1. CE 4228 Data Communications and Networking
Data Link Control

2. Data Link Control – Outline
Transmission timing
Flow control
Channel coding
Error control
HDLC

3. Interface

4. Interface DTE DCE DCE communicates data and control info with DTE
Data terminal equipment or data processing devices
Do not usually include data transmission facilities
Need another interface
DCE
Data circuit terminating equipment
Modem, NIC
Transmit bits on medium
DCE communicates data and control info with DTE
Done over interchange circuits with interface standards

5. Interface – Characteristics
Mechanical
Connection plugs
Electrical
Voltage, timing, encoding
Functional
Data, control, timing, grounding
Procedural
Sequence of events

6. Digital Transmission Timing
Timing problems require a mechanism to synchronize the transmitter and receiver
Two solutions
Asynchronous
Synchronous

7. Asynchronous Data transmitted one character at a time
5 to 8 bits
Timing only within each character
Re-synchronized with each character

8. Asynchronous – Diagram

9. Asynchronous – Behavior
In a steady stream, interval between characters or length of stop element is uniform
In idle state, receiver looks for transition 1 to 0
Next seven intervals, the length of a character, are read
Then look for next 1 to 0 for next character
Simple, cheap
Overhead of 2 or 3 bits per character, roughly 20%
Good for data with large gaps such as keyboard

10. Synchronous – Bit Level
Block of data transmitted without start/stop bits
Clocks must be synchronized
Can use separate clock line
Good over short distances
Subject to impairments
Embed clock signal in data
Manchester encoding
Carrier frequency for analog transmission

11. Synchronous – Block Level
Need to indicate start and end of block
Use preamble and postamble
Bit patterns to indicate start and end
More efficient than asynchronous
Lower overhead

12. Synchronous – Diagram

13. Frame Transmission

14. Flow Control
Ensuring the sending entity does not overwhelm the receiving entity
Preventing buffer overflow
Transmission time
Time taken to emit all bits into medium
Propagation time
Time for a bit to traverse the link
Stop-and-wait
Sliding window

15. Stop-and-Wait Source transmits frame
Destination receives frame and replies with acknowledgement
Source waits for ACK before sending next frame
Destination can stop flow by not send ACK
Work well for a few large frames
May be necessary for a low quality link
High error/loss rate

16. Stop-and-Wait – 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
Prevent one station occupying medium for long periods
Stop-and-wait becomes inadequate for multiple smaller frames per message in a high quality link, such as LAN

17. Stop-and-Wait – Analysis
T = Tt + Tp + Tr + Tt(ACK) + Tp(ACK) + Ts
T = time between two data frame transmissions
Tt = transmission time of data frame = N/R
Tp = propagation time of data frame = L/c0
Tr = receiving node processing time
Tt(ACK) = transmission time of ACK = N(ACK)/R
Tp(ACK) = propagation time of ACK = Tp
Ts = sending node processing time

18. Stop-and-Wait – Efficiency
Efficiency  = Tt/T = Tt/(Tt + 2Tp) = 1/(1+2a)
T  Tt + 2Tp when N(ACK) << N, Tr + Ts << Tp
a = propagation time/transmission time = (L/c0)/(N/R)
A long link, a small frame size, or a high link speed will yield a low efficiency  a

19. Stop-and-Wait – Example
Suppose
R = 10 Mbps
L = 2000 meters
c0 = 2  108 m/s
N = bits
N(ACK) = 400 bits
Tr = Ts = 2 sec
T  Tt + Tt(ACK) + 2Tp
msec
T  1.06 msec
Efficiency
 = 1/1.06  94.3%
If N =1000 bits
 = 0.1/0.16  62.5%
If R = 100 Mbps
 = 0.1/0.128  78.1%
If R = 1 Gbps
 = 0.01/0.034  29.1%

20. Stop-and-Wait – Link Utilization

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

22. Sliding Window – Diagram

23. Sliding Window – Example

24. Sliding Window – Enhancements
RNR
Receive not ready
Receiver can acknowledge frames without permitting further transmission
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 as in TCP

25. Sliding Window – Analysis
Efficiency depends on
Window size W
The value of a
T = 1 + 2a
Two cases
Case I
W ≥ 1 + 2a
  1
Case II
W < 1 + 2a
 = W/(1 + 2a)

26. Sliding Window – W ≥ 1 + 2a

27. Sliding Window – W < 1 + 2a

28. Sliding Window – Efficiency

29. Errors
An error occurs when a bit is altered between transmission and reception
Caused by thermal noise, electromagnetic interference, loss of synchronization, etc.
Impossible to guarantee that any single bit will be received correctly, no matter what communication equipment used

