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Physical Layer

Physical Layer. Physical Layer. Provide Physical connections Activate Maintain Deactivate Provide bit transmission b/w Data Link Layer May involve intermediate(relay) system Operates above the physical layer DTE 와 DCE 사이의 전기적 신호 전달 : digital or analog signal < 기능 > - 데이타 (bit) 의 전달

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Physical Layer

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  1. Physical Layer

  2. Physical Layer • Provide Physical connections • Activate • Maintain • Deactivate • Provide bit transmission b/w Data Link Layer • May involve intermediate(relay) system • Operates above the physical layer • DTE와 DCE사이의 전기적 신호 전달: digital or analog signal • < 기능 > • - 데이타(bit)의 전달 • - control signal transfer • - clock 제공( for synchronous transfer) • - 전기 접지 제공 • - 기계적 접속을 제공 : connector spec.

  3. 통신 이론 • 신호 유형 l애널로그 신호 x(t) / 이산 신호 x(kT) l주기 신호 x(t)=x(t + T) / 비주기 신호 • SNR(Signal to Noise Ratio in dB) 정보신호를 전송할 때 잡음이 들어가 품질이 나빠지는 정도 l신호 대 잡음 비 = l SNR(dB) = V1, V2 : 전압, 전류 P1, P2 : 전력(Watt)

  4. 통신 이론 • 기준 전력(W)과 상대 전력의 비에 따른 dB 기준 전력 상대 전력 dB ------------------------------------------------- 1 1 0 1 2 3 1 10 10 1 100 20 1 1000 30 2 1 - 3 10 1 - 10 ------------------------------------------------- if V2= 2* V1 : 6 dB if V2= 10* V1 : 20 dB if V2= 100*V1 : 40 dB 전화 : 30dB, radio: 40dB, TV : 45dB PCM : 8bit coding 은 30dB 보장

  5. 통신 이론 정보 표현 l bit l byte(octet) : 8 bit l Kbit : 210 bit l Mbit : 220 bit l Gbit : 230 bit 전송속도 l bps(bit per second) : 매초당 전송되는 비트수 l baud : 매초당 전송되는 신호 요소의 수 오율 l비트 오율(BER : Bit Error Rate) l블록 오율(block error rate) l잉여 오율(RER : Residual Error Rate)

  6. a y +A a A  180 x 360  90  -A Signal • Electromagnetic Waves : data transmission • Sine Waves : a=A sin() • a : 반지름 A인 원을 따라 움직이는 선이 Y축에 투영되는 값 • : X축과 반경이 형성하는 각의 크기 • maximum amplitude : sine wave 의 최대값 • frequency f : 초당 회전 수(cycle/sec) (Hertz) = 1/T(period) • a(t) =A sin(360°ft) = A sin(360°t/T) Radian frequency General Form of sine waves : Phase Sine wave 특징: 신호처리 과정 중에도 sine wave의 구조가 그대로 유지 모든 주기 신호들은 sine wave의 합 (Fourier theorem)

  7. Fourier Theorem • 모든 주기적인 파형은 그 파형의 주파수인 fundamental frequency를 갖는 파형과 fundamental frequency(fo)의 정수배 주파수를 갖는 파형들의 합으로 나타낼 수 있다.

  8. Sin 파의 주파수성분

  9. 0.5 1 1.5 2 • Harmonics : fundamental frequency(fo) 의 정수배 n fo • Example : square wave

  10. Bandwidth-Limited Signals 전송 용량에 따른 디지털 신호의 형태 - Broad Bandwidth(고용량 전송채널) 는 양질의 통신 서비스를 가능하게 하는 필요 조건이다.

  11. Bandwidth - signal bandwidth : signal이 가지고 있는 frequency의 범위 - medium bandwidth : transfer media가 통과시킬 수 있는 frequency 범위 - equalizer : distortion이 발생한 신호를 가능한 원래의 신호로 보상하기 위한 장치 - Hz : cycle/sec : sin파가 초당 진동하는 회수 -> analog signal - bps : bit/sec -> digital signal 주파수 대역폭 B Hz 일 때 digital signal speed B bps Baud : 매 초당 신호 변화의 상태가 몇 개의 다른 상태로 있었는가를 나타내는 통신속도의 차이 baud rate = bps / # of bits per signal bit rate = log2(회선 상태수) * baud rate Packet : 데이타를 전송할 때 일정한 크기의 블럭 단위로 분할하여 전송단위로 사용 1 packet = [ 1024 bits(128byte) or 2048 bits(256byte) ]

