1. EEC-484/584 Computer Networks
Lecture 12
Wenbing Zhao
Cleveland State University

2. Outline
Quiz#3 Result
Reminder: Initial Draft due 4/26/2010 midnight by
Data Link layer
Data Link Layer Design Issues
Framing
Error Detection and Correction
DLL
Data Link
Medium Access Control (MAC)
We Cover
This SubLayer in this lecture
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EEC-484/584: Computer Networks

3. EEC-484/584: Computer Networks
EEC584 Quiz#3 Result
Average: 81, high: 100, low: 58
Q1-34.8, Q2-17.8, Q3-14.9, Q4-13.5
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4. Data Link Layer Design Issues
Services Provided to the Network Layer
Point-to-point, source-to-destination
Framing
Physical bit stream divided up into frames
Error Control
Acknowledgements (acks), retransmission, duplicate suppression
Flow Control
Throttle sender so sends no faster than receiver can receive them
Note that DLL provides a point-to-point, source-to-destination service to the network layer, even though it use a broadcast technology. The multiaccess control issue is dealt with in the MAC sublayer
Some DLL protocol also provides multicast/broadcast service => but not main concern here
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5. Functions of Data Link Layer
Provide a virtual source-to-destination communication channel to the network layer
Dealing with transmission errors
Regulating data flow
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6. Services Provided to Network Layer
(a) Virtual communication
(b) Actual communication
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7. Types of Services Provided to Network Layer
Unacked connectionless
Ok if low error rate, real time applications
Acked connectionless
Unacked connection-oriented
Acked connection-oriented
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8. EEC-484/584: Computer Networks
Framing
DL layer divides physical bit stream into frames
Checksum computed by source included in frame
Checksum recomputed by destination and checked against checksum included in the frame
Discard/recover bad frame, notify source
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9. How Does DL Layer Form Frames?
Insert time gaps between frames: too risky
Character count
Flag bytes with byte stuffing
Starting and ending flags, with bit stuffing
Physical layer coding violations
Example: encode 1 bit of data with 2 bits
1 => 10
0 => 01
Can use 00 or 11 to delimit frames
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10. Framing Based on Character Count
Use field in header to indicate number of characters in frame
Problem: Count might be garbled
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11. Framing Based on Byte Stuffing
Each frame starts and ends with a special flag byte
Problem: flag byte might appear in data
Solution:
Source inserts ESC (DL escape) before each flag byte; ESC before each ESC
Destination removes inserted ESC bytes
Disadvantage: depends on 8-bits characters in ASCII
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12. Framing Based on Byte Stuffing
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13. Framing Based on Bit Stuffing
Each frame begins and ends with special bit patterns, (in fact, a flag byte)
When source’s data contains 11111, stuff 0
When destination receives , deletes 0
Advantages:
Allows arbitrary number of bits per frame
Allows arbitrary number of bits per character
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14. Framing Based on Bit Stuffing: Example
Original Frame (payload only)
Frame after bit stuffing
Received Frame after taken stuffed bit out
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15. Error Detection and Correction
Causes of errors
Transmission errors on phone lines due to thermal noise
Data transmission errors due to impulse noise
Signals are separated, distorted, recombined
Crosstalk between physically adjacent wires
Compression and decompression
Receiver out of synch with sender
Errors usually occur in bursts
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EEC-484/584: Computer Networks
Wenbing Zhao

16. Error-Correcting Codes
n-bit codeword – an n-bit unit containing data and check bits
m bits of data, r bits redundant/check bits (n = m+r)
How to measure the differences between two codewords (num of different bits)
Using exclusive OR and counting number of 1 bits in the result
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EEC-484/584: Computer Networks
Wenbing Zhao

17. Error-Correcting Codes
Hamming distance – number of bit positions in which two codewords differ
If two codewords are a Hamming distance d apart, it will require d single-bit errors to convert one into the other
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EEC-484/584: Computer Networks
Wenbing Zhao

18. Error-Correcting Codes
Complete code
Complete list of all legal codewords: 2m possible data messages
Recall that there are m bits of data
Hamming distance of the complete code
Find two codewords whose Hamming distance is minimum
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19. Error-Detection Codes
A distance d+1 code can detect up to d errors, why?
If there are d+1 errors, one valid codeword might be turned into another valid codeword
≤ d errors will change a valid codeword into an illegal codeword  can be detected!
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EEC-484/584: Computer Networks
Wenbing Zhao

20. Error-Correcting Codes
To correct d errors, need a distance 2d+1 code
Legal codewords are so far part that even with d changes, original codeword is still closer than any other codeword, so it can be uniquely determined
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EEC-484/584: Computer Networks
Wenbing Zhao

