香港浸会大学：《Data Communications and Networking》课程教学资源（PPT讲稿）Chapter 3 Data Transmission

Data communications and Networking Chapter 3 Data Transmission Reading Book Chapter 3 Data and Computer Communications, 8th edition By wIlliam Stallings

1 Data Communications and Networking Chapter 3 Data Transmission Reading: Book Chapter 3 Data and Computer Communications, 8th edition By William Stallings

Outline Concepts and Terminology Analog and Digital Data Transmission Transmission Impairments Channel Capacit

2 Outline • Concepts and Terminology • Analog and Digital Data Transmission • Transmission Impairments • Channel Capacity

Part 1: Concepts and Terminology Simplified Communications Model Digital bit balog Analog Digital bit stream signal sian al stream Text Text Tral Source Trans mission Receiver Destination mitter Syste ① Input data Transmitte Received Output data Output information g(t) ignal g(t information m

3 Part 1: Concepts and Terminology Simplified Communications Model

Digital Data Transmission Each single bit can be represented by a signal element Each signal element takes some time to send Bit rate the number of bits that can be sent out per unit of time 110011100 time

4 Digital Data Transmission time 1 1 0 0 1 1 1 0 0 Each single bit can be represented by a signal element. Each signal element takes some time to send. Bit rate: the number of bits that can be sent out per unit of time

What is the objective? Maximize the data rate number of bits that the system can transmit in a unit of time -within an acceptable bit error rate Why there could be bit errors? The signal received by the receiver is different from the signal sent from the sender Usually, if data rate becomes higher, it is more difficult for the receiver to recognize the signal higher data rate results in higher bit error rate In order to achieve high data rate with low bit error rate, we need to study the principle of data communications 5

5 What is the objective? • Maximize the data rate: number of bits that the system can transmit in a unit of time —within an acceptable bit error rate • Why there could be bit errors? —The signal received by the receiver is different from the signal sent from the sender • Usually, if data rate becomes higher, it is more difficult for the receiver to recognize the signal —higher data rate results in higher bit error rate • In order to achieve high data rate with low bit error rate, we need to study the principle of data communications

Terminology (1) Data transmission occurs between transmitter and receiver over some transmission medium Signal: electromagnetic waves -Can propagate along the transmission medium Transmission medium -Guided medium: the signals are guided along a physical path e.g, twisted pair, coaxial cable, optical fiber -Unguided medium: wireless e.g., air, water, vacuum

6 Terminology (1) • Data transmission occurs between transmitter and receiver over some transmission medium. • Signal: electromagnetic waves —Can propagate along the transmission medium • Transmission Medium —Guided medium: the signals are guided along a physical path • e.g., twisted pair, coaxial cable, optical fiber —Unguided medium: wireless • e.g., air, water, vacuum

Terminology(2) · Direct link Refer to the transmission path between the transmitter and receiver in which signals propagate directly with no intermediate devices, other than amplifiers or repeaters used to increase signal strength Note that it can apply to both guided and unguided media A transmission medium is point-to-point if Point-to-point -Direct link Only 2 devices share the medium a transmission medium is multipoint if More than two devices share the same medium Multipoint

7 Terminology (2) • Direct link —Refer to the transmission path between the transmitter and receiver in which signals propagate directly with no intermediate devices, other than amplifiers or repeaters used to increase signal strength. —Note that it can apply to both guided and unguided media • A transmission medium is point-to-point if: —Direct link —Only 2 devices share the medium • A transmission medium is multipoint if: —More than two devices share the same medium Point-to-point Multipoint

Terminology (3) Simplex transmission -Signals are transmitted in only one direction e.g. Television · Half duplex -Signals can be transmitted in either direction, but only one way at a time e.g. police radio Full duplex -Both stations may transmit simultaneously e.g. telephone 8

8 Terminology (3) • Simplex transmission —Signals are transmitted in only one direction • e.g. Television • Half duplex —Signals can be transmitted in either direction, but only one way at a time. • e.g. police radio • Full duplex —Both stations may transmit simultaneously. • e.g. telephone

Signals: Time Domain We are concerned with electromagnetic signals used as a means to transmit data A signal is generated by the transmitter and transmitted over a medium The signal is a function of time but it can also be expressed as a function of frequency Time domain concepts: an electromagnetic signal can be either analog or gita Analog signal The signal intensity varies in a smooth fashion over time. Or, there is no breaks or discontinuities in the signal Digital signal The signal intensity maintains a constant level for some period of time and then changes to another constant level Time domain function of a signal: s(t/ pecifies the amplitude(in volts )of the signal at each instant in time 9

9 Signals: Time Domain • We are concerned with electromagnetic signals used as a means to transmit data. • A signal is generated by the transmitter and transmitted over a medium. • The signal is a function of time, but it can also be expressed as a function of frequency. • Time domain concepts: an electromagnetic signal can be either analog or digital — Analog signal • The signal intensity varies in a smooth fashion over time. Or, there is no breaks or discontinuities in the signal. — Digital signal • The signal intensity maintains a constant level for some period of time and then changes to another constant level. • Time domain function of a signal: s(t) — Specifies the amplitude (in volts) of the signal at each instant in time

Analogue Digital signals Amplitude (volts) (a)Analog Amplitude Time 10 Digital

10 Analogue & Digital Signals