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Chapter 3 - William Stallings, Data and Computer Communications ...
데이터 전송 전송 매체 신호 부호화 기술 디지털 데이터 통신 기술 데이터 링크 제어 프로토콜 다중화 확산 대역 회선 교환과 패킷 교환 교환망의 경로배정 데이터망의 혼잡제어 셀룰러 무선망 근거리망 개관 무선 근거리망
Kyungpook National University
Computer Engineering
Data and Computer Communications Chapter 3 – Data Transmission
Data Communication
통신 (通信): 떨어진 두 지점간에 정보를 전달하는 행위 정보 (information) -> 부호 (data) 표현 -> 신호 (signal) 변환 -> 전송 매체로 전달
Kyungpook National University
Computer Engineering
Example
일종의 데이터 통신 디지털 통신의 기본 원리 사용
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Computer Engineering
Data and Signal Data
¾ Entities that convey meaning Signals
¾ Electric or electromagnetic representations of data, physically propagates along medium Transmission
¾ Propagation and processing of signals Coding
: Data -> Signal
¾ How is information coded in a format suitable for transmission?
Kyungpook National University
Computer Engineering
Signal Representation 시간 영역 또는 주파수 영역에서 표현 Time
domain concepts
¾Analog 9Various in a smooth way over time
¾Digital signal 9Maintains a constant level then changes to another constant level
¾Discrete signal : 시간적으로 불연속적인 신호 ¾Periodic vs aperiodic signal Kyungpook National University
Computer Engineering
Example
Kyungpook National University
Computer Engineering
Sine Wave s(t) = A sin(2πft +Φ)
Peak amplitude (A) ¾ Maximum strength of signal ¾ Volts
Frequency (f) ¾ Rate of change of signal ¾ Hertz (hz) or cycles per second ¾ Period = time for one repetition (T) ¾ T = 1/f
Phase (φ) ¾ Relative position in time
Kyungpook National University
Computer Engineering
Varying Sine Waves s(t) = A sin(2πft +Φ)
Kyungpook National University
Computer Engineering
Frequency Domain Concepts Signal
are made up of many frequencies Fourier analysis can show that any signal is made up of component sine waves ¾ 주기 신호는 정현파 신호의 합으로 표현 가능 9 각 정현파 성분의 주파수는 기본 주파수 (fundamental frequency) 의 정수 배
∞
s (t ) = C0 + ∑ Cn cos(2πnf 0t + θ n ), n =1
Kyungpook National University
1 f0 = T Computer Engineering
Frequency Domain Representations A sin (2πft)
f
Freq
1/3 A sin (2π(3f)t)
A sin (2πft) + 1/3 A sin (2π(3f)t) Kyungpook National University
f
3f
Freq
3f
Freq
Computer Engineering
Example : Rectangular Pulse
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Freq
f
3f
f
3f 5f 7f
f
3f 5f 7f 9f 11f
Computer Engineering
Audio Signals
소리를 내는 물체가 공기 중의 압력을 변화시키면=> 공기 진동 => 공기 밀도의 차이 발생 => 공기의 파동 발생 Freq range 20hz-20khz (speech 100hz-7khz) http://www.ultrasonic-ringtones.com/
Kyungpook National University
Computer Engineering
Spectrum & Bandwidth
Spectrum ¾ Range of frequencies contained in signal
Absolute bandwidth ¾ Width of spectrum
Effective bandwidth ¾ Often just bandwidth ¾ Narrow band of frequencies containing most energy
DC component ¾ Component of zero frequency
Kyungpook National University
Computer Engineering
Baud Rate vs Data Rate
Baud rate : 신호의 속도 (signal components per second) Data rate : 데이터의 속도 (bits per second) ¾ 예: 1초 당 2400개의 신호를 전송하는 송신기에서 각 신호가 2비트씩 을 운반할 경우, baud rate = 2400, bit rate= 4800
Kyungpook National University
Computer Engineering
Baud Rate vs Data Rate
하나의 신호가 N비트를 운반하면 각 신호는 2N 개의 상태를 가진다 Bit
Signal
하나의 신호가 2개의 상태를 가지면 각 신호는 1 비트를 운반(0, 1) 하나의 신호가 4개의 상태를 가지면 각 신호는 2 비트를 운반(00, 01, 10, 11) 하나의 신호가 M개의 상태를 가지면 각 신호는 log2 M 개의 비트를 운반 Kyungpook National University
