강좌개요 목표 : 데이터통신의기본개념과동작원리를습득 교재 : 강의자료 (http:// http://netopia.knu.ac.kr/) Data and Computer Communications, 8 th ed. by William Stalling 또는번역본 평가방법 중간고사 40% 기말고사 50% 출석 10% 4학년 1학기에개설되는 ' 컴퓨터망 ' 과연계하여강좌가진행되므로, 향후 ' 컴퓨터망 ' 과목을수강하고자하는학생은반드시이강좌를먼저수강하여야함.
강좌내용 3장 데이터전송 4장 전송매체 5장 신호부호화기술 6장 디지털데이터통신기술 7장 데이터링크제어프로토콜 8장 다중화 9장 확산대역 10 장 회선교환과패킷교환 12 장 교환망의경로배정 13 장 데이터망의혼잡제어 14 장 셀룰러무선망 15 장 근거리망개관 17 장 무선근거리망
Data and Computer Communications Chapter 3 Data Transmission
Data Communication 통신 ( 通信 ): 떨어진두지점간에정보를전달하는행위 정보 (information) -> 부호 (data) 표현 -> 신호 (signal) 변환 -> 전송매체로전달
Example 일종의데이터통신 디지털통신의기본원리사용
Transmission Terminology Data transmission occurs between a transmitter & receiver via some medium Guided medium Eg.. Twisted pair, coaxial cable, optical fiber Unguided / wireless medium Eg.. Air, water, vacuum
Transmission Terminology Simplex One direction Eg.. Television Half duplex Either direction, but only one way at a time Eg.. Police radio Full duplex Both directions at the same time Eg.. Telephone
Data and Signal Data Entities that convey meaning Signals Electric or electromagnetic representations of data, physically propagates along medium Transmission Propagation and processing of signals Coding : Data -> > Signal How is information coded in a format suitable for transmission?
Signal Representation 시간영역또는주파수영역에서표현 Time domain concepts Analog vs Digital signal Discrete signal : 시간적으로불연속적인신호 Periodic vs aperiodic signal
Analog Signals Various in a smooth way over time
Digital Signals Maintains a constant level then changes to another constant level
Periodic Signals Periodic signal : Pattern repeated over time Aperiodic signal : Pattern not repeated over time
Sine Wave Peak amplitude (A) Maximum strength of signal Volts Frequency (f) Rate of change of signal Hertz (hz( hz) ) or cycles per second Period = time for one repetition (T) T = 1/f Phase (φ)( s(t) ) = A sin(2πft +Φ) + Relative position in time
Varying Sine Waves s(t) ) = A sin(2πft +Φ) +
Frequency Domain Concepts Signal are made up of many frequencies Fourier analysis can show that any signal is made up of component sine waves Can plot frequency domain functions 주기신호는정현파신호의합으로표현가능 각정현파성분의주파수는기본주파수 (fundamental frequency) 의정수배 s t) = C + n= C cos(2πnf ( 0 n 0 + θn), 0 = 1 t f 1 T
Frequency Domain Representations A sin (2πft) f Freq 1/3 A sin (2π(3f)t) 3f Freq A sin (2πft) + 1/3 A sin (2π(3f)t) f 3f Freq
Example : Rectangular Pulse f 3f Freq f 3f 5f 7f f 3f 5f 7f 9f 11f
Spectrum & Bandwidth Spectrum Range of frequencies contained in signal Absolute bandwidth Width of spectrum Effective bandwidth Often just bandwidth Narrow band of frequencies containing most energy DC component Component of zero frequency
Audio Signals 소리를내는물체가공기중의압력을변화시키면 => 공기진동 => 공기밀도의차이발생 => 공기의파동발생 공기중의압력변화로같은진동인데도음색이다른이유는파동의모양, 즉파형이다르기때문
Audio Signals Freq range 20hz-20khz 20khz (speech 100hz-7khz) Easily converted into electromagnetic signals Varying volume converted to varying voltage http://www.ultrasonic-ringtones.com ringtones.com/
Baud Rate vs Data Rate Baud rate : 신호의속도 (signal components per second) Data rate : 데이터의속도 (bits per second) 예 : 1 초당 2400 개의신호를전송하는송신기에서각신호가 2비트씩을운반할경우, baud rate = 2400, bit rate= 4800
Baud Rate vs Data Rate 하나의신호가 N비트를운반하면각신호는 2 N 개의상태를가진다 Bit Signal 하나의신호가 2개의상태를가지면각신호는 1 비트를운반 (0, 1) 하나의신호가 4개의상태를가지면각신호는 2 비트를운반 (00, 01, 10, 11) 하나의신호가 M개의상태를가지면각신호는 log 2 M 개의비트를운반
