CHAP 2. The Physical Layer: Part 2

강의내용 - Stalling책

• Chap 4. 전송 매체, 10th ed.	(7th ed.)

• 강의 내용

- Signal을 전송하는 매체(media)의 이해: 유선과 무선

Topics

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

⇒ The "physical" path between the transmitter and receiver.

⇒ The characteristics and quality of data transmission are determined both	① by the nature of the "signal" - signal 자체의 성질

(analog/digital, frequency)

② by the nature of the "medium" - signal을 전달하는 매체

• The media

① Guided media: wired transmission

(ex) twisted pair, coaxial cable, optical fiber

⇒ The "medium itself" is important

⇒ (Stallings 10th ed.) p.113 fig. 4.1, p.115 fig. 4.2, p391(LAN)

(Stallings 7th ed.) p.111 fig. 4.1, p.113 fig. 4.2, p562

② Unguided media: wireless transmission

(ex) LF, MF, HF, VHF, UHF, SHF, EHF

(ex) 전파 전달: space & antenna (물질: Aluminum 합금, 무기재료 등)

⇒ The "spectrum" or "frequency band" of the signal is important.

⇒ (Stallings 10th ed.) p.113 fig. 4.1

(Stallings 7th ed.) p.111 fig. 4.1

• 전송속도: bps (bits/sec.)

(ex) Off-line: movable magnetic media

- magnetic tapes, floppy disks, movable disks

- adv.: often high bandwidth, security

disadv.: inconvenient, off-line connection, hours not milliseconds

(적은 데이터를 보내기 불편)

180 Mbytes/tape

200 tapes/truck

1 hour/trip

180 × 200 Mbytes/trip

⇒	288,000 Mbits/trip ÷ 3600 sec./trip = 80 Mbps

(c.f. modem = 56 Kbps)

Transmission Media: Wired

1. Twisted pair

- two insulated copper wires, typically about 1mm thick: 절연된 두 개의 동선

⇒ twisted: to reduce electrical interface

(ex) telephone, LAN (10BaseT Ethernet/Fast Ethernet)

(Stallings 10th ed.) p.381 tab. 12.1, p.382 tab. 12.2

(Stallings 7th ed.) p.603 tab. 16.2, p.592 tab. 16.1

- signal can run 2 Km without amplification.

⇒ for longer distance, repeaters are needed.

- 1～4 Mbps

(Stallings 10th ed.) p.115 fig. 4.2 (a), p.119 tab. 4.2

(Stallings 7th ed.) p.115 fig. 4.3, p.117 tab. 4.2

2. Coaxial cable

- baseband: one channel - 동시에 하나의 signal만을 전송.

broadband: multiple channels - 동시에 여러 signal들을 전송.

⇒ 50 ohm cable: baseband - used for 'digital' transmission (e.g. LAN)

70 ohm cable: broadband - used for 'analog' transmission

(e.g. cable TV)

(Stallings 10th ed.) p.115 fig. 4.2(b)(c), p.381 tab. 12.1

(Stallings 7th ed.) p562, p603 tab 16.2

- high bandwidth and excellent noise immunity

⇒ 10～500 Mbps

(ex) Baseband LAN (10Base5/10Base2 Ethernet)

(Stallings 10th ed.) p.381 tab. 12.1

(Stallings 7th ed.) p603 tab. 16.2

Long-distance transmission within the PSTN

→ 해저 동축(submarine) cable (1920년대)

- broadband coaxial cable

‣ analog transmission on standard cable TV cabling

‣ frequency band is divided up into multiple channels.

⇒ Each channel can be used for analog TV (e.g. 54MHz), FM (e.g. 91MHz) or a digital bit stream (e.g. 300MHz).

⇒ TV and data can be mixed on the same cable.

(ex) CATV: 50 이상의 TV 신호 전송

optical fiber: 1000 channels

3. Optical fiber

- Transmit data by "pulses" of light

⇒ a light pulse: 1

the absence of a pulse: 0

- ① Light source: emit 'light pulses' when an electrical current is applied.

(ex) LED(Light Emitting Diode), semiconductor laser

② Detector: generates an 'electrical pulse' when light falls on it.

(ex) photodiode

source

destination

a fiber of glass

(Stallings 10th ed.) p.125 fig. 4.6

(Stallings 7th ed.) p.113 fig. 4.2

- 60년대초: LASER (Light Amplification by Stimulated Emission of Radiation)

유도방사에 의한 광증폭 (단파장의 적외선 혹은 광) 발명.

66년: Kao 박사(영국) - 유리섬유에 의한 광도파에 대한 이론 발표.

(2009 Nobel prize, Wikipedia: single mode optical fiber)

70년: 코닝사(미국) - 저손실 광섬유 제조

⇒ 전달원리: '전반사' 이용 (다중모드 - multi-mode)

(Stallings 10th ed.) p.125 fig. 4.7

(ex) LANs: 10BaseFP Ethernet/10G Ethernet

(Stallings 10th ed.) p.387 fig. 12.6, p.388 fig. 12.7

(Stallings 7th ed.) p.592 tab. 16.1

Long-distance telephone lines: 육상, 해저(40 Km마다 repeater 설치)

- SONET(Synchronous Optical Network): Bellcore에서 제안하고 ANSI에 의해 표준화된 광전송 interface

(Stallings 10th ed.) p.260 fig. 8.10, p.259 tab. 8.4

(Stallings 7th ed.) p.305 fig. 8.10, p.305 fig 8.11

- Adv: NOT affected by electromagnetic interference.

