Week 2 Lecture 1. Introduction to Communication Networks. Review: Analog and digital communications

Transcription

1 Week 2 Lecture 1 Introduction to Communication Networks Review: Analog and digital communications Topic: Internet Trend, Protocol, Transmission Principle Digital Communications is the foundation of Internet ( a biggest communication network!) 1

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4 protocols (rules) control sending, receiving of msgs e.g., TCP, IP, HTTP, FTP, PPP 4

5 What s a protocol? human protocols: what s the time? I have a question introductions specific msgs sent specific actions taken when msgs received, or other events network protocols: machines rather than humans all communication activity in Internet governed by protocols protocols define format, order of msgs sent and received among network entities, and actions taken on msg transmission, receipt 5

6 A closer look at Internet structure: network edge: applications and hosts network core: routers network of networks physical media: communication links Communication Signals, Communication Media, And Data Transmission 6

7 Use what physics principle to send Data? From physics Energy Electromagnetic wave propagation Communication Media: How many types? Copper wire Need two wires Possibilities Twisted pair Coaxial cable Optical fiber Flexible Light stays in Air / space Used for electromagnetic transmission 7

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9 Forms of Energy Used To Transmit Data: Electric current Audible sounds Omni-directional electromagnetic waves Radio Frequency (RF) Infrared Directional electromagnetic waves Point-to-point satellite channel Limited broadcast (spot beam) Microwave Laser beam 9

10 Two Important Physical Limits Of a Communication System Propagation delay Time required for signal to travel across media Example: electromagnetic radiation travels through space at the speed of light (c = meters per second) Bandwidth Maximum times per second the signal can change Transmission of Analog/Digital Data Network hardware encodes information for transmission Two types of encoding Analog (amount of energy proportional to value of item) Digital (two forms of energy to encode 0 and 1 Computer networks use the latter 10

11 Example Digital Encoding Medium Copper wire Energy form Electric current Encoding Negative voltage encodes 1 Positive voltage encodes 0 Illustration Of Digital Encoding Known as waveform diagram X-axis corresponds to time Y-axis corresponds to voltage Asynchronous Communication Sender and receiver must agree on Number of bits per character Receiver Duration of each bit Does not know when a character will arrive May wait forever 11

12 To ensure meaningful exchange, the sender should send Start bit before character One or more stop bits after character Example: The RS-232C Standard Example use Connection to keyboard / mouse Serial port on PC Voltage is +15 or 15 Cable limited to ~50 feet Latest standard is RS-422 (ITU standard is V.24) Uses asynchronous communication Two-Way Communication Desirable in practice Requires each side to have transmitter and receiver Called full duplex Illustration Of Full-Duplex Communication 12

13 Transmitter on one side connected to receiver on other Separate wires needed to carry current in each direction Common ground wire DB-25 connector used Pin 2 is transmit Pin 3 is receive Next question: why use MODEM (Modulation/DeModulation) in Digital communication systems? Electrical Transmission (The Bad News) It s an ugly world Electrical energy dissipates as it travels along the wire Wires have resistance, capacitance, and inductance which distort signals Magnetic or electrical interference distorts signals Distortion can result in loss or misinterpretation 13

14 Illustration of Distorted Signal for a Single Bit In practice Distortion can be much worse than illustrated Consequences: Cannot use electrical current for long-distance transmission Long-Distance Communication USE MODEM! Important fact: an oscillating signal travels farther than Direct Current (DC) For long-distance communication Send a sine wave (called a carrier wave) Change (modulate) the carrier to encode data Note: modulated carrier technique used for radio and television 14

15 Illustration of a Carrier: Carrier Usually a sine wave Oscillates continuously Frequency of carrier fixed Types of Modulation Amplitude modulation (used in AM radio) Frequency modulation (used in FM radio) Phase shift modulation (used for data) Illustration of Amplitude Modulation Strength of signal encodes 0 or 1 One cycle of wave needed for each bit Data rate limited by carrier bandwidth Modem 15

16 MODEM: Hardware device Used for long-distance communication Contains separate circuitry for Modulation of outgoing signal Demodulation of incoming signal Name abbreviates modulator / demodulator Illustration of Modems Used Over a Long Distance One modem at each end Separate wires carry signals in each direction Modulator on one modem connects to demodulator on other ***************** Explain TDMA, FDMA, CDMA, Statistical Multiplexing 16