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1 Links - Optical Links - Optical Single mode Optical Media - PDF document

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Direct Link Networks Direct Link Networks Two hosts connected directly No issues of contention, routing, Key points: Physical Connections Encoding and Modulation Framing Error Detection 9/6/06 UIUC -

  1. Direct Link Networks Direct Link Networks Two hosts connected directly  No issues of contention, routing, …  Key points:  Physical Connections  Encoding and Modulation  Framing  Error Detection  9/6/06 UIUC - CS/ECE438, Fall 2006 2 Internet Protocols Outline  Hardware building blocks Application  Encoding User-level software Presentation  Framing Session Transport Kernel software Network Framing, error detection, Data Link medium access control Hardware (network adapter) Physical Encoding 9/6/06 UIUC - CS/ECE438, Fall 2006 3 9/6/06 UIUC - CS/ECE438, Fall 2006 4 Hardware Building Blocks Links - Copper Copper-based Media  Nodes  Category 5 Twisted Pair 10-100Mbps 100m  Hosts: general purpose computers  ThinNet Coaxial Cable 10-100Mbps 200m  Switches: typically special purpose hardware  ThickNet Coaxial Cable 10-100Mbps 500m  Routers: varied   Links twisted pair Copper wire with electronic signaling  copper core coaxial Glass fiber with optical signaling  insulation cable Wireless with electromagnetic (radio, infrared, braided outer conductor  (coax) microwave, signaling) outer insulation 9/6/06 UIUC - CS/ECE438, Fall 2006 5 9/6/06 UIUC - CS/ECE438, Fall 2006 6 1

  2. Links - Optical Links - Optical Single mode  Optical Media  Lower attenuation (longer distances)  Multimode Fiber 100Mbps 2km  Lower dispersion (higher data rates)  Single Mode Fiber 100-2400Mbps 40km  Multimode fiber  Cheap to drive (LED’s) vs. lasers for single mode  Easier to terminate  core of single mode fiber ~1 wavelength thick = glass core (the fiber) ~1 micron optical glass cladding fiber core of multimode fiber (same frequency; colors for clarity) plastic jacket O(100 microns) thick 9/6/06 UIUC - CS/ECE438, Fall 2006 7 9/6/06 UIUC - CS/ECE438, Fall 2006 8 Links - Optical Leased Lines  POTS 64Kbps  Advantages of optical communication  ISDN 128Kbps  Higher bandwidths  ADSL 1.5-8Mbps/16-640Kbps  Superior attenuation properties  Cable Modem 0.5-2Mbps  Immune from electromagnetic  DS1/T1 1.544Mbps interference  DS3/T3 44.736Mbps  STS-1 51.840Mbps  No crosstalk between fibers  STS-3 155.250Mbps (ATM)  Thin, lightweight, and cheap (the fiber,  STS-12 622.080Mbps (ATM) not the optical-electrical interfaces) 9/6/06 UIUC - CS/ECE438, Fall 2006 9 9/6/06 UIUC - CS/ECE438, Fall 2006 10 Wireless Encoding Cellular  AMPS 13Kbps 3km  digital data digital data PCS, GSM 300Kbps 3km  3G 2-3Mbps 3km (a string of (a string of modulator modulator demodulator demodulator  Wireless Local Area Networks (WLAN) symbols) symbols)  a string Infrared 4Mbps 10m  of signals 900Mhz 2Mbps 150m  2.4GHz 2Mbps 150m  2.4GHz 11Mbps 80m  Problems with signal transmission  Bluetooth 700Kbps 10m  Attenuation: Signal power absorbed by medium Satellites   Geosynchronous satellite 600-1000 Mbps continent  Dispersion: A discrete signal spreads in space  Low Earth orbit (LEO) ~400 Mbps world  Noise: Random background “signals”  9/6/06 UIUC - CS/ECE438, Fall 2006 11 9/6/06 UIUC - CS/ECE438, Fall 2006 12 2