  12. Nyquist: If bandwidth W 1/2T, where T is the symbol interval, one can receive symbols without inter symbol interference (ISI). 1/T = 2W is called the Nyquist rate Bandwidth without noise maximum symbol rate (baud) 1/T = 2W symbol/sec maximum data rate (bps) = 1/T log2V = 2W log2V bps for V -ary signaling V를 크게 할 수록 얼마든지 높은 정보율을 얻는다. 데이타의 전송 속도(bps) - serial transfer S= 1/T log2V - parallel transfer S= SUM of ( 1/Ti log2Vi ) V: discrete levels / signal T: min interval time of each pulse 1/T : # of pulses / sec

  13. Bandwidth with noise Signal to noise ratio S/N = signal power/noise power Shannon: over a channel with bandwidth H and additive white Gaussian noise (AWGN), the maximum rate at which data can be transmitted with high reliability (called ‘channel capacity’) is given by Cmax = Wlog2(1 + S/N)bps. eq. W = 3100Hz for a voice channel and (S/N)db = 30 = 10log10(S/N). S/N = 1000 max. data rate achievable = 3,100log2(1+103) = 30,894 bps

  14. 전송매체 열화 현상 • 감쇄 • 왜곡 • 잡음 l열잡음 l상호변조 잡음 l누화 l반향 l충격성 잡음 l페이딩 • 감쇄 및 왜곡 특성

  15. Transmission Media • Hardwire typical data rate (Bandwidth) typical repeater spacing medium note twisted pair 1 - 2 Mbps (250Mhz) 2 - 10 Km cheap/easy to install low/medium data rate low noise immunity used in ‘local loop’ in PSTN coaxial cable (baseband) (50 cable) 10 - 20 Mbps (300Hhz) 1 - 2 Km moderate cost/rate good noise immunity baseband (digital) signaling easy to tap -> widely used in LAN coaxial cable (broadband) (75 cable) 150 Mbps - 500Mbps (300-450MHz) 10 Km higher cost/rate than baseband broadband (analog) signaling normally multiple channels are derived (FDMA) - used in CATV optical fiber 1 Gbps and higher (2GHz) 10 - 100 Km small size high noise immunity difficult to splice or tap

  16. Fiber Optics Attenuation I dB = Coaxial Cable (b) Single cable inband traffic(5-30Mhz) outband traffic(40-300Mhz)

  17. Fiber Optic Network A passive star connection in fiber optic network Broadcasting Hardware

  18. Wireless transmissionmicrowave, ground radio, infrared, laser • - Terrestrial microwave - long distance communication, directional antenna/repeaters (up to 100 Km spacing) • Fundamental relation in vacuum • c : speed of light(3*108 m/sec) • EX: 1-Mhz waves ->300m, 1-cm waves->30Ghz ------------------------------------------------------------------ 주파수 대역 명칭 용도 ------------------------------------------------------------------ 30 - 300 kHz LF(Low Frequency) 해상 통신 300 - 3000 kHz MF(Medium Frequency) AM 방송 3 - 30 MHz HF(High Frequency) 단파 라디오 30 - 300 MHz VHF(Very High Frequency) FM 방송, VHF TV 300 - 3000 MHz UHF(Ultra High Frequency) UHF TV, 지상 M/W 3 - 30 GHz SHF(Super High Frequency) 통신위성, 지상 M/W 30 - 300GHz EHF(Extremely High Frequency) 단거리 점대점 통신 ------------------------------------------------------------------

  19. Electromagnetic spectrum

  20. Radio transmission Lightwave transmission : Laser Microwave Transmission : satellite TV dish Infrared and Millimeter waves : Local LAN(적외선) portable computer

  21. The Telephone system(PSTN) Relationship of LETA, LEC, IXC Structure of PSTN LATA : Local Access & Transport Area LEC : Local Exchange Carrier IXC : IntereXchange Carrier

