Data Link Control Protocols

1
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 gt tf a gt 1
Link is shorter than frame tp lt tf a lt 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 lt 1
a gt 1
Overhead of an ACK frame in both cases !
tp gt tf
tp lt tf
Useful TX time
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 shorter than frame a lt 1
Link is longer than frame a gt 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 81 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/(2a1))

15
Sliding Windows Flow Control

• Avoids the low efficiency of Stop-and-wait when a
  gt 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
FCS
HDLC Frame
17
Sliding Window Send/Receive Cycle
Window covers frames to be received
Window covers frames to be sent
RX
TX
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
Shrink
W 7 Max of frames TXed without ACKed
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
Expand
Delete
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 agt1
• 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

• 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 a1) ? (2 x 0.125 1) ? 1.25
• i.e. W 2 (A window of just 2 frames!)
• - easily achieved in practice!

• 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 a1) ? (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/(2a1) 7/(61)
  11.5 gt 1.6 for Stop and wait.

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
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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,
  1 (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
RR 1
ACK
RR 0
Lost Frame Scenario
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
F1 now expected
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 waiting to be received
Frames received correctly. Have or will soon be
acknowledged
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, i1,
• RX gets frame (i1) 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 i1) 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
  (i1)
• This RR frame is lost on its way to TX
• Two Scenarios
• Scenario 3.A
• A later Acknowledgement from RX, e.g. RR (in)
  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
Got 5 not 4 ? REJ 4 and beyond
3
4
5
6
7
0
1
2
2
1
0
7
Go back in window until you meet the
rejected Frame , and resume transmission from
there
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
W ? 2k 1

• Size of the window must be W ? 2k 1 i.e. W lt 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 lt 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 4 - 4 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
How are such flow and error mechanisms
implementedHigh-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
k 3 bits
Control Field
k 7 bits
58
HDLC Frame Format

• Flag
• Size 1 Byte
• Special pattern 0 1 1 1 1 1 1 0 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 01111110 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
• Inserts a 0 after each consecutive five 1s of
  non-flag data
• (e.g. 01111100.or 01111101)
• 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 1s)

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.

01111100
Frame Splitting
(Bit stuffed)
01111110
lost frame
01111110
Frame Merging
(Bit stuffed)
01111100
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 3 bits

66
HDLC I-Frames
Duplex Link
- Source - Destination
Carries 2 sequence numbers
8-bit Control filed
With Piggybacking
of next Frame expected Positive ACK RR N(R)
of Frame sent (This frame)

• 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
0
1
1
0
Assume
RR RNR REJ SREJ
Carries only 1 sequence number
First 2 bits 10 identifies frame as an S-Frame
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 (11111111) (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

• FCS field
• 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 X16X12X51), 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
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 128
  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
mode7-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 F1
• 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
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

• 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