CCM 4300 Lecture 13 Computer Networks, Wireless and Mobile Communication Systems

Transcription

1 CCM 4300 Lecture 13 Computer Networks, Wireless and Mobile Communication Systems Introduction to Wireless Networks - I Dr S Rahman 1

2 Session Content Recap of last session Lesson Objectives Wired vs. Wireless networks Wireless networks concepts Wireless access technologies Intro to Spread Spectrum techniques Direct Sequence Spread Spectrum (DS-SS) 2

3 Recap of Last Sessions (1-12) Network Topologies, Protocols & OSI Internet, TCP/IP, UDP, DNS & Physical, Data link Layers, Internetworking, etc. Computer Security, WWW, HTML, HTTP, etc. 3

4 Lecture objectives At the completion of this lecture you should be able to Understand why we use wireless networks Understand the concept of wireless networks Know the existing wireless access technologies Understand what is Spread Spectrum & Understand Direct Sequence Spread Spectrum (DS-SS) 4

5 Supporting wireless and mobile systems Connectivity transmission modulation media access Support in the network infrastructure connectivity between the wireless and the wired world Protocols specifically for dealing with mobility 7 application 6 presentation 5 session 4 transport 3 network 2 data link 1 physical Application layer 5

6 Wireless Networks: Why? Mobility:Users can access files, network resources, and the Internet without being physically connected to the network with wires. Users can be mobile while maintaining high speed and real-time access to the network. This increases productivity of the users. Minimise required infrastructure & length of wires Disaster recovery: Continuity of Operations Network Services Re-route wired network thru wireless to data vault/ ID Long distance, low data rate links 6

7 Mobile and Wireless Networks: Background What is wireless? Brief history.. The physical phenomena known as radio waves were first known as Hertzian Waves. Hertz showed that the electromagnetic phenomena (under study by Tesla) could be used to transfer energy between locations without a physical connection. Guglielmo Marconi began work in 1894 to reproduce the Hertz laboratory experiment over greater distances. His study and efforts brought about the first radio link in the form of wireless telegraph. The combined works of Tesla, Hertz, and Marconi proved that electromagnetic phenomena (such as a large spark) generated at one location could be detected at another location without a direct physical connection between locations. Thus, the ability to communicate without wires i.e. Wireless. 7

8 Mobile and Wireless networks: Key Concepts χ Wireless links inherently are more complex than wireline links χ Wireless links suffer from unfavorable channel characteristics χ There is a very limited spectrum for wireless communication χwireless communication is susceptible to interception/interference (see next slide) χhigh error rates (electrical noise, signal reflections) χwireless networks generate electrical interference themselves χpower range is low, why? To minimise interference χintrinsically insecure, (authentication) 8

9 Mobile and Wireless networks: Key Concepts 9

10 Mobile and Wireless Networks : Concept Next Generation Internet (NGI) MPLS, QoS, multimedia support, group communication, accounting Telematics (TM): Protocols, services, standards, LAN, Internet, TCP/IP, WWW, security, ISDN, management, interworking units Microprocessor Lab (MPP) Practical assignments for RO (also RA and TM) Wireless/mobile System Networked System Computer Processor Logic Physics Mobile Communications Lab (MCL) Practical assignments for MC, NGI and TM Mobile Communications (MC): Wireless transmission, medium access, GSM, 3G, WLAN, Mobile IP, Ad-hoc-networks, WAP Computer Architecture (RA) Multi processor systems, pipelining, vector processing, interconnections, multithreading Computer Organization (RO): CPU, RISC/CISC, assembler, I/O, bus, controller, PIO, DMA, interrupt, memory, peripherals Computer structures: Boolean algebra, combinational and sequential circuits, computer arithmetic, von Neumann machine Physical electrical basics: semiconductors, TTL, CMOS, gates, memory, programmable logic, discrete elements 10

11 Wireless Connectivity Transmission radio-based systems (IR currently of limited use) noise: modulation techniques and error correction Available wireless networks: of interest are IEEE x Access network provide connectivity between mobile devices cell-based systems: Mobility support Infrastructure wireless Mobile and ad hoc network (MANET) 11

12 Key questions wireless networks. How can we provide connectivity for mobile systems? What kind of network structure do we need to support wireless mobility? What changes might we need to make to the existing infrastructure (mechanisms and systems) to support wireless mobility? 12

13 Wireless Networking Technologies Personal Area Network (WPAN) Bluetooth Infra-red Local Area Network (WLAN) 2.4 GHz waveband IEEE (WaveLAN) HomeRF WMAN Wide Area Network (WAN) (Wired/national) GSM (2G) (Global System for Mobile Communications) GPRS (2.5G) (General Packet Radio Service) UMTS (3G) (Universal Mobile Telecommunications System) 4G (Extremely high data rates) Wide Area Network (International) Satellite systems LEO, MEO, HEO and GEO 13

14 Wireless Networking Technologies con Transmission: Spread spectrum TDMA (Time Division) FDMA (Frequency Division) CDMA (Code Division) OFDM (Orthogonal Frequency Division) Networking: Mobile IP MANET 14

