Introduction

1. Chapter 2 Introduction to Data Communication Introduction • Information transfer between two computers occurs in one of two types signals: digital or analog.

2. Chapter 2 Introduction to Data Communication 2.5 Transmission Modes • When data is transferred, the receiving computer must determine when one signal ends and when the next one begins • Two types: Asynchronous transmission and Synchronous transmission.

3. 2.5 Transmission Modes Asynchronoustransmission • adds a start bit, one or two end bits and a parity bit (for error detection) to achieve synchronization • Occurs character by character and used for serial communication, such as by modem.

4. 2.5 Transmission Modes Synchronoustransmission • use a clock pulse for synchronization • Transfer large blocks of data and used for parallel transmission, such as reading a file or printing information.

5. Chapter 2 Introduction to Data Communication 2.6 Transmission Methods • Serial communication is a method by which data is transmitted one bit at a time over a single transmission media • Parallel transmission is a method by which data is transmitted byte by byte (eight bits).

6. Chapter 2 Introduction to Data Communication 2.7 Communication Modes • Simplex Mode：one direction only • Half-Duplex Mode：two devices exchange information in only one direction at a time • Full-Duplex Mode：both computers can send and receive information simultaneously.

7. Chapter 2 Introduction to Data Communication 2.8 Bandwidth and Signal Transmission • bandwidth in communication is the range of frequencies that signal occupies.

8. 2.8 Bandwidth and Signal Transmission Signal transmission • Baseband mode uses the bandwidth of a transmission media to carry only one signal (Use digital signals) • Broadband mode uses the bandwidth of a transmission media to carry several signals (Use analog signals).

9. Chapter 2 Introduction to Data Communication 2.9 Digital signal encoding • Digital information can be represented by several forms of digital signal, such as NRZ (WAN) , NRZ-I, Manchester and Differential Manchester (LAN) .

10. 2.9 Digital signal encoding Encoding and Modulation • Analog Signal • Modulation : AM、FM、PM • Encoding ：PCM • Digital Signal • Encoding：NRZ、 Manchester • Modulation：ASK、FSK、PSK.

11. 2.9 Digital signal encoding Manchester Encoding • The clock pulse is embedded into the signal • Manchester Encoding： • One bit is divided into two parts, the first part is not changed, the second part is changed(0->1,1->0).

12. 2.9 Digital signal encoding Manchester Encoding

13. 2.9 Digital signal encoding Manchester Differential Encoding • The first bit is the same as Manchester Encoding • From the second bit: • 1->the first part is the same as the second part of previous;0-> the first part is opposite to the second part of previous • the second part is opposite to the first part.

14. 2.9 Digital signal encoding Manchester Differential Encoding

15. Chapter 2 Introduction to Data Communication 2.10 Error detection methods • When the transmitter sends a frame to the receiver, the receiver must first check it.

16. 2.10 Error detection methods Methods to detect error • Parity Check • BCC • One’s complement of sum (checksum) • CRC.

17. 2.10 Error detection methods Parity Check • Simplest error detection method • Can detect one error • Before transmitting to the receiver, a parity bit (an extra bit) adds to the information • The receiver checks it .

18. 2.10 Error detection methods Even Parity • 1000011 ：parity bit is 1 • 0101101 ：parity bit is 0 • Used in serial communication, limited to detection only one error and is used for transmitting single characters.

19. 2.10 Error detection methods Block Check Character • When a block of characters is transmitted, BCC can be used to detect two errors • BCC uses vertical and horizontal parity bits in order to detect double errors.

20. TABLE 2.6

21. 2.10 Error detection methods One’s Complement of the Sum • Used for error detection of TCP header and IP header • Disadvantage： 0001 0011 0010 0000 0011 0001 0001 0011 0111 0111

22. 2.10 Error detection methods Cyclic Redundancy Check • If frame M = 10010110 (K=8 bits) M(X)=X7+X4+X2+X （K-1=7） • If divisor P =1010101 (N+1 bits) P(X)=X6+X4+X2+1 （N=6) • Modulo-2 addition to calculate FCS： 2n*M= X6 M = X13+X10+X8+ X7（10010110000000）.

23. CRC FCS • FCS= 2n*M/P= X3+X2+X+1 • 0-1=1 • 1-0=1 • 0-0=0 • 1-1=0 10110011 1010101|10010110000000 1010101 1111000 1010101 1011010 1010101 1111000 1010101 1011010 1010101 FCS----1111

24. CRC The transmit frame • Transmit frame T = 2n*M+FCS： • 2n*M= X13+X10+X8+ X7 • FCS= 2n*M/P= X3+X2+X+1 • T(X)=X13+X10+X8+X7+ X3+X2+X+1 10010110001111.

25. CRC Check • Divides T by P using modulo-2 division, if FCS=0 , no error 10110011 1010101|10010110001111 1010101 1111000 1010101 1011010 1010101 1111111 1010101 1010101 1010101 0

26. CRC P(X) • IEEE and ITU standards: • CRC12=X12+X11+X3+X2+1 • CRC16 =X16+X15+X2+1 • CRC-ITU =X16+X12+X5+1 • CRC32 =X32+X26+X23+X22+ X16+X11+X10+X8+ X7+X5+X4+X2 +X+1.

27. Chapter 2 Introduction to Data Communication Short answer questions • 27, 28, 31, 33 • 36, and the transmits frame T