Chapter 4 - William Stallings, Data and Computer Communications, 8/e

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1 Data and Computer Communications Chapter 4 Transmission Media

2 Design Factors Characteristics and quality determined by medium and signal Key concerns are data rate and distance Higher bandwidth gives higher data rate Transmission impairments Eg.. Attenuation, interference Unguided - wireless In unguided media - bandwidth produced by the antenna is more important Guided - wire / optical fiber In guided media - medium is more important More receivers introduces more attenuation

3 Transmission Characteristics of Guided Media Frequency Range Typical Attenuation Typical Delay Repeater Spacing Twisted pair (with loading) Twisted pairs (multipair cables) 0 to 3.5 khz khz 0 to 1 MHz khz 50 µs/km 2 km 5 µs/km 2 km Coaxial cable 0 to 500 MHz 7 10 MHz 4 µs/km 1 to 9 km Optical fiber 186 to 370 THz 0.2 to 0.5 db/km 5 µs/km 40 km

Twisted Pair Can use either analog or digital signals Needs a repeater every 2-3km 2 for digital signal Needs an amplifiers every 5km to 6km for analog signal Limited distance, limited bandwidth

4 Twisted Pair Can use either analog or digital signals Needs a repeater every 2-3km 2 for digital signal Needs an amplifiers every 5km to 6km for analog signal Limited distance, limited bandwidth (1Mhz), Limited data rate (100Mhz) Susceptible to interference and noise Near End Crosstalk Coupling of signal from one pair to another Occurs when transmit signal entering the link couples back to receiving pair Ie.. near transmitted signal is picked up by near receiving pair

5 Unshielded vs Shielded TP Unshielded twisted pair (UTP) Ordinary telephone wire Cheapest Easiest to install Suffers from external EM (Electromagnetic) interference Shielded twisted pair (STP) Metal braid or sheathing that reduces interference More expensive Harder to handle (thick, heavy)

6 Coaxial Cable Superior frequency characteristics to TP Performance limited by attenuation & noise Analog signals Amplifiers every few km Up to 500Mhz Digital signals Repeater every 1km

7 Optical Fiber Greater capacity Data rates of hundreds of Gbps Smaller size & weight Lower attenuation Electromagnetic isolation Greater repeater spacing 10s of km at least

8 Optical Fiber Transmission Modes

9 Optical Fiber Uses total internal reflection to transmit light Effectively acts as wave guide for to hz Can use several different light sources Light emitting diode (LED) Cheaper, wider operating temp range, lasts longer Injection laser diode (ILD) More efficient, has greater data rate

10 Electromagnetic Wave Electromagnetic Wave 무선통신을위한매개체로서주파수에의해그특성이구분 광의적 : 전자기파는전파, 적외선, 가시광선, 자외선등을포괄하는개념 협의적 : 적외선이상의파장을갖는전자기파 ( 즉 3kHz~3THz 의주파수를갖는전자기파 ) 전파법제 2조 : " 전파란인공적매개물이없이공간에전파하는 3,000GHz 보다낮은주파수의전자파 Maxwell 이 1864 년전자기파의존재를수학적으로예측 Hertz 는 1874 년전자기파의존재를실험으로증명 Marconi 는 1896 년무선전신을이용해대서양횡단무선통신에성공

11 Electromagnetic Wave 전기장과자기장이서로수직을이루면서, 파동 ( 波動 ) 으로서공간전파 시간에따라변화하는전기장은자기장을유도하고, 다시시간에따라변하는자기장은전기장을유도 즉, 전파는전류의변화로생기며, 파동형태로빛의속도로진행 1 도선에전류가흐르면, 자계발생 2 자계가발생하면, 자계를중심으로전계형성 3 형성된전계는시간에따라변하기때문에, 전계를중심으로다시자계가형성 4 이자계에의해다시전계가발생하고, 전계에의해자계가다시발생 전기장 자기장 전파의방향 v = 3*10 8 m/s