30. Errors – Single Bit Errors
One bit altered
Adjacent bits not affected
White noise

31. Errors – Burst Errors
Contiguous sequence of B bits in which first last and any number of intermediate bits in error
Impulse noise, or fading in wireless
Effect greater at higher data rates

32. Channel Coding
BER is a performance metric of a digital communication system
Channel coding
Coding for channel error reduction
A process by which redundant digits are inserted into the source data in such a fashion that the transmitted data exhibit certain patterns which can be used by the receiver to do error detection or error correction
Block coding
Convolution coding

33. Error Detection
Additional bits added by transmitter for error detection code
Parity
Value of parity bit is such that character has even or odd number of ones
Even parity or odd parity
Even number of bit errors goes undetected

34. Error Detection – Process

35. Error Detection – Parity

36. Error Detection – CRC Cyclic redundancy check
Cyclical shifting of a valid code produces another valid code
For a block of k-bit data, transmitter generates n-bit codeword
Exactly divisible by some number
Receiver divides frame by that number
If no remainder, assume no error
Capable of detecting various error patterns including most error bursts

37. Error Detection – CRC Example

38. Error Detection – CRC Process

39. Error Detection – CRC Division

40. Error Detection – CRC Division

41. Error Detection – CRC Hardware
Shift register

42. Error Detection – CRC Hardware

43. Error Detection – CRC Polynomial
Bit pattern can be represented as a polynomial

44. Error Detection – CRC Polynomial
Bit pattern can be represented as a polynomial
Simpler analysis

45. Error Detection – CRC Standard
Parity bit
CRC-16
16 bits per data frame
x16 + x12 + x5 + 1
Also called CRC-CCITT, used in HDLC FCS, Bluetooth
CRC-32
32 bits per data frame
x32 + x26 + x23 + x22 + x16 + x12 + x11 + x10 + x8 + x7 + x5 +x4 + x2 + x + 1
IEEE MAC frame

46. Error – Treatment
Correction of detected errors usually requires data block to be retransmitted
Not appropriate for wireless applications
Bit error rate is high
Lots of retransmissions
Propagation delay can be long compared with frame transmission time
Satellite
Would result in retransmission of frame in error plus many subsequent frames
Need to correct errors on basis of bits received

47. Error Correction Each k bit block mapped to an n bit block, n>k
Forward error correction encoder
Codeword sent
Received bit string may contain errors
Received code word passed to FEC decoder
If no errors, original data block output
Some error patterns can be detected and corrected
Some error patterns can be detected but not corrected
Some rare error patterns are not detected
Result in incorrect data output from FEC

48. Error Correction – Process

49. Error Correction – Operation
Add redundancy to transmitted message
Can deduce original in face of certain level of error rate
Block error correction code
In general, add n−k bits to end of block
Gives n bit block, called codeword
All of original k bits included in codeword
Some FEC map k bit input onto n bit codeword such that original k bits do not appear

50. Error Correction – Hamming Code
2n−k = n +1
Single bit error
0100
1101
0110
1111
Error at 2
Error at 1,2, or 3
0100
1101
0110
Error at 1,2, or 4
1111
No error at 1,3, and 4

51. Error Correction – Hamming Code

52. Error Correction – Hamming Code
Burst error correction by interleaving

53. Channel Coding – Performance
Redundant digit insertion leads to either increased bit rate and bandwidth requirement or reduced information rate for fixed bandwidth
Percentage of errors that can be detected or corrected represents the performance of a channel code
In general, channel coding results in a drastically reduced BER
Trade-off between bandwidth and BER

54. Channel Coding – Performance

55. Channel Coding Theorem
Shannon’s 2nd theorem
For R  C, there exists a channel coding scheme such that the information from the data source can be transmitted over the channel with an arbitrarily small error rate despite the presence of noise
Channel capacity C bps
Data source with information rate R bps
If R > C, this cannot be true

56. Channel Coding Theorem
No matter how small the BER is specified, a channel code can be designed to beat it
With codec complexity and delay as tradeoff
Reliable communication concept
It guarantees the existence of an elegant coding method which supports the benefit of channel coding
It does not offer a means of constructing it

57. Error Control Detection and correction of errors Lost frames
Damaged frames
Automatic repeat request
Error detection
Positive acknowledgment
Retransmission after timeout
Negative acknowledgement and retransmission

58. ARQ
Automatic repeat request
Stop-and-wait
Go-back-N
Selective reject

59. 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
Receiver gets two copies of the same frame
Use ACK0 and ACK1