  22. Typical Circuit route for medium distance call • Local loop

  23. Transmission Impairment • Attenuation • the loss of energy as the signal propagates outward • delay distortion • different Fourier components travel at different speeds • noise • unwanted energy from sources other than the transmitter • Due to these problem : • baseband (DC) signaling unsuitable except at slow speed and over short distances • telephone line Analog carrier(AC) signaling is used • MODEM (DC<->AC) • modulation (변조) : DC signal -> AC signal • demodulation(복조) : AC signal -> DC signal

  24. Modulation Methods 4bits/baud modulation (9600bps over 2400baud) QAM (Quadrature Amplitude Modulation) ITU V.32 3bits/baud modulation V.32 bis : 14,400bps (6bits/baud) V.34 : 28,800 bps Trellis coding : maximize the chance of detecting errors

  25. Echo suppressor • Filtering : 주파수 영역중 특정 범위의 주파수 요소만을 선택하거나 배제하는 과정

  26. Asynchronous transmissions character - represented by a fixed number of buts (5 - 8 bits) Two states: - state 1: e.q. negative voltage in RS-232-C - state 0: e.q. positive voltage in RS-232-C Idle state, the line is in state 1. To start transmission, the line goes to state 0 for one bit time (start bit). During the next 5 to 8 bits, the line is set at state 1 or 0 representing the character to be transmitted. After each character, the line is maintained at state 1 for a minimum duration of 1, 1.5 or 2 bit times (stop interval).

  27. 5 - 8 bits idle state stop interval start bit remain idle or start next character optional parity bit Because the receiver (re)synchronizes at the beginning of every character, a moderate timing error can be tolerated. Overhead for start bit and stop interval is significant e.q. 7 bit character + 1 bit parity; 1 bit stop interval overhead = 2/10 = 20%

  28. Synchronous transmissions A block of bits are transmitted without start bit or stop interval. Bit timing is provided by - separate clock line (e.q. X.21 - S line; EIA-232-D pin 15, 17). - sufficient state transitions for receiver synchronization. Manchester encoding used in Ethernet - every bit has a state transition for bit synchronization. (no dc component, higher bandwidth requirement) Usually a block of data includes preamble, block error check and post-preamble for defining the block boundary and error control. frame = preamble+data block+block error check+post-preamble

  29. RS-232 인터페이스 l기계적 특성 : 핀수 등 물리적 기능 l전기적 특성 : 전압 레벨 및 타이밍 l기능적 특성 : 수행되는 기능 l절차적 특성 : 기능에 근거한 데이터 전송의 시퀀스

  30. RS-232 인터페이스 • 특성 l유효 거리 : 15m 이내 l전송 속도 : 20 Kbps 이내 l전압 레벨 : 데이터 1, OFF 상태(-3V 이하) 데이터 0, ON 상태(+3V 이상) l전압 측정 : 불평형 모드 l전송 방식 : 비동기식 l핀수 : 25핀

  31. RS-232 인터페이스 • 핀 기능 ------------------------------------------------------------------ 핀 번호 기능 핀 번호 기능 ------------------------------------------------------------------ 1 ground 7(AB) signal ground 2(BA) transmitted data 8(CF) carrier detect 3(BB) received data 20(CD) DTE ready 4(CA) RTS(request to send) 25(TM) test mode 5(CB) CTS(clear to send) 18(LL) local loopback 6(CC) DCE ready 21(RL) remote loopback ------------------------------------------------------------------

  32. Fiber in the local Loop Using Tel line Using CATV cable

  33. Trunk and Multiplexing • Multiplexing techniques - a number of lines share a common channel. • Static - FDM, TDM (STDM, synchronous TDM) • Dynamic - ATDM (asynchronous TDM, or statistical MUX) - allocates time slots on demand FDM(Frequency Division Multiplexing) WDM(Wavelength Division Multiplexing)

  34. 8000 samples/sec (125 ) TDM(Time Division Multiplexing) PCM(Pulse Code Modulation) - T1carrier (1.544 Mbps), E1(2.048 Mbps) 24ch/frame*8bit/ch+1bit=193bit/frame 193bit/frame*8000frame/sec=1.544Mbps Multiplexing T1 streams onto higher carriers

  35. Needs a buffer in case demand exceeds channel capacity momentarily. Each data unit has a header containing its source address. Good for burst traffic - for which static MUX tends to waste capacity while a user is idle. Comparison of response time (FDM and ATDM): N line traffic is multiplexed. Assume message arrival process is Poisson with rate / N per line and message length is exponentially distributed with mean . Channel capacity available to a line in FDM = C/N Channel capacity available to N lines ATDM = C Assume that the overhead for address in ATDM is negligible.

