CHAP 2. The Physical Layer

강의내용 - Stalling책

Chap. 3. 데이터 전송, 10th ed. (7th ed.)

Chap. 5. 신호 인코딩 기술, 10th ed. (7th ed.)

• 강의 내용

- 전송의 개념: Data와 Signal의 이해.

Topics

• 전송(Transmission): 서로 떨어져 있는 computer들간에 어떻게 전기적(electric, 전류-current, 전압-voltage)/전자기적(electromagnetic, 전파) 신호(signal)를 전달하는가?

- Data

- Signal

- Transmission

(ex) 정보→언어→목소리(변조, modulation)→음파(340m/sec)→귀→언어→정보

(info.) (data)	(signal)	<transmission> (signal)(data)(info.)

[‘의미(invisible)’ → 의미를 담음(visible)]

• 매체(Media): 신호를 전달하기 위해 서로 떨어져 있는 computer들을 “물리적”으로 어떻게 연결하는가?

- 유선(wired): 유도(guided)

- 무선(wireless): 비유도(unguided)

Introduction

• Communication system

computer	LAN card	LAN card     computer

• Basic Definitions

- Information: intelligible meaning to human (invisible)

- Data: entities that 'represent/convey' meaning (visible)

① analog data: 음성(audio), 그림(image), 영상(video)

⇒ 'Continuous' (연속)

② digital data: 문자(text/symbol), 숫자(number)

⇒ 'Discrete' (비연속/이산)

(Wikipedia) ASCII code

(Stallings 7th ed.) ASCII code

- Signal: electric/electromagnetic encoding of data

① 'Analog' signal: represent data with "continuous" varying electric /electromagnetic quantity. - "Wave" form

ex)	PSTN(가입자→전화국), audio tape, LP, VTR, broadcasting

② 'Digital' signal: represent data with "discrete" (e.g. two levels) of electric /electromagnetic quantity. - "Pulse" form

ex)	computer: two voltage level (0 volt - 0, 1 volt - 1) - ASCII

PSTN(전화국→전화국)

CD, CD-ROM, HDD, digital audio tape, digital TV

(Stallings 10th ed.) p.72 fig. 3.1    (Stallings 7th ed.) p.69 fig. 3.1

․ Noise: additional "unwanted" signals that are inserted somewhere between transmission and reception.

(Stallings 10th ed.) p.96 fig. 3.15   (Stallings 7th ed.) p.94 fig. 3.16

- Transmission: the communication of data

⇒ By the 'propagation' and 'processing' of signals along a suitable medium.

① analog data → analog signal

(ex) PSTN(가입자→전화국), broadcasting(radio, TV)

② analog data → digital signal

(ex) PSTN(전화국간, codec)

③ digital data → analog signal

(ex) modem(MOdulator-DEModulator, 변복조기)

④ digital data → digital signal

(ex) LAN

(Stallings 10th ed.) p.88 fig. 3.13	(Stallings 7th ed.) p.86 fig. 3.14

- Transmission media

․ wired: twisted pair, coaxial cable, optical fiber

․ wireless: radio, infrared, light, laser, microwave(terrestrial/satellite)

Data

• analog data: continuous values on some interval

(ex) 음성(audio), 그림(picture), 영상(video)

audio: 주파수 - 20Hz ～ 20,000Hz < supersonic wave(초음파)

• digital data: discrete values

(ex) ASCII code:	문자(text/symbol): ㄱㄴㄷ …, a b c … , & * # …

숫자(number): 1 2 3 …

Signal

• Def.: 물리적인 양(전기적 - 전류, 전압, 전자기적 - 전파)의 시간에 따른 변화.