  3. Analog vs. Digital Encoding Transmission  Goal: Advantages of digital transmission over analog  Reasonably low-error rates over arbitrary distances Understand how to connect nodes in such a   Calculate/measure effects of transmission problems way that bits can be transmitted from one node  Periodically interpret and regenerate signal  to another Simpler for multiplexing distinct data types (audio, video,   Idea: e-mail, etc.) Two examples based on modulator-demodulators The physical medium is used to propagate   (modems) signals Electronic Industries Association (EIA) standard: RS-232(- Modulate electromagnetic waves   C) Vary voltage, frequency, wavelength  International Telecommunications Union (ITU) Data is encoded in the signal   V.32 9600 bps modem standard 9/6/06 UIUC - CS/ECE438, Fall 2006 13 9/6/06 UIUC - CS/ECE438, Fall 2006 14 RS-232 RS-232 Timing Diagram Communication between computer and modem  +15 Uses two voltage levels (+15V, -15V),  a binary voltage encoding Data rate limited to 19.2 kbps (RS-232-C); raised in  Voltage later standards + Characteristics  Serial: one signaling wire, one bit at a time  Asynchronous: line can be idle, clock generated from data  Character-based: send data in 7- or 8-bit characters -15  idle start 1 stop idle 0 0 1 1 0 0 Time 9/6/06 UIUC - CS/ECE438, Fall 2006 15 9/6/06 UIUC - CS/ECE438, Fall 2006 16 RS-232 Voltage Encoding One bit per clock  Common binary voltage encodings  Voltage never returns to 0V   Non-return to zero (NRZ) 0V is a dead/disconnected line  -15V is both idle and “1”  NRZ inverted (NRZI)  initiates send by pushing to 15V for one clock (start bit)   Manchester (used by IEEE 802.3—10 Minimum delay between character transmissions  Mbps Ethernet) Idle for one clock at -15V (stop bit)   4B/5B One character leads to 2+ voltage transitions  Total of 9 bits for 7 bits of data (78% efficient)  Start and stop bits also provide framing  9/6/06 UIUC - CS/ECE438, Fall 2006 17 9/6/06 UIUC - CS/ECE438, Fall 2006 18 3

  4. Non-Return to Zero Inverted Non-Return to Zero (NRZ) (NRZI)  Signal to Data Signal to Data  Transition  1  High  1  Low  0 Maintain  0   Comments  Bits 0 0 1 0 1 1 1 1 0 1 0 0 0 0 1 0 Transitions maintain clock synchronization  Long strings of 0s confused with no signal  NRZ Long strings of 1s causes baseline wander  Both inhibit clock recovery  NRZI Bits 0 0 1 0 1 1 1 1 0 1 0 0 0 0 1 0  Comments Strings of 0’s still a problem  NRZ 9/6/06 UIUC - CS/ECE438, Fall 2006 19 9/6/06 UIUC - CS/ECE438, Fall 2006 20 Manchester Encoding 4B/5B Signal to Data  Signal to Data  XOR NRZ data with clock  Encode every 4 consecutive bits as a 5 bit  High to low transition  1  symbol Low to high transition  0   Symbols Comments  Solves clock recovery problem  At most 1 leading 0  Only 50% efficient ( 1/2 bit per transition)  At most 2 trailing 0s  Bits 0 0 1 0 1 1 1 1 0 1 0 0 0 0 1 0 Never more than 3 consecutive 0s  Transmit with NRZI  NRZ  Comments Clock 80% efficient  Manchester 9/6/06 UIUC - CS/ECE438, Fall 2006 21 9/6/06 UIUC - CS/ECE438, Fall 2006 22 Binary Voltage Encodings Amplitude Modulation Problem with binary voltage (square wave)  encodings: Wide frequency range required, implying  Significant dispersion  Uneven attenuation  Prefer to use narrow frequency band (carrier  frequency) Types of modulation  Amplitude (AM)  Frequency (FM)  Phase/phase shift  Combinations of these  1 0 idle 9/6/06 UIUC - CS/ECE438, Fall 2006 23 9/6/06 UIUC - CS/ECE438, Fall 2006 24 4

  5. Frequency Modulation Phase Modulation 1 0 1 0 idle idle 9/6/06 UIUC - CS/ECE438, Fall 2006 25 9/6/06 UIUC - CS/ECE438, Fall 2006 26 Phase Modulation Phase Modulation Algorithm  Send carrier frequency 8-symbol for one period example Perform phase shift  90º Shift value encodes  108º difference in phase phase shift symbol 135º 45º in carrier collapse for 108º shift Value in range [0, 360º) frequency  180º 0º Multiple values for  multiple symbols 225º 315º Represent as circle  270º 9/6/06 UIUC - CS/ECE438, Fall 2006 27 9/6/06 UIUC - CS/ECE438, Fall 2006 28 Constellation Pattern for V.32 V.32 9600 bps QAM  Communication between modems  Analog phone line  Uses a combination of amplitude and 45º phase modulation 15º  Known as Quadrature Amplitude For a given symbol: Modulation (QAM) Perform phase shift and  Sends one of 16 signals each clock change to new amplitude cycle 9/6/06 UIUC - CS/ECE438, Fall 2006 29 9/6/06 UIUC - CS/ECE438, Fall 2006 30 5

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