1Direct Link Networks9/6 /06 UIUC - CS/ECE438, Fal l 200 6 2Direct Link Networks Two hosts connected directly No issues of contention, routing, … Key points: Physical Connections Encoding and Modulation Framing Error Detection9/6 /06 UIUC - CS/ECE438, Fal l 200 6 3Internet ProtocolsPhysicalData LinkHardware(network adapter)Framing, error detection,medium access controlEncodingNetworkTransportKernel softwareApplicationPresentationSessionUser-level software9/6 /06 UIUC - CS/ECE438, Fal l 200 6 4Outline Hardware building blocks Encoding Framing9/6 /06 UIUC - CS/ECE438, Fal l 200 6 5Hardware Building Blocks Nodes Hosts: general purpose computers Switches: typically special purpose hardware Routers: varied Links Copper wire with electronic signaling Glass fiber with optical signaling Wireless with electromagnetic (radio, infrared,microwave, signaling)9/6 /06 UIUC - CS/ECE438, Fal l 200 6 6Links - Copper Copper-based Media Category 5 Twisted Pair 10-100Mbps 100m ThinNet Coaxial Cable 10-100Mbps 200m ThickNet Coaxial Cable 10-100Mbps 500mtwisted paircopper coreinsulationbraided outer conductorouter insulationcoaxialcable(coax)29/6 /06 UIUC - CS/ECE438, Fal l 200 6 7Links - Optical Optical Media Multimode Fiber 100Mbps 2km Single Mode Fiber 100-2400Mbps 40kmglass core (the fiber)glass claddingplastic jacketopticalfiber9/6 /06 UIUC - CS/ECE438, Fal l 200 6 8Links - Optical Single mode Lower attenuation (longer distances) Lower dispersion (higher data rates) Multimode fiber Cheap to drive (LED’s) vs. lasers for single mode Easier to terminateO(100 microns) thickcore of multimode fiber (same frequency; colors for clarity)~1 wavelength thick =~1 microncore of single mode fiber9/6 /06 UIUC - CS/ECE438, Fal l 200 6 9Links - Optical Advantages of optical communication Higher bandwidths Superior attenuation properties Immune from electromagneticinterference No crosstalk between fibers Thin, lightweight, and cheap (the fiber,not the optical-electrical interfaces)9/6 /06 UIUC - CS/ECE438, Fal l 200 6 10Leased Lines POTS 64Kbps ISDN 128Kbps ADSL 1.5-8Mbps/16-640Kbps Cable Modem 0.5-2Mbps DS1/T1 1.544Mbps DS3/T3 44.736Mbps STS-1 51.840Mbps STS-3 155.250Mbps (ATM) STS-12 622.080Mbps (ATM)9/6 /06 UIUC - CS/ECE438, Fal l 200 6 11Wireless Cellular AMPS 13Kbps 3km PCS, GSM 300Kbps 3km 3G 2-3Mbps 3km Wireless Local Area Networks (WLAN) Infrared 4Mbps 10m 900Mhz 2Mbps 150m 2.4GHz 2Mbps 150m 2.4GHz 11Mbps 80m Bluetooth 700Kbps 10m Satellites Geosynchronous satellite 600-1000 Mbps continent Low Earth orbit (LEO) ~400 Mbps world9/6 /06 UIUC - CS/ECE438, Fal l 200 6 12Encoding Problems with signal transmission Attenuation: Signal power absorbed by medium Dispersion: A discrete signal spreads in space Noise: Random background “signals”digital data(a string of symbols)digital data(a string of symbols)modulator demodulatora stringof signalsmodulator demodulator39/6 /06 UIUC - CS/ECE438, Fal l 200 6 13Encoding Goal: Understand how to connect nodes in such away that bits can be transmitted from one nodeto another Idea: The physical medium is used to propagatesignals Modulate electromagnetic waves Vary voltage, frequency, wavelength Data is encoded in the signal9/6 /06 UIUC - CS/ECE438, Fal l 200 6 14Analog vs. DigitalTransmission Advantages of digital transmission over analog Reasonably low-error rates over arbitrary distances Calculate/measure effects of transmission problems Periodically interpret and regenerate signal Simpler for multiplexing distinct data types (audio, video,e-mail, etc.) Two examples based on modulator-demodulators(modems) Electronic Industries Association (EIA) standard: RS-232(-C) International Telecommunications Union (ITU)V.32 9600 bps modem standard9/6 /06 UIUC - CS/ECE438, Fal l 200 6 15RS-232 Communication between computer and modem Uses two voltage levels (+15V, -15V),a binary voltage encoding Data rate limited to 19.2 kbps (RS-232-C); raised inlater 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 characters9/6 /06 UIUC - CS/ECE438, Fal l 200 6 16RS-232 Timing Diagramidle start1 110 0 0 0stop idle-15++15TimeVoltage9/6 /06 UIUC - CS/ECE438, Fal l 200 6 17RS-232 One bit per clock Voltage never returns to 0V 0V is a dead/disconnected line -15V is both idle and “1” initiates send by pushing to 15V for one clock (start bit) Minimum delay between character transmissions Idle for one clock at -15V (stop bit) One character leads to 2+ voltage transitions Total of 9 bits for 7 bits of data (78% efficient) Start and stop bits also provide framing9/6 /06 UIUC - CS/ECE438, Fal l 200 6 18Voltage Encoding Common binary voltage encodings Non-return to zero (NRZ) NRZ inverted (NRZI) Manchester (used by IEEE 802.3—10Mbps Ethernet) 4B/5B49/6 /06 UIUC - CS/ECE438, Fal l 200 6 19Non-Return to Zero (NRZ) Signal to Data High 1 Low 0 Comments Transitions maintain clock synchronization Long strings of 0s confused with no signal Long strings of 1s causes baseline wander Both inhibit clock recoveryBits 0 0 1 0 1 1 1 1 0 1 0 0 0 0 1 0NRZ9/6 /06 UIUC - CS/ECE438, Fal l 200 6 20Non-Return to Zero Inverted(NRZI) Signal to Data Transition 1 Maintain 0Bits 0 0 1 0 1 1 1 1 0 1 0 0 0 0 1 0NRZNRZI Comments Strings of 0’s still a problem9/6 /06 UIUC - CS/ECE438, Fal l 200 6 21Manchester Encoding Signal to Data XOR NRZ data with clock High to low transition 1 Low to high transition 0 Comments Solves clock recovery problem Only 50% efficient ( 1/2 bit per transition)Bits 0 0 1 0 1 1 1 1 0 1 0 0 0 0 1 0NRZClockManchester9/6 /06 UIUC - CS/ECE438, Fal l 200 6 224B/5B Signal to Data Encode every 4 consecutive bits as a 5 bitsymbol Symbols At most 1 leading 0 At most 2 trailing 0s Never more than 3 consecutive 0s Transmit with NRZI Comments 80% efficient9/6 /06 UIUC - CS/ECE438, Fal l 200 6 23Binary Voltage Encodings Problem with binary voltage (square wave)encodings: Wide frequency range required, implying Significant dispersion Uneven attenuation Prefer to use
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