- electric current(전류): 전자의 흐름 ⇒ 전화기, 마이크(Microphone)

- electric voltage(전압): 전류의 압력 ⇒ computer

- electromagnetic wave(전자기파 - 전파): 전자의 가속운동에 의해 공간에 퍼지는 전자기 복사 (자기장의 변화) ⇒ 통신(communication)

⇒ 발생원: radio, microwave - 도체 내에서 '가속운동'하는 전자, antenna

visible light - 대단히 뜨거운 물체

ultraviolet - 아아크 광이나 기체방전

X-ray: 표적을 때리는 전자

γ-ray: 방사성 원자의 핵

⇒ 전달속도: 3×108 m/sec  (c.f. voice(음파) - 340m/sec)

(St. 10th ed.) p.113 fig. 4.1	(St. 7th ed.) p.111 fig. 4.1

• Characteristics of signal

- wave(파): frequency(주파수, Hz - cycles/sec), amplitude(진폭), phase(위상)

(ex) (Stallings 10th ed.) p.73 fig. 3.2, p.74 fig. 3.3, p.76 fig. 3.4

(ex) (Stallings 7th ed.) p.70 fig. 3.2, p.71 fig. 3.3, p.72 fig. 3.4

- frequency band(주파수대역),

bandwidth(대역폭(어떤 대역(band)의 넓이) = 최대주파수-최소주파수)

⇒ bps와 관련

(ex)	(Stallings 10th ed.) p.82 fig. 3.8, p.113 fig 4.1

(ex)	(Stallings 7th ed.) p.78 fig. 3.8, p.111 fig 4.1

- channel(통로): data의 통로, 특정 주파수 대역

(ex) AM 방송: 540～1600Khz

FM 방송: 88～108MHz

(ex) TV: channel 2-4 - 54～72MHz

channel 5-6 - 76～88MHz

channel 7-13 - 174～216MHz

channel 14-83 - 420～890MHz

- multiplexing(다중화): 여러 사용자의 data를 동시에 전송.

⇒ 동시에 여러 채널 연결

(ex) TDMA: 시분할 다중화

PCM - p.122 fig.2-26

FDMA: 주파수분할 다중화

CDMA: 코드분할 다중화

• analog/digital signal

① analog signal: 연속적인 변화 - wave form

- original data에 충실

- 복사할 때마다 error 발생 → 100% 복사 불가능

- 잡음(noise)에 약함

(ex) LP, audio tape 등

② digital signal: 이산적인 변화 - pulse form

- 100% original data로 환원 불가능

- 여러번 복사해도 100% 정확

- 잡음(noise)에 강함

(ex) CD

- required bandwidth of the digital signal for transmission

⇒ the ‘greater‘ the bandwidth of the signal, the ’more faithfully‘ it represent a digital pulse stream.

⇒ for a data rate x bits/sec,

x/2 Hz: minimum

x   Hz: quite good

2x  Hz: very good

(ex) (Stallings 10th ed.) p.82 fig. 3.8

(ex) (Stallings 7th ed.) p.78 fig. 3.8

• Fourier 급수 (series)

- 전자기적 주기 신호: 진폭(A), 주파수(f), 위상(ψ)

(ex) (Stallings 10th ed.) p.73 fig. 3.2, p.74 fig. 3.3

(ex) (Stallings 7th ed.) p.70 fig. 3.2, p.71 fig. 3.3

- Fourier 급수: 어떠한 주기적 신호도 정현파(sine wave)의 합으로 나타낼 수 있다.

x(t) = ∑∞n=0 ancos(2πnf0t) + ∑∞n=1 bnsin(2πnf0t)

(ex) (Stallings 10th ed.) p.76 fig. 3.4

(ex) (Stallings 7th ed.) p.72 fig. 3.4

Transmission

• Terminologies

- 4가지 방식

(Stallings 10th ed.) p.89 tab. 3.2, p.153 fig. 5.1, p.154 tab. 5.1

(Stallings 7th ed.) p.87 tab. 3.1, p.153 fig. 5.1, p.155 tab. 5.1

1. digital data → digital signal

- Strings of 0's and 1's are transmitted instead of continuous signals.

