Lecture 5 TransmissionPhysical and Datalink Layers: 3 LecturesFrom Signals to PacketsToday’s LectureModulationAmplitude and Frequency ModulationThe Frequency DomainSignal = Sum of WavesWhy Do We Care?Transmission Channel ConsiderationsThe Nyquist LimitPast the Nyquist LimitCapacity of a Noisy ChannelExample: Modem RatesLimits to Speed and DistanceSupporting Multiple ChannelsTime Division MultiplexingBaseband versus Carrier ModulationAmplitude Carrier ModulationFrequency Division Multiplexing: Multiple ChannelsFrequency versus Time-division MultiplexingCopper WireLight Transmission in FiberRay PropagationFiber TypesGigabit Ethernet: Physical Layer ComparisonRegeneration and AmplificationWavelength Division MultiplexingWireless TechnologiesThings to Remember1Lecture 5TransmissionHui ZhangDepartment of Computer ScienceCarnegie Mellon Universityhttp://www.cs.cmu.edu/~srini/15-441/F072Physical and Datalink Layers:3 Lectures1. Physical layer.2. Datalink layer introduction, framing, error coding, switched networks.3. Broadcast-networks, home networking.ApplicationApplicationPresentationPresentationSessionSessionTransportTransportNetworkNetworkDatalinkDatalinkPhysicalPhysical3From Signals to PacketsAnalog Signal“Digital” SignalBit Stream0 0 1 0 1 1 1 0 0 0 1Packets0100010101011100101010101011101110000001111010101110101010101101011010111001Header/BodyHeader/BodyHeader/BodyReceiverSenderPacketTransmission4Today’s Lecturez Modulation.z Bandwidth limitations.z Frequency spectrum and its use.z Multiplexing.z Media: Copper, Fiber, Optical, Wireless.z Coding.z Framing.5Modulationz Sender changes the nature of the signal in a way that the receiver can recognize.» Similar to radio: AM or FMzDigital transmission: encodes the values 0 or 1 in the signal.» It is also possible to encode multi-valued symbolszAmplitude modulation: change the strength of the signal, typically between on and off.» Sender and receiver agree on a “rate”» On means 1, Off means 0zSimilar: frequency or phase modulation.z Can also combine method modulation types.6Amplitude and FrequencyModulation0 0 1 1 0 0 1 1 0 0 0 1 1 1 0 0 0 1 1 0 0 0 1 1 1 00 1 1 0 1 1 0 0 0 17The Frequency Domainz A (periodic) signal can be viewed as a sum of sine waves of different strengths.» Corresponds to energy at a certain frequencyz Every signal has an equivalent representation in the frequency domain.» What frequencies are present and what is their strength (energy)z Again: Similar to radio and TV signals.TimeFrequencyAmplitude8Signal = Sum of Waves=+ 1.3 X+ 0.56 X+ 1.15 X9Why Do We Care?z How much bandwidth can I get out of a specific wire (transmission medium)?z What limits the physical size of the network?z How can multiple hosts communicate over the same wire at the same time?z How can I manage bandwidth on a transmission medium?z How do the properties of copper, fiber, and wireless compare?10Transmission Channel Considerationsz Every medium supports transmission in a certain frequency range.» Outside this range, effects such as attenuation, .. degrade the signal too muchz Transmission and receive hardware will try to maximize the useful bandwidth in this frequency band.» Tradeoffs between cost, distance, bit ratez As technology improves, these parameters change, even for the same wire.» Thanks to our EE friendsFrequencyGood BadSignal11The Nyquist Limitz A noiseless channel of width H can at most transmit a binary signal at a rate 2 x H.» E.g. a 3000 Hz channel can transmit data at a rate of at most 6000 bits/second» Assumes binary amplitude encoding12Past the Nyquist Limitz More aggressive encoding can increase the channel bandwidth.» Example: modems– Same frequency - number of symbols per second– Symbols have more possible valuesz Every transmission medium supports transmission in a certain frequency range.» The channel bandwidth is determined by the transmission medium and the quality of the transmitter and receivers» Channel capacity increases over timepskPsk+ AM13Capacity of a Noisy Channelz Can’t add infinite symbols - you have to be able to tell them apart. This is where noise comes in.z Shannon’s theorem:» C = B x log(1 + S/N)» C: maximum capacity (bps)» B: channel bandwidth (Hz)» S/N: signal to noise ratio of the channel– Often expressed in decibels (db). 10 log(S/N).z Example:» Local loop bandwidth: 3200 Hz» Typical S/N: 1000 (30db)» What is the upper limit on capacity?– Modems: Teleco internally converts to 56kbit/s digital signal, which sets a limit on B and the S/N.14Example: Modem Rates1001000100001000001975 1980 1985 1990 1995 2000YearModem rate15Limits to Speed and Distancez Noise: “random” energy is added to the signal.z Attenuation: some of the energy in the signal leaks away.z Dispersion: attenuation and propagation speed are frequency dependent.» Changes the shape of the signalz Effects limit the data rate that a channel can sustain.» But affects different technologies in different waysz Effects become worse with distance.» Tradeoff between data rate and distance16Supporting Multiple Channelsz Multiple channels can coexist if they transmit at a different frequency, or at a different time, or in a different part of the space.» Three dimensional space: frequency, space, timezSpace can be limited using wires or using transmit power of wireless transmitters.z Frequency multiplexing means that different users use a different part of the spectrum.» Again, similar to radio: 95.5 versus 102.5 stationzControlling time is a datalink protocol issue.» Media Access Control (MAC): who gets to send when?17Time Division Multiplexingz Different users use the wire at different points in time.z Aggregate bandwidth also requires more spectrum.FrequencyFrequency18Baseband versus Carrier Modulationz Baseband modulation: send the “bare”signal.z Carrier modulation: use the signal to modulate a higher frequency signal (carrier).» Can be viewed as the product of the two signals» Corresponds to a shift in the frequency domainzSame idea applies to frequency and phase modulation.» E.g. change frequency of the carrier instead of its amplitude19Amplitude Carrier ModulationAmplitudeSignalCarrierFrequencyAmplitudeModulatedCarrier20Frequency Division Multiplexing:Multiple ChannelsAmplitudeDifferent CarrierFrequenciesDeterminesBandwidthof ChannelDetermines Bandwidth of Link21Frequency versus Time-division Multiplexingz With
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