21. Error-Correcting Codes: Example
Consider a code with only four valid codewords
, , ,
This code has a distance 5  can correct double errors
If arrives, receiver knows the original must have been
However, if triple error changes to , the error will not be corrected properly
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EEC-484/584: Computer Networks
Wenbing Zhao

22. EEC-484/584: Computer Networks
Parity Bit
Parity bit – a single bit is appended to the data
Parity bit is chosen so that number of 1 bits in the codeword is even or odd
Example: Given
With even parity 
With odd parity 
A code with a single parity bit has a distance 2
Since any single-bit error produces a codeword with wrong parity  can be used to detect single bit errors
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EEC-484/584: Computer Networks
Wenbing Zhao

23. Error-Detecting Codes
If a single parity bit is appended to a block, error detecting probability is only 0.5 if burst error occurs (why?)
This can be improved by treating a block as a matrix, n bits wide and k bits high,
A parity bit is computed for each column and affixed to the matrix as the last row
The matrix is transmitted one row at a time
Probability of accepting bad block is 2-n
If a burst error occurs, it is equally likely to have single-bit error, double-bit error, etc.
If odd number of bits error => can detect error
If eve number of bits error => cannot detect error
Therefore, the probability of detecting error is 50%
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24. Error-Detecting Codes: CRC
Polynomial code, also known as CRC (Cyclic Redundant Code)
Treat bit string as polynomial with 0 and 1 coefficients
m-bit frame: M(x) = bm-1xm-1 + … + b0
E.g.: => M(x) = x7 + x6 + x4 + x3 + x1
Use modulo 2 arithmetic
No carries or borrows: XOR
The degree of a polynomial is the maximum of the degrees of all terms in the polynomial
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EEC-484/584: Computer Networks
Wenbing Zhao

25. EEC-484/584: Computer Networks
Cyclic Redundant Code
Sender and receiver agree on generator polynomial G(x) (High & low order bits must be 1)
For a frame with m bits corresponding to M(x), m > deg G(x) = r
Append checksum to end of frame so polynomial T(x) corresponding to checksummed frame is divisible by G(x)
When receiver gets checksummed frame, divides T(x) by G(x)
If remainder R(x) != 0, then transmission error
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EEC-484/584: Computer Networks
Wenbing Zhao

26. Algorithm to Compute CRC Checksum
Let m = deg M(x), r = deg G(x)
Append r 0 bits to lower-order end of frame: xrM(x)
Divide bit string corresponding to xrM(x) by bit string corresponding to G(x)
Subtract remainder R(x) from bit string corresponding to xrM(x), result is checksummed frame. Let T(x) be its polynomial
xrM(x) = Q(x)G(x) + R(x)
xrM(x) – R(x) = Q(x)G(x) = T(x)
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EEC-484/584: Computer Networks
Wenbing Zhao

27. EEC-484/584: Computer Networks
Compute CRC Checksum
XOR
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Wenbing Zhao

28. International Standard Polynomials
CRC-12 G(x) = x12 + x11 + x3 + x2 + x1 + 1
Used for 6-bit characters
CRC-16 G(x) = x16 + x15 + x2 + 1 CRC-CCITT G(x) = x16 + x12 + x5 + 1
Used for 8-bit characters
CRC-32 G(x) = x32 + x26 + x23 + x22 + x16 + x11 + x10 + x8 + x7 + x5 + x4 + x2 + x1 + 1
Used in IEEE 802
Detects all bursts of length 32 or less and all bursts affecting an odd number of bits
No need to remember these generator polynomials
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29. EEC-484/584: Computer Networks
Exercise
Q1. The following character encoding is used in a data link protocol: A: ; B: ; FLAG: ; ESC: Show the bit sequence transmitted (in binary) for the four-character frame: A B ESC FLAG when each of the following framing methods are used:
(a) Character count.
(b) Flag bytes with byte stuffing.
(c) Starting and ending flag bytes, with bit stuffing
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30. EEC-484/584: Computer Networks
Exercise
Q2. To provide more reliability than a single parity bit can give, an error-detecting coding scheme uses one parity bit for checking all the odd-numbered bits and a second parity bit for all the even-numbered bits. What is the Hamming distance of this code?
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EEC-484/584: Computer Networks
Wenbing Zhao

31. EEC-484/584: Computer Networks
Exercise
Q3. A bit stream is to be transmitted using the standard CRC method described in the text. The generator polynomial is x Show the actual bit string transmitted. Suppose the third bit from the left is inverted during transmission. Show that this error is detected at the receiver's end.
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Wenbing Zhao