(ex) LAN: Ethernet

Digital data

Digital signal

- digital data: represent data with discrete (e.g. two - binary) levels of electromagnetic quantity.

․ binary digital data inside computers: uses two constant voltage levels.

⇒ one level for binary 1 and the other for binary 0

4

3

2

0.5

0

-0.5

binary 1

binary 0

volts

(ex) the transfer of characters into binary digital data

In CPU, - A shift of the previous character to the next 8-cells on its left.

- When a transfer of 4 characters is completed, the CPU register is full, and its contents are transferred into a memory word.

- Digital transmission

① Binary encoding (NRZ-L, Nonreturn-to-Zero-Level)

․ gives the receiver NO way of determining when each bit starts and ends.

⇒ NO 'synchronization'

② Manchester encoding

․ each bit period is divided into two equal intervals.

․ every bit period has a transition in the "middle".

1: bottom-up,  0: top-down (반대로도 가능)

․ makes it easy	for the receiver to "synchronize" with the sender.

③ Differential Manchester encoding

․ always a transition in the "middle"  - synch.

․ at the "start" of the interval,	- data

0: the presence of a transition,	1: the absence of a transition

․ data와 synch.를 분리

④ 기타

(Stallings 10th ed.) p.156 fig. 5.2, p.154 tab. 5.2

(Stallings 7th ed.) p.156 fig. 5.2, p.155 tab. 5.2

2. digital data → analog signal

- modem: modulator (pulse → wave) + demodulator (wave → pulse)

․ Inserted between the (digital) computer and the (analog) telephone system.

․ Accepts a serial stream of bits as input and produces a modulated carrier as output or vice versa.

⇒① Amplitude modulation: two binary values → two different amplitudes.

② Frequency modulation: two binary values → two different frequencies. (i.e., two different tones → high and low)

③ Phase shift modulation: The phase of the carrier signal is "shifted" to represent data.

- 2위상편이 변조 방식: Each phase shift transmits 1 bits of data.

- 4위상편이 변조 방식: Each phase shift transmits 2 bits of data.

(Stallings 10th ed.) p.165 fig. 5.7	(Stallings 7th ed.) p.167 fig. 5.7

․ 두 가지 4위상편이 변조 방식

① PSK(Phase Shift Keying): 절대적 기준 위상을 사용.

(ex)	cos(x)	→ 00,

cos(x+π/2)  → 01,

cos(x+π)    → 10,

cos(x+3π/2) → 11

② DPSK(Differential(차등) Phase Shift Keying): 바로 앞에 전송된 위상을 기준으로 하여 위상 편이를 줌.

(ex) 0o(0)→ 00, 90o(π/2) → 01, 180o(π) → 11, 270o(3π/2) → 10

․ baud(보오): 1초에 파형이 몇 번 변하였는가?

(ex) 4위상편이 변조시 1200 baud → 2400 bps (1번 변조에 2 bit 씩 전송)

- How to increase the data rate of a modem

․ For a given bandwidth, the data rate can be increased by increasing the “no. of different” signals.

(ex) ITU-T V.26 protocol, 1200 baud & 4위상편이 변조방식을 사용

(International Telecomm. Union - Telecomm. standardization sector)

⇒ 4개의 서로 다른 signal 사용

⇒ log24 bits/baud = 2 bits/baud 전송가능

⇒ 1200 baud/sec×2 bits/baud = 2400 bps

ITU-T V.29 protocol, 2400 baud & 8위상 2진폭 변조방식을 사용

⇒ (8×2)개의 서로 다른 signal 사용

⇒ log216 bits/baud = 4 bits/baud 전송가능

⇒ 2400 baud/sec×4 bits/baud = 9600 bps

- 서로 다른 “신호의 갯수”를 어떻게 늘리는가?

On a noisy channel, a receiver must "distinguish" one of V different signals.

⇒ Noise on a transmission line will "limit" the practical value of V.