Data Rate and Bandwidth Any transmission system has a limited band of frequencies This limits the data rate that can be carried Limited bandwidth increases distortion Example Square have infinite components and hence bandwidth But most energy in first few components Have a direct relationship between data rate & bandwidth 높은데이터전송률은넓은주파수대역폭을필요 전송시스템의대역폭이클수록고속의데이터전송이가능
Nyquist Bandwidth Consider noise free channels 최대데이터전송률 (Nyquist rate) : C = 2B B : 전송매체의대역폭 [Hz] Given bandwidth B, the highest signal rate is 2B If rate of signal transmission is 2B, then we we can carry signal with frequencies no greater than B For binary signals, 2B bps needs bandwidth B Hz 신호가 M개의상태를가질경우, 하나의신호는 log 2 M 개의비트를전송할수있으므로 C = 2B log 2 M [bits/s] So increase rate by increasing signals at cost of receiver complexity Limited by noise & other impairments
Shannon Capacity Formula Max possible data rate on comm channel Consider relation of data rate, noise & error rate Faster data rate shortens each bit, so bursts of noise affects more bits => Given noise level, higher rates means higher errors Channel capacity is a function of Data rate - in bits per second Bandwidth - in cycles per second or hertz Noise - on comm link Error rate - of corrupted bits Capacity C = B log 2 (1 + SNR) SNR = db 10 log 10 (signal/noise) : Signal S to Noise Ratio (in decibels) Theoretical maximum capacity Get lower in practise
Transmission Impairments Signal received may differ from signal transmitted causing: Analog - degradation of signal quality Digital - bit errors Most significant impairments are Attenuation and attenuation distortion Delay distortion Noise Attenuation Regular pulse Delay Noise
Attenuation Where signal strength falls off with distance Depends on medium Received signal strength must be: Strong enough to be detected Sufficiently higher than noise to receive without error So increase strength using amplifiers/repeaters Is also an increasing function of frequency So equalize attenuation across band of frequencies used Eg.. Using loading coils or amplifiers
Bandwidth vs Received Signal Input signal 전송매체 Output signal Bandwidth
Example Pulse after transmission Bandwidth 500 Hz Pulse before transmission Bit rate: 2000 bits per second Bandwidth 1700 Hz 전송매체의대역폭 Bandwidth 2500 Hz Bandwidth 4000 Hz
Delay Distortion Only occurs in guided media Propagation velocity varies with frequency Hence various frequency components arrive at different times Zero-crossing point 의왜곡 ( 위상변화 ) Particularly critical for digital data Since parts of one bit spill over into others Causing intersymbol interference 디지탈신호의전송속도를제한하는주요요인
Noise Additional signals inserted between transmitter and receiver Thermal Due to thermal agitation of electrons Uniformly distributed White noise Intermodulation Signals that are the sum and difference of original frequencies sharing a medium
Noise Crosstalk A signal from one line is picked up by another Impulse Irregular pulses or spikes Eg.. External electromagnetic interference Short duration High amplitude A minor annoyance for analog signals But a major source of error in digital data A noise spike could corrupt many bits
Noise 전송데이터 0 1 0 1 1 0 0 1 1 0 0 1 0 1 0 디지털신호 잡음 신호 + 잡음 표본화시간 수신데이터 원래데이터 0 1 0 1 1 0 1 1 1 0 0 1 0 0 0 0 1 0 1 1 0 0 1 1 0 0 1 0 1 0 비트오류 비트오류
Example Transmitted data Transmitted signal Attenuation Attenuation distortion Delay distortion Noise Received signal Sampling pulse Received data
Advantages & Disadvantages of Digital Signals Cheaper Less susceptible to noise But greater attenuation Digital now preferred choice