【導讀】PhysicalLayer. Propagation:. UTPandOpticalFiber. Chapter3. UpdatedJanuary2020. XUZhengchuan. FudanUniversity. 2. Orientation. Chapter2. Chapter3. –Physicallayer(Layer1). methods,propagationeffects. –Chapter3:Signaling,UTP,opticalfiber,andtopologies. 3. SenderReceiver. TransmissionMedium. Propagation. Transmitted. Signal. ReceivedSignal. (Attenuated&. Distorted). 4. P141. Data. Representation. 6. Binary-EncodedData. representations. –Binarymeans―two‖。–Calledbits. 7. Binary-EncodedData. –Text. –Integers(wholenumbers). –Decimalnumbers. –Alternatives(North,South,East,orWest,etc.). –Graphics. –Humanvoice. –etc.Hello11011001…8. Binary-EncodedData. –48-bitEtheraddresses. –32-bitIPaddresses. 9. Numbers. Integer. 0. 1. 2. 3. 4. 5. 6. 7. 8. Binary. 0. 1. 10. 11. 100. 101. 110. 111. 1000. 10. Numbers,Continued. 1. 0011+1. +0+1+0+1+1. =0=1=1=10=11. BasicRules. 11. Numbers,Continued. BinaryDecimal. 10008. +1+1. =1001=9. +1+1. =1010=10. +1+1. =1011=11. +1+1. =1100=12. Examples. 12. Alternatives. NumberofBits. InField(N). 1. 2. 3. 4. 8. 16. ThatCanbeEncoded. withNbits. 2(21). 4(22). 8(23). 16(24). 256(28). 65,536(216). 13. Alternatives. 121=2Male=0,Female=1. 222=4Spring=00,Summer=01,Autumn=10,Winter=11. keyboards.Space=00000000,etc.14. Powersof2. BitsAlternatives. 12. 24. 38. 416. 532. 664. 7128. 8256. 101,024. 1665,536. Eachadditionalbit. doublesthenumberof. possibilities. youhavewhatyouneed. .,ifyouknow8is256,10mustbe4timesas. largeor1,024.Memorizefor1,4,8,and. 16bits. 15. Alternatives. Quiz. abyteofstorage?cream?–Howmanybitsdoyoune

　　

【正文】 Fiber – Carrier fiber must span long distances – This requires expensive longwavelength laser light sources (1,310 and 1,550 nm) – It also requires expensive ―singlemode‖ fiber with a very narrow core ( microns) 77 Figure 324: Multimode and SingleMode Optical Fiber , Continued Light Source SingleMode Fiber Cladding Core Single Mode Light enters only at certain angles called modes Singlemode fiber cores are so thin that only one mode can propagate—the one traveling straight through No modal dispersion (discussed earlier), so can span long distances without this distortion Expensive but necessary in WANs 78 ? Test Your Understanding ? P 174 ? 18 79 Figure 324: Carrier Fiber and LAN Fiber ? Carrier Fiber – Main propagation effect for singlemode fiber is attenuation, which is very low ? For 850 nm light, attenuation is around dB/km ? At 1,310 nm, attenuation is lower—about dB/km ? At 1,550 nm, attenuation falls even lower—about dB/km – Longer wavelengths carry farther but cost more – Carrier fiber uses wavelengths of 1,310 or 1,550 nm 80 Figure 324: Carrier Fiber and LAN Fiber ? Noise and Electromagic Interference (EMI) Are Not Problems for Either LAN or Carrier Fiber – Noise from moving electrons cannot interfere with light signals – EMI would have to be light signals ? Wrapping the cladding in an opaque covering prevents light from ing in 81 Figure 324: Carrier Fiber and LAN Fiber Corporate LAN Multimode Fiber Carrier (WAN) SingleMode Fiber Needed Distance Only 200300 meters Many kilometers Cost Much Lower ($) Very high ($$$$) Fiber Type Multimode ($) Singlemode ($$$$) Wavelength Usually 850 nm ($) Usually 1,310 or 1,550 nm ($$$$) Typical Core 50/ microns ($) microns ($$$) Propagation Limit Modal Distortion Attenuation Is Modal Bandwidth Important? Yes No. Only attenuation matters 82 ? Test Your Understanding ? P 174 ? 19 Topology 84 Figure 326: Major Topologies ? Topology – Network topology refers to the physical arrangement of a work’s puters, switches, routers, and transmission lines – Topology is a physical layer concept – Different work (and inter) standards specify different topologies PointtoPoint Topology (Telephone Modem Communication, Private Lines) 85 Figure 326: Major Topologies, Continued Star (Modern Ether) Example: Pat Lee’s House in Chapter 1a 86 Figure 326: Major Topologies, Continued Extended Star or Hierarchy (Modern Ether) Only one possible path between any two puters For puters X and Y, the path is XBACDY A B C D E X Y Z 87 Figure 326: Major Topologies, Continued Mesh (Routers, Frame Relay, ATM) Multiple alternative paths between two puters A B C D Path ABD Path ACD 88 Figure 326: Major Topologies, Continued Ring (SONET/SDH) 89 ? Test Your Understanding ? P 176 ? 20 90 Figure 326: Major Topologies, Continued Bus Topology (Broadcasting) Used in Wireless LANs Topics Covered 92 Topics Covered ? Binary Data Encoding ? Inherently binary data (IP addresses, etc.) ? Integers (binary arithmetic) ? Alternatives (N bits can represent 2N Alternatives) ? Text (ASCII and Extended ASCII) ? Graphics (pixels, bits per pixel color) ? … ? For transmission the sender converts bits to signals (on/off, voltage levels, etc.) 93 Topics Covered, Continued ? Digital Transmission (Box) ? A few states instead of just two states (binary) ? All binary transmission is digital transmission ? Only some digital transmission (transmission with two states) is binary ? In the box: bit rates and baud rates 94 Topics Covered, Continued ? UTP – 4pair UTP cords and RJ45 connectors and jacks – Attenuation (often expressed in decibels) and noise ? Limit UTP cords to 100 meters – Electromagic interference, crosstalk interference, and terminal crosstalk interference ? Limit wire unwinding to cm (a half inch) to limit terminal crosstalk interference – Serial versus parallel transmission 95 Topics Covered, Continued ? Optical Fiber – On/off light pulses from transceiver – Core and cladding。 perfect internal reflection – Dominates for trunk lines among core switches – 2 fiber strands/fiber cord for fullduplex transmission – SC and ST connectors are the most mon – Carriers use singlemode fiber and long wavelengths – LANs use multimode fiber and short wavelengths 96 Topics Covered, Continued ? Multimode Optical Fiber Distance Increases With … – Greater Wavelength ? 850 nm 1310 nm 1550 nm ―windows‖ ? But largerwavelength transceivers cost more – Smaller Core Diameter ? 50 microns microns – Greater Modal Bandwidth () ? Measure of multimode fiber quality 97 Topics Covered, Continued ? Topologies – Organization of devices and transmission links – Physical layer concept – Pointtopoint, star, hierarchy, ring, etc.