․ 진폭변조의 경우

① 2진폭 변조: 2가지 signals ⇒ log22 bits/signal = 1 bits/signal

② 4진폭 변조: 4가지 signals ⇒ log24 bits/signal = 2 bits/signal

⇒ 8가지/16가지 진폭을 사용할 수 있는가? 수신자가 구별하기 어려움.

․ 주파수변조의 경우,

마찬가지로 여러 주파수를 구별하기 어려움.

⇒ 저속 데이터 전송에 사용 (2000 bps 이하)

․ 위상변조의 경우

여러 개의 서로 다른 signal들을 전송 가능.

⇒ 고속 modem에 사용

․ 두 가지 DPSK 방법

DPSK	Quaternary-DPSK(4등분)	Octal-DPSK(8등분)

① A방식:

0

1

00

10

01

11

000

001

010

011

111

110

101

100

0

1

00

01

11

10

000

001

010

111

110

101

100

011

② B방식:

ITU-T V.26(1200baud, 2400bps)  (π/8, 3π/8, …)

⇒ Which one is better? B방식

Why? 언제나 위상변이→수신자와의 “동기화”에 유리.(Manchester encoding)

․ APSK(Amplitude Phase Shift Keying): 진폭위상 편이 변조

⇒ 진폭과 위상을 함께 변환.

4위상-2진폭	8위상 2진폭(동심원)	12위상-1진폭/2진폭(사각형)

8가지 signal	16가지 signal	16가지 signal

⇒ 3 bits/signal	⇒ 4 bits/signal	⇒ 4 bits/signal

ITU-T V.29 protocol	직교진폭변조

2400baud×4bits/signal	(QAM - Quadrature

= 9600bps	Amplitude Modulation)

3. analog data → digital signal

- Codec(Coder-Decoder): The analog signals are 'digitized'.

⇒ Producing a 7- or 8-bit number.

⇒ p.108 fig. 2-17

(Stallings 10th ed.) p.177 fig. 5.17

(Stallings 7th ed.) p.180 fig. 5.16

- PCM(Pulse-Code Modulation): analog signal → digital signal

(ex) Sampling: 4-bit sampling,  sampling rate = 8 samples/sec = 8 Hz

(Stallings 10th ed.) p.177 fig. 5.17 (c.f.)

(Stallings 7th ed.) p.180 fig. 5.16 (c.f.)

①표본화(sampling) - PAM(Pulse Amplitude Modulation)

②양자화(quantization)

③부호화(encoding)

(ex) Sound Blaster Pro: 8-bit sampling,  max. sampling rate = 44,000Hz

CD: 16-bit sampling,  sampling rate = 44,000Hz

⇒ 'Music'

(ex) T1-carrier

⇒ 'Voice'

8000Hz sampling ⇔ sampling period = 1 sec/8000 samples

= 125 μsec/sample

⇒ This is sufficient to capture all the info. from the 4Khz telephone channel bandwidth.

⇒ At a lower sampling rate, info. would be lost, and at a higher one, no extra info. would be gained.

⇒ All time intervals within the PSTN are multiples of 125μsec.

․ T1-carrier: PCM + multiplexing(TDMA)

The T1-carrier consists of 24 voice channels multiplexed together.

⇒ Each of the 24 channels, in turn, gets to insert 8 bits into the output stream.

⇒ p.122 fig. 2-26

⇒ 193 bits/125×10-6 sec = (193/125)×106 bps = 1.544 Mbps

Multiplexing T1 streams onto higher carriers

⇒ p.124 fig. 2-28

․ E1-carrier

(1 control frame + 31 voice channels) are multiplexed together.

⇒ (8×32) bits/125×10-6 sec = (256/125)×106 bps = 2.048 Mbps

4. analog data → analog signal

- analog signal 전송방식은 이제 거의 사용되지 않음

e.g., 과거의 radio/TV 전송방식