60. Stop-and-Wait
Simple
Inefficient

61. Stop-and-Wait – Analysis
Stop-and-wait flow control
Not account for retransmissions that due to errors
q = probability of retransmission of erroneous frame
nr = average number of transmissions for each successful frame
nr = 1×(1−q) + 2×q(1−q) + 3×q2(1−q) + …
nr = 1/(1−q)
  1/[nr(1+2a)] = (1−q)/(1+2a)
With retransmissions

62. 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

63. Go-Back-N – Damaged Frame
Receiver detects error in frame i
Receiver sends rejection-i
Transmitter gets rejection-i
Transmitter retransmits frame i and all subsequent

64. Go-Back-N – Lost Frame Frame i lost Transmitter sends i+1
Receiver gets frame i+1 out of sequence
Receiver sends rejection-i
Transmitter goes back to frame i and retransmits

65. Go-Back-N – Lost Last Frame
Frame i lost and no additional frame sent
Receiver gets nothing and returns neither acknowledgement nor rejection
Transmitter times out and sends acknowledgement frame with P bit set to 1
Receiver interprets this as command which it acknowledges with the number of the next frame it expects, which is frame i
Transmitter then retransmits frame i
Same scenario as in case of damaged rejection

66. Go-Back-N – Damaged ACK
Receiver gets frame i and send acknowledgement i+1 which is lost
Acknowledgements are cumulative, so next acknowledgement i+n may arrive before transmitter times out on frame i
If transmitter times out, it sends acknowledgement with P bit set as before
This can be repeated a number of times before a reset procedure is initiated

67. Go-Back-N

68. Go-Back-N – Analysis p(k) = qk−1(1−q)
Probability that a frame takes k transmissions to succeed
q = probability of retransmission
nr = average number of transmissions for each successful frame
An erroneous frame causes N retransmissions
N  1 + 2a provided that W  1 + 2a
W = window size
At least this many frames have been transmitted when the ACK of the 1st of these frames is supposed to arrive

69. Go-Back-N – Analysis   1/nr  (1−q)/(1+2aq)
  (1−q)/(1+2a) in case of stop-and-wait
Benefit of go-back-N decreases when q increases

70. Selective Reject Also called selective retransmission
Only rejected frames are retransmitted
Subsequent frames are accepted by the receiver and buffered
Minimize retransmission
Receiver must maintain large enough buffer
More complex login in transmitter

71. ARQ – Efficiency

72. HDLC High-level Data Link Control Bit-oriented protocol Widely used
ISO 33009, ISO 4335
Bit-oriented protocol
Widely used
Basis for many other important data link control protocols
Similar formats
Similar mechanisms
LAPB, LAPD, LAPF

73. HDLC – Station Types Primary station Secondary station
Control operation of link
Frames issued are called commands
Maintain 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

74. HDLC – Link Configurations
Unbalanced
One primary and one or more secondary stations
Support full duplex and half duplex
Balanced
Two combined stations

75. HDLC – Transfer Modes NRM Normal response mode
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

76. HDLC – Transfer Modes ABM ARM Asynchronous balanced mode
Balanced configuration
Either station may initiate transmission without receiving permission
No polling overhead
Most widely used
ARM
Asynchronous response mode
Unbalanced configuration
Secondary may initiate transmission without permission form primary
Primary responsible for line
Rarely used

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

78. HDLC – Flag Field 01111110 Delimit frame at both ends
May close one frame and open another
Receiver hunts for flag sequence to synchronize

79. HDLC – 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

80. HDLC – Bit Stuffing Error

81. HDLC – Address Field
Identify 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 or not by 1 or 0
All ones, , is broadcast

82. HDLC – Control Field Different for different frame type
Information
Data to be transmitted to user
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

83. HDLC – Control Field

84. HDLC – Poll/Final Bit Depend on context Command frame Response frame
P bit
1 to solicit, or poll, response from peer
Response frame
F bit
1 indicates response to soliciting command

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

86. HDLC – FCS Field Frame check sequence Error detection 16-bit CRC
Optional 32-bit CRC

87. HDLC – Operation
Exchange of information, supervisory and unnumbered frames
Three phases
Initialization
Data transfer
Disconnect

88. HDLC – Operation

89. HDLC – Operation

90. PPP Point-to-point protocol Used in Internet Dial-up modem
Router to router
Dial-up modem
Home to ISP
Byte oriented
Byte stuffing

91. Data Link Control – Conclusion
Several procedures needed to ensure the data integrity
Additional bits inserted
Different techniques for different kinds of transmissions
Tasks can be divided