1. ICOM 6115©Manuel Rodriguez-Martinez ICOM 6115 – Computer Networks and the WWW Manuel Rodriguez-Martinez, Ph.D. Lecture 10

2. ICOM 6115©Manuel Rodriguez-Martinez Lecture Objectives Understand the properties of technologies used to implement the physical layer –Called Layer 1 by networking “techies” Major technologies –Cooper Twisted Pair vs Coaxial –Fiber –Radio Waves –Phone Lines SONET

3. ICOM 6115©Manuel Rodriguez-Martinez Cooper-based technology Cooper is mainly used for –LANs Ethernet (10Mbps, 100Mbps, 1000Mbps) –Telephone Last link to your home (“Last mile”) DSL –Cable TV Last link to your home (“Last mile”) Cheap, well-understood –Cannot span long distances

4. ICOM 6115©Manuel Rodriguez-Martinez Unshielded Twisted Pair Cooper Cables are twisted like DNA molecule –Make cable radiate less Used in telephone, and in many LANs (Ethernet) Most common cables are Cat. 3 and now Cat. 5 (most popular currently) –New comers (Cat 5e (GigaEthernet), Cat 6 and Cat 7) CAT 3 – 16MHz CAT 5 – 100MHz

5. ICOM 6115©Manuel Rodriguez-Martinez Coaxial Cable (Original Ethernet) Can span longer distances (shielding) –75 ohm – Cable TV –50 ohm – Digital Transmission (old timer Ethernet) Bandwidth – 1GHz (modern days)

6. ICOM 6115©Manuel Rodriguez-Martinez Coaxial, Cat 5, and Ethernet

7. ICOM 6115©Manuel Rodriguez-Martinez Fiber Optics Way of the future –1Gbps (“cheap fiber”) –10Gbps (“expensive fiber”) Limited by the ability to convert between light and electrical signals. Fiber gives the possibility of infinite bandwidth –Old days: avoid moving data over network –The Future: Spread data around network It is cheap!

8. ICOM 6115©Manuel Rodriguez-Martinez Basics of Fiber Bits are converted into light pulses by light source (1 is a light pulse, 0 is lack of light) Then moved by fiber of glass. Detector maps light to electrical signal Put two lines: one to send, one to receive –Full duplex fiber Light Source Detector Light Source Detector Fiber glass

9. ICOM 6115©Manuel Rodriguez-Martinez Connectivity scheme Because of current costs, fiber mostly connects routers and switches –Backbone link Hosts might use 1000Gbps Ethernet to directly use the bandwidth of this backbone Network Card Network Card

10. ICOM 6115©Manuel Rodriguez-Martinez Physics of Fiber Prepare the glass so it can make light reflect completely into the wire.

11. ICOM 6115©Manuel Rodriguez-Martinez Types of Fiber Multi-mode fiber –Light pulses hit the glass at various angles Need repeaters to amplify signal Single-mode fiber –Very thin glass – light goes almost on a straight line –Can go longer distances (100km) without repeaters

12. ICOM 6115©Manuel Rodriguez-Martinez Fiber Networks: Active Repeaters - Failure of 1 link breaks the network + Links can be kilometers in length (campus backbone)

13. ICOM 6115©Manuel Rodriguez-Martinez Fiber Networks: Passive Star

14. ICOM 6115©Manuel Rodriguez-Martinez Passive Star: Tradeoff Benefit – Failure in one interface won’t break network Disadvantage –Light is broadcasted, so need good photodiodes Limits the number of nodes you can have on the network

15. ICOM 6115©Manuel Rodriguez-Martinez Radio Waves Use air as the medium for data transmission –Unguided –Natural Broadcast network –Security issue here How to protect your data? Need good encryption mechanism Network card has radio transmitter and receiver

16. ICOM 6115©Manuel Rodriguez-Martinez Example: 801.11 Family Base Station Ad-hoc Network

17. ICOM 6115©Manuel Rodriguez-Martinez Some Issues Lack of coverage Overlapping coverage

18. ICOM 6115©Manuel Rodriguez-Martinez Radio Waves Issues Lack of Coverage –Signal might degrade so much it might reach certain areas Pocket of disconnected operation Overlapping Coverage –One or more stations might be sending over the same frequency 2.4GHz is the band for 802.11b –Unregulated –Need mechanism to prevent interference Ex: Change frequencies

