1. 1 Part III Packet Transmission Chapter 7 Packets, Frames, and Error Detection

2. 2 Scope  Discuss the concept of packet  Explain how a sender and receiver coordinate to transfer a packet  Show how packet can be implemented in a character-oriented network using a simple frame format  Discuss mechanisms used for error detection

3. 3 The Problem  Cannot afford individual network connection per pair of computers  Reasons Installing wires consumes time and money Maintaining wires consumes money (esp. long- distance connections) C(N,2)=N(N-1)/2

4. 4 Solution  Network has Shared central core Many attached stations  Problems Some applications have large transfers Some applications cannot wait (e.g., interactive) Need mechanism for fairness Quality of Service (QoS) guarantee

5. 5 Packet Switching Principle  Solution for fairness Divide data into small units called packets, which it sends individually Packet (switching) networks Allow each station opportunity to send a packet before any station sends another Form of time-division multiplexing

6. 6 Illustration of Packet Switching

7. 7 Packets and Hardware Frames  Each H.W. technology defines the details of packets that can be transfer on that H.W. Format Minimum / maximum size  Hardware packet called a frame

8. 8 Example Frame Format Used with RS-232  RS-232 is character-oriented permits arbitrary delay between characters  Special characters (i.e., unused character in a text document) are picked as frame delimits Start of header ( soh )(e.g., ^) End of text ( eot )(e.g., ~)

9. 9 When Data Contains Special Characters  Translate to alternative form by inserting extra bits or bytes  Called byte stuffing  Example

10. 10 Illustration of Frame With Byte Stuffing  Stuffed frame longer than original  Necessary evil

11. 11 Transmission Errors  Interference can change signal used for transmission Bits lost Bit values changed Unsent data to appear  Frame includes additional information to detect / correct error Set by sender Checked by receiver  Statistical guarantee

12. 12 Error Detection And Recovery Techniques (1)  Parity bit One additional bit per character Can use  Even parity  Odd parity Cannot handle error that changes two bits 1011011  1011011 1 1010011 1 (1-bit error) 1110011 1 (2-bit error)

13. 13 Error Detection And Recovery Techniques (2)  Checksum Computation Example  Checksum appended to frame Handles multiple bit errors Cannot handle all errors

14. 14 Error Detection And Recovery Techniques (2)  Illustration Of Errors: Checksum Fails to Detect  Second bit reversed in each item

15. 15 Error Detection And Recovery Techniques (3)  Cyclic Redundancy Check ( CRC ) Mathematical function for data More complex to compute Detect more errors than a checksum  CRC covers data only

16. 16 Building Blocks For CRC  Exclusive or  Shift register

17. 17 Example Of CRC Hardware  Computes the 16-bit CRC  Registers initialized to zero  Bits of message shifted in one at a time  CRC for the message found in registers  A receiver calculates the CRC for an incoming message

18. 18 Example CRC Computation  若欲傳送資料 10 ，試計算其 CRC 值 0 xxx 000000000000000 1 0 xxx 000000000000000 0 00 00 1 100 0 xxx 000000000 1 00000 First bit:

19. 19 Second bit: Example CRC Computation (ctn.) 0 xxx 000000000 1 00000 0 0 xxx 000000000 1 00000 0 000 0 0000 0 xxx 000000001 0 00000

20. 20 Exercise  What is the content of register?

21. 21 The End

22. 22 Example CRC Computation 0 + 3210 Power of x Incoming bit string 11000001010Initial 000 + 1 + 0001010step 0 100 + 0 + 001010step 1 001 + 0 + 01010step 2 010 + 0 + 1010step 3 100 + 1 + 010step 4 001 + 1 + 1010step 5 011 + 1 + 0step 6 110 + 0 + step 7 101 + + Exclusive OR 1 1 0 0 0 0 0 1 0 1 0 1 1 0 0 1 0 0 0 1 0 0 0 0 0 0 0 0 1 0 0 0 0 0 0 0 0 1 0 0 1 0 0 0 0 0 1 0 0 1 0 1 1 0 0 1 1 0 1 1 1 1 1 0 0 1 1 1 1 0 0 1 1 0 0 1 0 1 0 1 1 1 0 0 1 1 0 0 1 0 1 0 step1 step7 step2 step3 step4 step5 step6

23. 23 Error Detection And Recovery Techniques (3)  CRC Arithmetic Background X 4 +X 3 +1 X 10 +X 9 +X 7 +X 5 +X 4 X 6 +X 3 +X X 10 +X 9 +X 6 +X 7 +X 6 +X 5 +X 4 +X 7 +X 6 +X 3 X 5 +X 4 +X 3 X 5 +X 4 +X X 3 +X remainder 1 1 0 1 0 1 1 0 0 0 0 1 1 0 0 1 1 0 0 1 0 1 0 0 0 1 1 1 0 0 0 0 0 0 1 1 1 1 0 0 0 0 0 1 1 1 1 0 1 1 0 0 1 0 1 1 1 0 0 0 0 0 0 1 1 1 0 0 1 1 0 0 1 0 1 0 1 0 0 0 0 0 0 remainder 1 0 1 0 0+0=00-0=0 1+0=11-0=1 0+1=10-1=1 1+1=01-1=0

24. 24 Error Detection And Recovery Techniques (3)  How CRC Works Sending  Given the generator polynomial G(x)  B(x): sending bit string+ n “0”s (degree of G(x))  R(x): remainder  T(x): B(x) - R(x) Receiving  G(x) | T’(x)  T’(x)  T(x)  G(x) | T’(x)  T’(x)  T(x) 1 1 0 1 0 1 1 0 0 0 0 1 1 0 0 1 1 0 0 1 0 1 0 0 0 1 1 1 0 0 0 0 0 0 1 1 1 1 0 0 0 0 0 1 1 1 1 0 1 1 0 0 1 0 1 1 1 0 0 0 0 0 0 1 1 1 0 0 1 1 0 0 1 0 1 0 1 0 0 0 0 0 0 remainder 1 0 1 0

25. 25 Error Detection And Recovery Techniques (3)  CRC Example 1 1 0 1 0 1 1 1 0 1 0 1 1 0 0 1 0 0 1 1 1 0 0 0 0 0 0 1 1 1 1 0 0 0 0 0 1 1 1 1 1 1 1 0 0 1 0 1 1 0 0 0 0 0 0 0 1 1 0 0 1 0 0 0 0 0 0 0 0 remainder Correct! 1 1 0 0 1 1 0 0 1 0 1 0 1 1 0 0 0 0 0 1 0 1 0 1 1 0 0 1 0 0 0 1 0 0 0 0 0 0 0 0 1 0 0 0 0 0 0 0 0 1 0 0 1 0 0 0 0 0 1 0 0 1 0 1 1 0 0 1 1 0 1 1 1 1 1 0 0 1 1 1 1 0 0 1 1 0 0 1 0 1 0 1 remainder Incorrect! 1 1 0 0 1 1 0 0 1 0 1 0