15 Wireless Transmission Radio Spectrum regulated ISM channels used by other applications Radio broadcast Signal confinement Propagation problems loss interference multi-path - fading industrial, scientific and medical (ISM) Infra-red Not regulated Line of site scattering diffusion satellite LEDs Low power: 10 Mb/s Laser diode: > 10 Mb/s more complex transceiver 15

16 Infra-red and radio diffusion modes Point-to-point passive satellite active satellite typical 1Mb/s Non active requires high transmitter power power limited? Directional antennas, detectors and emitters reduce multi-path effects 16

17 Radio broadcast connectivity Multiple host, multiple channels? TDM and FDM (fixed allocation) impractical not scaleable Many hosts, single channel? Shared Media: but when to transmit? was there a collision? is the receiver listening? can we ensure Rx listening? host host host host 17

18 Radio Systems I Easy to set-up network: High wiring data rates possible Mature technology: mobile still maturing Local and nationwide Radio spectrum is subject to international regulatory control, so it is not possible to use just any part of the spectrum at will you have to obtain a license. Description Frequency Wavelength High frequency 3-30MHz m VHF MHz 6-3m UHF MHz 75-30cm Global satellites Microwaves Hz 10cm -3mm χ Interference Millimetre waves Hz 3mm -0.3mm Infra-red Hz 0.3mm - χ Security 0.5mm Visible light Hz χ Spectrum regulation 0.5mm -0.4mm Ultra-violet Hz 0.4mm m χ Safety X-rays Hz 10-9 m m Gamma rays > 1019Hz < m Note: For each doubling of the distance between the source and receiver, a 6dB loss is experienced. 18

19 Radio Systems II Different propagation characteristics Effects: 1- reflection (meeting plan object) 2- refraction (medium with different wave speed) 3- diffraction (wave encounters an edge) LF 4- scattering (any other waves other than the above) Interface - multi-path Effects: e.g. TV ghosting diffraction reflection refraction 19

20 Typical Wireless Channel Simulation 20

21 RF Behaviour (key for slide 20) Reflection: occurs when a propagating electromagnetic wave strikes an object that has very large dimensions in comparison to the wavelength of the propagating wave. Reflection occurs from the surface of earth, buildings, walls, and many other obstacles (this reflection is referred to as multipath) Refraction: describes the bending of the wave as it passes through a medium of different density, i.e., as an RF wave passes into a denser medium the wave will be bent such that its direction changes where some of it will be reflected and some will be bent through the medium in different direction. Eg, atmospheric conditions change Diffraction: occurs when the radio path between the TX and RX is obstructed by a surface that has sharp irregularities or an otherwise a rough surface, i.e., the wave is bending around an obstacle Scattering: here if an RF wave strikes an even surface and is reflected in many directions with small amplitude reflections and destroys the main RF signal or if it encounters heavy dust it gets reflected into tiny particals. 21

22 Radio - ISM Existing spectrum allocation e.g. radio and TV, (mobile), telecommunication, satellite ISM Industrial, scientific and medical (ISM) 3 bands some frequencies already occupied uses include military Bands available MHz (26 MHz) GHz (83.5 MHz), unlicensed, 100mW GHz (125MHz), licence required, 2W Typical high noise: interference from other users 22

23 Modulation and Media Access techniques Spread Spectrum bits are not transmitted over a single frequency because of electrical interference in 2.4GHz frequency band transmitted bandwidth >> minimum bandwidth that the signal requires Therefore the source signal bandwidth must be spread across a much wider frequency range. very low signal to noise ratio (SNR) possible typical < 1 (0dB) Overall signal bandwidth: spread source signal C = Blog 2 (1 + S/N) C B [ln(1 + S/N) / ln 2] = 1.44 B S/N B 0.7 CN/S 32Kb/s, +30dB B 22 Hz 32Kb/s, -30dB B 22MHz Good noise immunity Hard to jam & snoop Works with low S/N Complex Hartley-Shannon Law Because Spread Spectrum signals are noise-like, they are hard to detect, hard to Intercept or demodulate are harder to jam (interfere with) than narrowband signals. 23

24 Log formula The logarithm log b (x) can be computed from the logarithms of x and b with respect to an arbitrary base k using the following formula: Taylor series For any real number z that satisfies, the following formula holds: 24

25 Direct sequence spread spectrum DS-SS Square pulse train: smaller, T higher signal bandwidth Combine data with pseudorandom binary sequence: pseudorandom noise (PN) spreading sequence Combine with carrier: e.g. BPSK, QPSK Chip - bit in PN sequence Chipping rate t b user data 0 1 XOR t c t b : bit period t c : chip period chipping sequence = resulting signal 25

26 DS - SS At Tx: synchronisation bits all 1 s At Rx: local copy of PN XOR with Rx autocorrelation correct sync generates preamble signal Synchronisation: preamble periodically DSSS typically fixed 22MHz, that makes about 14 channels avalaible to users (varies!) preamble PN: Tx No sync Rx PN XOR In sync Rx PN XOR 26

27 DS SS - example 27

28 28

29 Summary Wireless networks basic concept Radio diffusion modes and connectivity Wired and Wireless - Pros and Cons Basic propagation characteristics for mobile channels Modulation and Media Access Control Techniques (MAC) Direct Sequence Spread Spectrum (DS-SS) 29