Computer Engineering
Data Rate and Bandwidth Any
transmission system allows a limited band of frequencies ¾ This limits the data rate that can be carried
Limited
bandwidth increases distortion 높은 데이터 전송률은 넓은 주파수 대역폭을 필요 ¾ 전송 시스템의 대역폭이 클수록 고속의 데이터 전송이 가 능
Kyungpook National University
Computer Engineering
Nyquist Bandwidth Consider noise free channels 2진 신호의 경우 최대 데이터 전송률 (Nyquist
rate) :
C = 2B
¾ B : 전송매체의 대역폭 [Hz] ¾ Given bandwidth B, the highest signal rate is 2B 신호가 M개의 상태를 가질 경우,
하나의 신호는 log2 M 개의 비트를 전송할 수 있으므로 ¾ C = 2B log2 M [bits/s]
Limited
by noise & other impairments
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Computer Engineering
Shannon Capacity Formula Max
possible data rate on comm channel Consider relation of data rate, noise & error rate ¾ Faster data rate shortens each bit, so bursts of noise affects more bits => Given noise level, higher rates means higher errors Capacity
C = B log2(1 + SNR)
¾ SNRdb= 10 log10 (signal/noise) : Signal to Noise Ratio (in decibels) ¾ Theoretical maximum capacity
Kyungpook National University
Computer Engineering
Transmission Impairments
Signal received may differ from signal transmitted causing: ¾ Analog - degradation of signal quality ¾ Digital - bit errors
Most significant impairments are ¾ Attenuation and attenuation distortion ¾ Delay distortion ¾ Noise Attenuation Delay Regular pulse Noise Kyungpook National University
Computer Engineering
Attenuation Where
signal strength falls off with distance Depends on medium Increase strength using amplifiers/repeaters Is also an increasing function of frequency
Kyungpook National University
Computer Engineering
Bandwidth and Received Signal Pulse after transmission
Bandwidth 500 Hz
Pulse before transmission Bit rate: 2000 bits per second Bandwidth 1700 Hz
전송매체의 대역폭
Bandwidth 2500 Hz
Bandwidth 4000 Hz Kyungpook National University
Computer Engineering
Delay Distortion Only
occurs in guided media Propagation velocity varies with frequency Hence various frequency components arrive at different times ¾ Zero-crossing point의 왜곡 (위상변화) Particularly
critical for digital data
¾ Since parts of one bit spill over into others ¾ Causing intersymbol interference
디지탈신호의 전송속도를 제한하는 주요 요인
Kyungpook National University
Computer Engineering
Noise Additional
signals inserted between transmitter and receiver Thermal ¾ Due to thermal agitation of electrons ¾ Uniformly distributed ¾ White noise Intermodulation
¾ Signals that are the sum and difference of original frequencies sharing a medium
Kyungpook National University
Computer Engineering
Noise Crosstalk
¾ A signal from one line is picked up by another Impulse
¾ Irregular pulses or spikes 9 Eg. External electromagnetic interference
¾ Short duration ¾ High amplitude ¾ A minor annoyance for analog signals ¾ But a major source of error in digital data 9 A noise spike could corrupt many bits
Kyungpook National University
Computer Engineering
Noise 0
1
0
1
1
0
0
1
1
0
0
1
0
1
0
수신 데이터
0
1
0
1
1
0
1
1
1
0
0
1
0
0
0
원래 데이터
0
1
0
1
1
0
0
1
1
0
0
1
0
1
0
전송 데이터 디지털 신호
잡음
신호+잡음
표본화 시간
비트 오류 Kyungpook National University
비트 오류 Computer Engineering
Example Transmitted data Transmitted signal Attenuation Attenuation distortion Delay distortion Noise Received signal Sampling pulse Received data Kyungpook National University
