1. CS4470 Computer Networking Protocols
2/10/2018
CS Computer Networking Protocols
3. Data Link Layer
Huiping Guo
Department of Computer Science
California State University, Los Angeles

2. Outline Overview of data link layer Framing Error detection
2/10/2018
Outline
Overview of data link layer
Framing
Error detection
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3. Data link layer overview
2/10/2018
Data link layer overview
Network layer
Network layer
Data
Data
Data H T
frame
Link layer
Link layer
Data H T … …
Physical layer
Physical layer
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4. Data link layer overview (cont.)
Achieve reliable, efficient communication between two physically connected machines
Make use of services provided by the physical layer
Get a series of bits
Provide services to the network layer
The kind of services depends on the link layer protocol
How to make a link appear reliable
A link may corrupt frame from time to time
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5. Design issues Services interface to the network layer
How to group bits into frames (framing)
How to deal with errors (error control)
Errors in the physical layer
How to mediate access to a link if the link is shared by multiple hosts
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6. Framing Break the bit stream into frames Why framing?
Raw bit stream cannot provide any error detection capability.
Bits may change (lightning strikes).
Bits may get lost (clock drift).
Challenge
How to delimitate frames?
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7. How to do framing Character count Flag bytes with byte stuffing
Starting and ending flags with bit stuffing
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8. Character count
Uses a field in the header to specify the number of characters in the frame
frame1
frame2
frame3
frame4
Problem: What if the count is corrupted
error
frame1
frame2: wrong !
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9. Flag bytes with byte stuffing
Each frame start and end with special bytes
Soh block of data in frame eot
Flag bytes
Start of header: soh
End of text: eot
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10. What if the data contains the flag?
Byte stuffing
Uses another special flag.
Translates the data to alternative form
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11. Illustration of Frame with byte stuffing
After byte stuffing, the characters soh and eot do not occur in the data
Stuffed frame longer than original
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12. Disadvantages of byte stuffing
It is closely tied to the use of 8-bit characters
Not all character codes use 8-bit characters
UNICODE uses 16-bit characters
How to support flags with arbitrary sized characters?
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13. Bit stuffing Allows data frames to contain an arbitrary number of bits
Allows character codes with an arbitrary number of bits per character
Each frame begins and ends with a special bit pattern ( )
It’s like flag byte
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14. Bit stuffing (cont.) Receiver hunts for flag sequence to synchronize
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
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15. Bit Stuffing
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16. Exercise 2. Suppose the following sequence of bits arrive over a link
Assuming a framing protocol that uses bit stuffing, show the bit sequence transmitted over the link when the frame contains the following bit sequence
2. Suppose the following sequence of bits arrive over a link
Show the resulting frame after any stuffed bits have been removed.
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17. Error Detection Parity bits Checksum Cyclic Redundancy Code (CRC)
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18. Parity Checking Single Bit Parity:
Detect single bit errors 1
Count the number of 1s in the received d+1 bits.
An odd number indicates at least one bit errors with an even parity scheme.
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19. Parity Checking Two Dimensional Bit Parity:
d bits are organized into ixj bit array
Parity checking for each column and row
Resulting i+j+1 parity bits
Detect and correct single bit errors
Forward error correction (FEC)
Can also detect any combinations of two bit errors
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20. Checksum Sender: Receiver: treat data as sequence of 16-bit integers
checksum: addition (1’s complement sum) of segment contents
Attach the checksum to the end of data segments
Receiver:
Sum up data and checksum
The results are all 1s?
Yes, no errors
No, errors.
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21. 2/10/2018
Checksum Example
Note
When adding numbers, a carryout from the most significant bit needs to be added to the result
Example: add two 16-bit integers
wraparound
Kurose and Ross forgot to say anything about wrapping the carry and adding it to low order bit
sum
checksum
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22. Exercise
Calculate the checksum for the following binary numbers. Give the result in binary. Use the 16-bit format.        
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23. Cyclic Redundancy Check
A major goal in designing error detection algorithm is to maximize the probability of detecting errors using only a small number of redundant bits
CRC uses some fairly powerful mathematics to achieve this goal
The theoretical foundation of the CRC is finite field
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24. Cyclic Redundancy Check
An (n+1) bit message is represented by an n degree polynomial, a polynomial whose highest-order term is xn.
The message is represented by a polynomial
The value of each bit in the message is as the coefficient for each term in the polynomial
The most significant bit represents the hightest-order term
Eg. An 8-bit message corresponds to the polynomial
M(x) = 1 * x7 + 0 * x6 + 0 * x5 + 1 * x4 + 1 * x3
+ 0 * x2 + 1 * x1 + 0 * x0
= x7 + x4 + x3 + x
We can think of a sender and a receiver as exchanging polynomials with each other
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25. Cyclic Redundancy Check
view data bits, D, as a binary number
choose r+1 bit pattern (generator), G
goal: choose r CRC bits, R, such that
<D,R> exactly divisible by G (modulo 2)
receiver knows G, divides <D,R> by G. If non-zero remainder: error detected!
can detect all burst errors less than r+1 bits
widely used in practice (ATM, HDCL)
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26. CRC Example D.2r R = remainder[ ] G Want: D.2r + R = nG equivalently:
if we divide D.2r by G, want remainder R
R = remainder[ ]
D.2r G
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27. Exercise D=11100110, G=11001, r=4 R=? The transmitted bits=?
If no error occurs, how the receiver knows the received bits are correct?
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