  36. Synchronous Transmission -1 • PDH(Plesiochronous Digital Hierarchy) • 기존의 디지털 계위 : 북미식과 유럽식 DS-1,DS-2,DS-3,DS-4등 • SDH(Synchronous Digital Hierarchy) • B-ISDN의 NNI(Network Node Interface)표준으로 사용할 수 있도록 일반화 시킨 것 • PDH • 북미식: DS-1(1.544Mbps), DS-1C(3.152Mbps),DS-2(6.312Mbps),DS-3(44.736Mbps),DS-4E(139.264) • 유럽식: DS-1E(2.048Mbps), DS-2E(8.448Mbps), DS-3E(34.368Mbps), DS-4E(139.264Mbps), DS-5E(564.992Mbps) • SDH • 북미,유럽을 통합하여 세계적 통신이 가능하도록 일반화한 것 • STM-n , n=1,4,16…. (155.520Mbps,622.080Mbps,2488.320Mbps...)

  37. Synchronous Transmission -2 • Digital Transmission • PDH신호를 구성하여 baseband 를 사용하여 전송하는 기존 방식 • Synchronous Transmission • SDH 에 의거한 전송 방식 • Synchronous Multiplexing 과정을 통해 기존의 DS-1,DS-2,DS-3,DS-4등 계위 신호들을 STM-n으로 Multiplexing • Synchronous ADD/DROP 장치나 Synchronous Cross-Connect 장치를 통해 망을 재구성 • Synchronous optical network을 사용하여 전송 • 장점 • multiplexing/demultiplexing용이 • OAM(operation, Administration, Maintenance)기능 향상 • 구조간단, 확장용이

  38. 전송방식의 표준화 • Metrobus : AT&T Bell lab. • Optical transmission system • 150Mbps를 표준신호, • Overhead , Box 개념 사용 • SONET : Bellcore. • CCITT T1commitee에서 북미 표준으로 채택 • multiplex multiple digital channels 제공 • 50Mbps(T3: 44.736Mbps) 토대로 계층화된 구조 • Overhead 구분 체계화

  39. SONET path • A SONET system consist of switches, multiplexers, and repeaters, all connected by fiber • A SONET path A SONET Architecture

  40. SONET frame • SONET frame (STS-1: Synchronous Transport Signal-1) • 90 col * 9 rows = 810bytes • 810bytes/125 = 8bits/byte*810byte/frame=6,480bit/frame • 6,480bits/frame*8,000frames/sec = 51.84Mbps (STS-1) • Payload : 87*9*8*8000= 50.112 Mbps

  41. SONET multiplexing • SONET & SDH multiplex rate

  42. Switching Circuit Packet

  43. Timing of event

  44. Narrowband ISDN-1/2 • Channel Type • A : 4kHz analog voice ch. • B : 64 kbps digital PCM ch. For voice and data • C : 8-16 kbps digital ch. • D : 16 kbps digital ch. for out-of-band signaling • E : 64 kbps digital ch. for internal ISDN signaling • H : 384-, 1536-, 1920-kbps digital ch. • ISDN • BASIC rate : 2B+1D • Primary Rate : 23B+1D or 30B+1D(유럽) • Hybrid : 1A + 1C

  45. Narrowband ISDN-2/2

  46. Broadband-ISDN & ATM • B-ISDN • a digital virtual circuit for moving fixed size packets(cell) at 155Mbps • Virtual Circuit • it is simply defined by table entries inside the switch • ATM : • a packet switch not a circuit switch technology

  47. Transmission in ATM Network • ATM standard • Primary rate : 155Mbps • Additional rate : 622.08 Mbps • Synchronous Transmission Mode • Asynchronous Transmission Mode

  48. 1.5 Mbps, DS1 • (24 bytes x 8 bits/byte)/125 usec = 1.536 Mbps of payload • Cell delineation by HEC detection as with STM-1 • Cell payload = 1.536 Mbps x (48/53) = 1.391 Mbps 125 usec ...F B B...B F B...B F... 24 bytes Framing Bit

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