity
Extendable – in multiples of 7 bits
The unique address ( ) (single octet) is used by the primary to broadcast to all secondary stations

71. HDLC Frame Structure – Control Field
Frame Type:
Extended frame sequence #s
(7 instead of 3 bits)
Data-carrying,
Allows Piggybacking
Why N(R) on an I frame?
ACKs such as:
REJ N(R)
Additional link
control functions
I or not
S or U
Poll/Final (P/F) bit:
In command frames (P): used to solicit response from peer entity
In response frames (F): indicate response is the result of soliciting command

72. HDLC Frame Structure – Information/FCS Fields
Information field: (in I and U frames only)
Present ONLY in I-frames (user data) and some U-frames (link management data)
Contains an integer number of octets (bytes)
Variable number of octets – up to some system defined maximum
m bytes

73. HDLC Frame Structure – Information/FCS Fields
CRC Error detection code
Calculated from ALL remaining bits in frame (excluding the two flags)
Normally 16 bits: (F is 1-bit shorter than P)
- (CRC-CCITT polynomial = X16+X12+X5+1), or
- 32-bit optional FCS using CRC-32

74. HDLC – Operation
HDLC Operation: Exchange of I-frames, S-frames, and U-frames between two stations
Table 7.1 (slide 69) lists types of Control/Response functions for various frame types
The operations of HDLC involve three phases:
Link Setup or Initialization (by either side): U-Frames
Both agree on various options
Actual Data Transfer (by the two sides): I- and S-Frames
Exchange of user data and control info for flow and error control
Link Disconnect (by either side): U-Frames
Indicating termination of operation

75. Some U-frame commands and responses
Command/response
Meaning
SNRM
Set normal response mode
SNRME
Set normal response mode (extended)
SABM
Set asynchronous balanced mode
SABME
Set asynchronous balanced mode (extended) UP
Unnumbered poll UI
Unnumbered information UA
Unnumbered acknowledgment RD
Request disconnect
DISC
Disconnect DM
Disconnect mode
RIM
Request information mode
SIM
Set initialization mode
RSET
Reset
XID
Exchange ID
FRMR
Frame reject
Subset of Table 7.1

76. HDLC – Operation 1. Initialization (Link Setup) 2. Data Transfer,
3. Link Disconnect
“A” issues one of 6 set mode commands: and sets a timer:
SNRM, SARM, SABM (k = 3 bits: Modulo 8 frame sequencing)  8-bit control field or
SNRME, SARME, SABME (k = 7 bits: Modulo frame sequencing)  16-bit control field
B responds with either:
UA (Unnumbered ACK) (if it agrees), or:
or DM (Disconnect mode) (if request is rejected)
A receives the UA, initializes its variables for data exchange, and data exchange begins
After finishing: to disconnect, A or B send DISC command
B responds with UA. Any outstanding unACKed I-frames may be lost. These can be recovered by higher layers
1. Link Setup
2. Data Transfer
3. Link Disconnect
SABME: Set Asynchronous balanced/extended mode;7-bit frame sequence

77. HDLC – Operation Normal Data Transfer:
Once initialization is complete, a logical path is established.
Both sides start sending I-frames (Full-duplex exchange) starting with sequence number 0
N(S), N(R) are sequence numbers to support flow and error control in the send and receive directions, respectively
N(R) is the ACK for the I-frame received; it allows the HDLC module to indicate the I-frame number it expects to receive next (Note: No control function used for this positive ACK- understood by default as RR)
When no reverse user data (I-frame) needs to be sent, a dedicated RR should be sent on an S-frame
If no new acknowledgement needs to be sent, the last N(R) value is repeated
same
Repeat N(R)
if no new Fs
received +1 +1 +1
S-Frame

78. HDLC – Operation Busy condition: Note use of P/F bit
When A is unable to keep up with the speed of the transmitter “B” or buffer is full
A sends RNR, to halt transmission from B
To check the readiness of A, B periodically sends RR frame with P set
Once the busy condition is cleared at A, it responds with an RR and F=1
An RR with F set indicates it is a response to a previous polling using RR, P +1
At last !!
B sending 4 and
waiting for 0

79. For any – ive ACK must use an S-Frame!
HDLC – Operation
Reject Recovery:
I-frame 4 was lost
B receives I-frame 5 (out of order) – responds with REJ 4
A resends I-frame 4 and all subsequent frames previously sent
(Go-back-N)
For any – ive ACK must use an S-Frame!

80. HDLC – Operation A sends I-frame 3 – but it is lost
Timeout Recovery:
A sends I-frame 3 – but it is lost
Timer expires before acknowledgement arrives
A polls B with RR, P = 1
B responds with RR, F =1, indicating it is still waiting for frame 3
A responds by retransmitting I-frame 3
This time it gets a + ive ACK from B