19. ICOM 6115©Manuel Rodriguez-Martinez The Telephone System Why should we care? Phone lines run through vast regions of the Earth –They reach homes, schools, offices Big Idea! –Move data over phone lines Build wide-area networks on top of leased lines from phone companies –Sprint, AT&T, Verizon

20. ICOM 6115©Manuel Rodriguez-Martinez Architecture of Phone System Local Loop End Office Backbone Switching Office End-line Trunks

21. ICOM 6115©Manuel Rodriguez-Martinez Elements of the System Local Loop – line arriving to subscriber End Office (Central Office) – center where subscriber line Toll Office – switching center that aggregates lines coming from end offices Intermediate Office – interconnects toll offices Toll connecting trunks – line connecting end office to toll offices Intertoll trunks – lines connecting toll offices

22. ICOM 6115©Manuel Rodriguez-Martinez Example organization

23. ICOM 6115©Manuel Rodriguez-Martinez Analog and Digital Transmission over Phone System

24. ICOM 6115©Manuel Rodriguez-Martinez Interesting Components Modem – converts from analog to digital signals Codec – converts from digital to analog Local Loop –made out of twisted pair cooper –Analog communication Trunks – made out of fiber optics –Digital communication

25. ICOM 6115©Manuel Rodriguez-Martinez Modem Convert from analog communications to digital and vice-versa –Modulator – demodulator Modem modulate the signal carrier to represent the appropriate values –Amplitude modulation –Frequency modulation –Phase modulation

26. ICOM 6115©Manuel Rodriguez-Martinez Modulation Performed by Modems

27. ICOM 6115©Manuel Rodriguez-Martinez Baud Rate Baud – number of samples per second done by the modem –Typically is 2400 samples/sec One symbol is send/received per baud A Binary scheme yields (2 voltages) –2400 symbols/sec at a bandwidth of 2400 bps 4 Voltages –2400 symbols but bandwidth is 4800 bps Now every symbols is more data 4Voltage map to: 00, 01, 10, 11

28. ICOM 6115©Manuel Rodriguez-Martinez Making Modems Faster Modem can be made to transmit more data per unit of time having more symbols –Each symbols represents a bit pattern Typically done by having different tones made out of combinations of –Amplitude Modulation –Phase Modulation Constellations Diagrams show possible combinations

29. ICOM 6115©Manuel Rodriguez-Martinez Constellation Diagram For each point: - Amplitude is given by distance from origin - Phase shift is given by angle between x-axis and line from origin

30. ICOM 6115©Manuel Rodriguez-Martinez Trunks and Multiplexing Trunks have the capacity to move large amounts of with small delay –Have big delay x bandwidth Question: –How does a high speed trunk carries traffic from slower trunks? –Answer: Multiplexing SONET

31. ICOM 6115©Manuel Rodriguez-Martinez Multiplex based on Frequency Frequency Division Multiplexing (FDM) –The spectrum of possible signal frequency is divided into bands (channels) –Each user is given possession of the band Lower level trunk might be assigned a given frequency Problem: Wastes bandwidth if a user does not have data to send

32. ICOM 6115©Manuel Rodriguez-Martinez FDM

33. ICOM 6115©Manuel Rodriguez-Martinez Multiplex based on Wavelength Wavelength Division Multiplexing (WDM) Each wavelength of the light pulse is used to represent a channel –Similar to FDM Some numbers –Products with 96 channels of 10Gbps –Kind of expensive

34. ICOM 6115©Manuel Rodriguez-Martinez WDM

35. ICOM 6115©Manuel Rodriguez-Martinez Multiplex based on Time Time Division Multiplexing (TDM) Given a unit of transmission T –Allow each user to send a block of data as part of T Let every one put a grain of sand Benefit: –Maximize use of link Important examples: T1, SONET

36. ICOM 6115©Manuel Rodriguez-Martinez Pulse Code Modulation Phone are analog Codec are used to map analog data to digital –Sample rate is 8000 per second 125 usec/sample T1 carrier –24 voice channels –Channel sampling is done in round robin Each channel gets to put 8 bits of information –Result is passed to the codec

37. ICOM 6115©Manuel Rodriguez-Martinez Example of T1 Carrier - Control bit is for synchronization of frames - Successive frames should alternate the bit - Synchronization pattern 01010101