Physical Layer - 1

Physical Layer

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without prior written permission of Professor Nen-Fu Huang (E-mail: nfhuang@cs.nthu.edu.tw).

Physical Layer - 2

Transmission Medium, Technologies, and Encoding SchemesTransmission Medium

Twisted PairCoaxial CableFiber OpticRadio

Transmission TechnologiesBasebandBroadband

Physical Layer - 3

Encoding Schemes

Encoding SchemesRS-232C encodingZero-Complemented Encoding

Manchester EncodingDifferential Manchester Encoding

Physical Layer - 4

Twisted Pair

Two insulated wires arranged in a spiral patternThe wires are copper or steel coated with copperA cable may have many pairsTwisting minimizes electromagnetic interference between

pairsNoise Immunity: Good at low frequencyLeast expensiveHigh data error rate : 1 x 10-5

Typical data rate: 10/100 Mbps (within 100m)

Physical Layer - 5

Twisted Pair

Physical Layer - 6

Coaxial CableConsist of two conductors, operate over a wider ra

nge of frequency50 Ohm cable: Baseband digital signaling

Manchester Encoding, 12 Mbps, a few KilometersSupport on the order of 100 devices per segmentLess signal loss in the insertion of the Taps

75 Ohm cable: Community Antenna Television (CATV)Broadband Analog signaling with FDM

300-400 MHz In CATV, each channel has 6 MHz bandwidth

Physical Layer - 7

Coaxial CableSingle-Channel Broadband for High Speed Analog or Digital Signaling (No FDM)

Digital Signaling : Up to 50 Mbps Analog Signaling (PSK): Up to 50 Mbps

Support thousands of devices depending on data rates

Tens of Kilometers depending on data rates

Moderate cost, components readily available

Physical Layer - 8

Coaxial Cable

中心導體絕緣層

外層網狀導體

保護外皮

中心導體 (Conducting Core)

絕緣層 (Insulation)

外層網狀導體 (Conducting Mesh or Sleeve)

保護外皮 (Protective Jacket)

Physical Layer - 9

Fiber OpticUse optical signals instead of electrical signalsLight sources : LED (發光二極體） Laser DiodeOptical Fibers

Single ModeStepped IndexGraded Index

Detector: Photoelectric Diodes 光纖

外圍材料

保護外皮

Physical Layer - 10

Fiber Optics Characteristics

Dielectricity (free from interference)Low Attenuation over distanceSmall size, light weightGood bandwidth (Up to 3.3 GHz)Support long distanceBendable (Minimum bend radius 200mm - 5

mm)Relative Expensive

Physical Layer - 11

強化彎曲物密集裝捆之光纖

管狀包裝層保護層

Physical Layer - 12

A

B

C

C

A

B

可接受入射角度

Physical Layer - 13

Optical Transmission System

傳送器 接收器

驅動器光源

光偵測器

接收器

（電子）

光纖纜線

光纖

光訊號連接器

數位訊號類比或

輸入

數位訊號類比或

輸出

電子訊號連接器

（電子）

Physical Layer - 14

Transmission Techniques

Baseband : Digital SignalingBroadband: Analog Signalin

g

Physical Layer - 15

BasebandSignals sent at their original formIn LAN, baseband usually refers to digital si

gnalingIn baseband coaxial cable (50 OHM)

500 meters50-OHM Terminators at both endDistance between two Taps - multiples of 2.5 m

to ensure no reflection from adjacent TapAt most 100 Taps

Physical Layer - 16

Baseband

A D

B C E

同軸電纜

收發器電纜

終端器

終端器連接拴

同軸電纜區段（最長 500 公尺）

（每區段最多接 100 個）（最長 50 公尺）

工作站

Physical Layer - 17

BroadbandSignals are modulated before transmission and

demodulated after receiving, 75-OHM coaxial cableNeed modem : Modulation/DemodulationUnidirectional transmission Multiple/Single channel broadbandCover a larger distanceCascaded amplifiers can result in loss of data

integrity at high data ratesSystem Configuration

Single-cable systemDual cable system

Physical Layer - 18

Broadband低速數據通道

交換聲音 /數據通道

高速數據通道

視訊通道

保留

保留

保留

10MHz

25MHz

55MHz

75MHz

175MHz

210MHz

240MHz

310MHz

寬頻同軸電纜 300MHz

Physical Layer - 19

Single Cable System

Headend (Central Retransmission Facility) Consist of •amplifiers, •filters, and •signal modulators

Physical Layer - 20

工作站

工作站

數據機

數據機

傳送頻道 10-110 MHz

接收頻道 175-310 MHz

頻率轉換設備

10-110 MHz175-310 MHz

緩衝頻帶

Physical Layer - 21

Dual Cable System

Entire bandwidth is available for

transmission in both directionsA passive connector is used to

connect two cablesHigher cost for cable and taps

Physical Layer - 22

工作站 工作站

300 MHz

數據機 數據機

300 MHz

傳送

接收

Physical Layer - 23

Comparison of Baseband and Broadband Technologies

基頻技術 寬頻技術較便宜（不須數據機） 資料容量大網路架構較簡單 網路架構較有彈性安裝容易 可傳送多種型態資料

傳送距離較長 較成熟的 CATV 技術傳送單一型態資料 需要數據機

資料容量較小 安裝維護較複雜傳送距離較短 二倍傳遞延遲時間 （二倍長的電纜線）

優點

缺點

Physical Layer - 24

Encoding Schemes

1 0 1 1 1 0 1 1 + 6 V

0 V

0 0 0 1 1 0 0 1

位元時間

0 1 1 1 0 1 0 1 + V

- V

Start Bit Data bits

0 1 2 3 4 5 6 7

Stop Bit

RS-232C

Zero Complemented

Physical Layer - 25

Encoding Schemes

0 1 0 0 1 1 0 1 0 0 + V

- V

位元時間

1 0 1 1 0 0 1 0 1 1 + V

- V

位元時間

Manchester

Differential Manchester

0 1 0 0 1 1 0 1 0 0 + V

- V

取樣時間

Sampling at

Receiver side

Physical Layer - 26

Access Control Methods

Network Topologies Star TopologyBus/Tree TopologyRing TopologyMesh Topology

Transmission ControlRandom Transmission ControlDistributed Transmission ControlCentralized Control

Physical Layer - 27

Transmission Control

Random Transmission ControlALOHA Network, Slotted Aloha, Wireless LAN

Carrier Sense multiple access with collision detection (CSMA/CD)

Slotted RingRegister Insertion Ring

Physical Layer - 28

Transmission Control

Distributed Transmission ControlToken Passing : Token-Ring (FDDI), Token-Bus (GM, MAP)

Carrier Sense multiple access with collision avoidance (CSMA/CA)

Distributed Queue Dual Bus (DQDB)

Physical Layer - 29

Transmission Control

Centralized ControlPollingCircuit Switching (X.25, Frame-Relay, ATM network)

Time-division Multiple Access (TDMA)Frequency-division Multiple Access (FDMA)Wavelength-division Multiple Access (WDMA)

Code-division Multiple Access (CDMA)