12 Frequency & Wavelength Frequency : 전파가 1초동안진동하는횟수 ( 단위 : Hz) Wavelength : distance occupied by one cycle Assuming signal velocity v, f = v/ λ v = 3*10 8 m/s (speed of light in free space) 전파의파장과주파수는반비례관계 주파수 ( 파장 ) 에따라직진성과회절성 ( 꺽임성 ), 투과성이다름 v m for 1s λ

13 Electromagnetic Spectrum 전파는공공자원 : 유한한주파수자원을효율적으로이용하기위해서는주파수할당을국제적, 국가적통제아래관리 ITU (International Telecommunication Union) 이주파수대의배치, 할당, 이용의조정등을총괄 => 주파수분배표

14 Characteristics of Electric Wave 회절성 회절성 : 장애물의후방까지도달하는성질 800 Mhz 1.7 Ghz 음영지역 ( 소 ) 음영지역 ( 대 )

15 Characteristics of Electric Wave 직진성 직진성이강하면곧게퍼짐 목적지로전파를집중복사가능 투과성 800 Mhz 1.7 Ghz 침투불가지역

16 Wave vs Frequency 고주파 : 강한직진성 / 투과성, 약한굴절성 / 회절성 회절성이약해장애물이있으면넓은음영지역발생 전파도달거리가짧아서신호의질이떨어짐 신호감쇠가주파수의제곱에비례함 고속데이터전송율 : 대량의정보전송이가능해초고속통신에적합 저주파 : 약한직진성 / 투과성, 강한굴절성 / 회절성 회절성이강하여장애물이있어도골고루전달 음영지역이적다 감쇠가적으므로수신감도우수 해상ㆍ항공통신등장거리통신에적합

17 Example 지형탐사또는어류탐지용초음파 바닥깊은곳의해저나고기떼와부딪쳐다시배로돌아와야하므로회절성보다는직진성이강해야함 초음파의파장은짧아야함 음파의경우, 주파수가낮은남자의목소리가멀리퍼짐

18 Example AM 531~1,602KHz 로서중파 KBS AM 711KHz 의파장 = 421.6m 회절성이강하다 => 산속이나지하도속에서도잘들린다 => 장애물에의한통화차단은없다 => 난청지역이없다 AM 방송송신탑은대구에 1개만있으면됨 ( 커버리지가넓으므로 ) FM 88.1~107.9MHz 로서초단파대에해당 MBC FM 95.9MHz 의파장 = 3.126m 회절성이낮아장애물을만나면방송이끊어짐 지하도나터널에들어가면먹통 더많은데이타량으로음악을표현하여전송하므로음질이좋음 TV 수 GHz 직진성이강하고회절성이약하다 가시청영역을넓히려면송수신안테나의위치를높여야함 ( 송신소는고지대에위치 ) 난시청지역이많다

19 Example 빛 약 Hz ( 파장 = 약 400nm~ 700nm) 직진성이강하고회절성이약하다. 회절성이약해장애물이있으면빛이도달하지않아그림자가생긴다. 소리 매질 : 공기 속도 : 340m/s 340Hz 일경우 ( 파장 = 1m) 회절성이매우강하고직진성은약하다. 소리 장애물이있어도잘들린다.

20 Example 016, 018, 019 기지국 : 1840 ~ 1870MHz, 이동단말 : 1750 ~ 1780MHz 011, 017 기지국 : 869 ~ 894MHz, 이동단말 : 824 ~ 849MHz 800MHz 대역이 1.8GHz 에비해감쇠가적고, 전파의도달범위가넓다.( 약두배차이 ) 1.8GHz 는 800MHz 에비하여 0.8~3dB 정도의우수한투과율제공 통화품질은도달거리, 회절율, 투과율순으로영향을미침 종합적으로는구석구석잘전달되는 800MHz 대역의신호품질이우수함 안테나의길이는파장에비례 016, 018, 019 : 휴대폰 (1765MHz) 의파장 = 0.17m 011, 017 전화기의안테나길이가 016, 018, 019 안테나보다 2 배길다

