1Direct Link Networks9/12/07 UIUC - CS/ECE438, Fall 2007 2Direct Link Networks Two hosts connected directly No issues of contention, routing, … Deliver bits between two computers Modulation Encoding Framing ... quickly Bandwidth Delay ... reliably Error detection Error correction9/12/07 UIUC - CS/ECE438, Fall 2007 3Outline Bandwidth vs. delay Hardware building blocks Encoding Framing9/12/07 UIUC - CS/ECE438, Fall 2007 4Performance Bandwidth/throughput Data transmitted per unit time Example: 10 Mbps Link bandwidth vs. end-to-end bandwidth Notation KB = 210 bytes Mbps = 106 bits per second9/12/07 UIUC - CS/ECE438, Fall 2007 5Performance Latency/delay Time from A to B Example: 30 msec (milliseconds) Many applications depend on round-trip time (RTT) Components Transmission time Propagation delay over links Queueing delays Software processing overheads9/12/07 UIUC - CS/ECE438, Fall 2007 6Performance Notes Speed of Light 3.0 x 108 meters/second in a vacuum 2.3 x 108 meters/second in a cable 2.0 x 108 meters/second in a fiber Comments No queueing delays in a direct link Bandwidth is not relevant if size = 1bit Software overhead can dominate when distance is small Key Point Latency dominates small transmissions Bandwidth dominates large29/12/07 UIUC - CS/ECE438, Fall 2007 7Delay x Bandwidth Product channel = pipe delay = length bandwidth = area of a cross section bandwidth x delay product = volumeBandwidthDelay9/12/07 UIUC - CS/ECE438, Fall 2007 8Delay x Bandwidth Product Example: Transcontinental Channel BW = 45 Mbps delay = 50ms bandwidth x delay product= (45 x 106 bits/sec) x (50 x 10–3 sec)= 2.25 x 106 bits Bandwidth x delay product How many bits the sender must transmit beforethe first bit arrives at the receiver if the senderkeeps the pipe full Takes another one-way latency to receive aresponse from the receiver9/12/07 UIUC - CS/ECE438, Fall 2007 9Bandwidth vs. Latency Relative importance 1-byte: Latency bound 1ms vs 100ms latency dominates 1Mbps vs 100Mbps BW 25MB: Bandwidth bound 1Mbps vs 100Mbps BW dominates 1ms vs 100ms latency25MB1 Mbps1b1Mbps1ms100 Mbps 100Mbps100ms9/12/07 UIUC - CS/ECE438, Fall 2007 10Bandwidth vs. Latency Infinite bandwidth RTT dominates Throughput = TransferSize / TransferTime TransferTime = RTT + 1/Bandwidth xTransferSize Its all relative 1-MB file to 1-Gbps link looks like a 1-KBpacket to 1-Mbps link9/12/07 UIUC - CS/ECE438, Fall 2007 11Hardware 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)9/12/07 UIUC - CS/ECE438, Fall 2007 12Links - 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)39/12/07 UIUC - CS/ECE438, Fall 2007 13Links - Optical Optical Media Multimode Fiber 100Mbps 2km Single Mode Fiber 100-2400Mbps 40kmglass core (the fiber)glass claddingplastic jacketopticalfiber9/12/07 UIUC - CS/ECE438, Fall 2007 14Links - 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 fiber9/12/07 UIUC - CS/ECE438, Fall 2007 15Links - 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)9/12/07 UIUC - CS/ECE438, Fall 2007 16Leased 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)9/12/07 UIUC - CS/ECE438, Fall 2007 17Wireless 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 5 GHz 74 Mbps 150m Bluetooth 700Kbps 10m Satellites Geosynchronous satellite 600-1000 Mbps continent Low Earth orbit (LEO) ~400 Mbps world9/12/07 UIUC - CS/ECE438, Fall 2007 18Encoding 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 demodulator49/12/07 UIUC - CS/ECE438, Fall 2007 19Encoding Goal: Understand how to connect nodes in such a waythat bits can be transmitted from one node toanother Idea: The physical medium is used to propagatesignals Modulate electromagnetic waves Vary voltage, frequency, wavelength Data is encoded in the signal9/12/07 UIUC - CS/ECE438, Fall 2007 20Analog 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 standard9/12/07 UIUC - CS/ECE438, Fall 2007 21RS-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 characters9/12/07 UIUC - CS/ECE438, Fall 2007 22RS-232 Timing Diagramidle start1 110 0 0 0stop idle-15++15TimeVoltage9/12/07 UIUC - CS/ECE438, Fall 2007
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