⇒ Can propagate for 10～100 Km with virtually NO loss.

⇒ Powerful lasers can drive a fiber 100 Km long without repeaters.

⇒ Can transmit data at about 1～2 Gbps

Transmission Media: Wireless

1. Terrestrial transmission

- send the electromagnetic wave into the air.

- radio, microwave, infrared, laser 이용

(Stallings 10th ed.) p.137 tab. 4.5, p.131 tab. 4.4, p139 fig. 4.11

(Stallings 7th ed.) p.136 tab. 4.7, p.129 tab. 4.6, p137 fig. 4.8

- Adv.: an alternative to coaxial cables.

⇒ much cheaper

(ex) LAN runs between several buildings or industrial park, etc.

⇒ dig up the street & cross a public road.

⇒ 무선 LAN

(ex) Microwave is used for both telephone (cellular phone) and TV as an alternative to coaxial cable.

100 Km

100 m

- Disadv.: interference

(ex) Thunderstorm can interfere with microwave.

2. Communication satellites

- 위성용도: 통신, 방송, 해상통신, 기상, 지구관측, 군사 등

- 위성궤도

①저궤도위성(Low Earth Satellite): 지상 700～1,500Km 상공, 90분 정도 주기

⇒ 지구에 근접, 사진촬영 → 군사목적

②중궤도위성(Middle Earth Satellite): 지상 10,000～20,000Km 상공, 7시간 정도 주기

③정지궤도위성(Stationary Satellite): 지상 36,000Km 상공, 24시간 주기

⇒ 통신, 방송

- 지구둘레를 도는 인공위성의 공전주기

v2/R = g  ⇒  v2 = g․R, where	v = 위성의 속도

a=v2/R : 원운동을 하고 있는 위성의

가속도의 크기

g : 지구의 중력가속도

g = 위성의 고도에서의 중력가속도

R = 궤도의 반지름

2π․R = v․T   ⇒   T = (2π․R)/v

(위성의 일주 = 이동거리)   = (2π․R)/ g․R

(ex) 궤도가 지상 400Km인 위성의 주기는?

지상 400Km에서의 중력가속도

v2 = g․R = 8.6 m/sec2 × (6400 Km + 400 Km)

= 8.6 × (6.8×106) m2/sec2

⇒ v = 7.6×103 m/sec

= 27,000 Km/hr

T = (2π․R)/v = (2π․6.8×106 m) / (7.6×103 m/sec)

= 5.6×103 sec

= 93 min

- 통신위성: a big microwave repeater in the sky.

① Listens to some frequency band.

② Amplifies the incoming signal.

③ Rebroadcasts it at another frequency to avoid interference with the incoming signal.

⇒ uplink: high frequency & downlink: low frequency

⇒ 상용위성통신에 사용되는 주파수대역

(Stallings 10th ed.) p.132 fig. 4.9

(Stallings 7th ed.) p.131 fig. 4.6

- Stationary satellite(정지궤도 위성): An altitude of approximately 36,000Km above the equator, the satellite period is 24 hrs.

(ex) IntelSatⅥ(1989), Ⅶ(1992), 무궁화 위성

⇒ 3개의 위성으로 전지구를 커버

속도=11,070Km/hr

17.34o

41,756Km

73,155Km

35,860Km

12,752Km

18,101Km

- 전파간섭 없으려면 4o마다 배치: 360/4 = 90개의 정지궤도 위성 배치 가능

⇒ Each of 90 possible satellites could have several data channels or

two or more satellites could occupy one orbit slot if they operate at different frequencies.

- Disadv.:

․ A large round trip distance introduces a "substantial" delay.

(ex) end-to-end delay: 250～300 msec

(c.f. microwave links: 3 μsec/Km,  coaxial cable links: 5 μsec/Km)

(2×35,787 Km) / (300,000 Km/sec)

= 0.23858 sec

= 238 msec

․ broadcasting: some form of encryption is required.

- Adv.:

․ high bandwidth: faster (50～622 Mbps)

․ the cost of transmitting a message is independent of the distance traveled.

⇒ The propagation delay is independent of the distance between the sender and receiver.

- telephone, data, TV

(ex) 한국통신 위성 Internet 서비스: Asymmetric (uploading < downloading)

⇒ 양방향(e.g. 화상회의 등)에는 한계,

단방향(e.g. 원격교육, VOD, Home shopping 등)에는 효과적.

uploading:

28～56Kbps

가입자

KORNet

Host

uplink:

10～45Mbps

downlink:

0.4～2Mbps

downloading

무궁화위성

(Stallings 10th ed.) p.134 fig. 4.10

(Stallings 7th ed.) p.132 fig. 4.7