21 Example B A 전파의도달거리 A: 800 Mhz 의서비스반경 B: 1.7 Ghz 의서비스반경 800MHz vs 1.8GHz, 휴대폰통화품질우수주파수는?

22 Ground Wave

23 Sky Wave

24 Line of Sight

25 Transmission Loss Free space loss Loss of signal with distance Atmospheric absorption From water vapour and oxygen absorption Multipath Multiple interfering signals from reflections Refraction Bending signal away from receiver

26 Free Space Loss

27 Multipath Interference 여러개의송신신호가공중에서여러경로를거쳐수신안테나에수신 서로다른경로를거쳐전달되는신호는서로다른진폭감쇠와위상변화를겪음

28 Refraction Velocity of electromagnetic wave is a function of density of material 3 x 10 8 m/s in vacuum, less in anything else Speed changes as move between media Index of refraction (refractive index) is Sin(incidence)/sin(refraction) Varies with wavelength Have gradual bending if medium density varies Density of atmosphere decreases with height Results in bending towards earth of radio waves Hence optical and radio horizons differ

29 Antennas Electrical conductor used to radiate or collect electromagnetic energy Transmission antenna Radio frequency energy from transmitter Converted to electromagnetic energy by antenna Radiated into surrounding environment Reception antenna Electromagnetic energy impinging on antenna Converted to radio frequency electrical energy Same antenna is often used for both purposes

30 Radiation Pattern Power radiated in all directions Not same performance in all directions An isotropic antenna is a (theoretical) point in space Radiates in all directions equally With a spherical radiation pattern

31 Parabolic Reflective Antenna 에너지원에서나오는파는포물면의축과평행한방향으로반사 이론적으로분산이없는평행한빔생성

32 Antenna Gain 이상적인등방성안테나에의하여특정방향으로분사되는출력전력과해당안테나에의해그방향으로방사되는전력의비 Measure of directionality of antenna Measured in decibels (db) Example : 안테나이득이 3dB 일경우, 해당안테나는등방성안테나에비해특정방향으로 3dB 만큼 ( 혹은 2배로 ) 더센전력으로에너지를방사 Results in loss in power in another direction

33 Wireless Transmission Frequencies 2Ghz to 40Ghz Microwave Highly directional Point to point Satellite 30Mhz to 1Ghz Omnidirectional Broadcast radio 3 x to 2 x Infrared

34 Terrestrial Microwave 1-40Ghz frequencies 직진성기반의기술 가시구역내의고정점사이의통신 ( 전화 텔레비전등의중계 ) 에사용 Microwave 는전파의회절이거의없으므로, 같은지역에서같은주파수를사용해도전파의방향이다르면혼신이생기지않는다 Requires fewer repeaters but line of sight Use a parabolic dish to focus a narrow beam onto a receiver antenna 문제점 Main source of loss is attenuation Distance, rainfall Also multipath interference

35 Satellite Microwave Satellite is relay station Receives on one frequency, amplifies or repeats signal and transmits on another frequency Eg. Uplink Ghz & downlink Ghz Typically requires geo-stationary orbit Height of 35,784km Spaced at least 3-4 apart Typical uses Long distance telephone Private business networks Global positioning

36 Satellite Link

37 Broadcast Radio 30Mhz 1Ghz FM radio, UHF and VHF television Omnidirectional Still need line of sight Suffers from multipath interference Reflections from land, water, other objects

38 Infrared No licenses required Typical uses - remote control ~ Hz 장점 높은주파수로인하여전송속도및속도개선의잠재력풍부 송수신에사용되는각종소자가주로반도체이기때문에처리속도가빠름 간섭이작고통신기기자체가가볍고, 작으며, 전력소모가적다. 단점 Are blocked by walls 사방으로산란되면서신호에너지가급격히감소

FTTH 기술발표

FTTH 기술발표 2 3 xdsl /UTP FTTH / 2002 2005 2010 2 Mbps 6 Mbps 100Mbps * 10 Mbps 45Mbps 155Mbps FTTO / FTTD / Digital (DBS) 53Mbps/4km LMDS ADSL : Asymmetric Digital Subscriber Line HDSL : High speed Digital Subscriber