Lecture 5 TransmissionPhysical and Datalink Layers: 3 LecturesFrom Signals to PacketsToday’s LectureWhy Do We Care?ModulationAmplitude and Frequency ModulationThe Frequency DomainSignal = Sum of WavesSlide 10Transmission 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 RememberSlide 32Analog versus Digital EncodingWhy Do We Need Encoding?EncodingNon-Return to Zero (NRZ)Non-Return to Zero Inverted (NRZI)Ethernet Manchester Encoding4B/5B EncodingSlide 40Other Encodings1Lecture 5TransmissionPeter SteenkisteDepartments of Computer Science andElectrical and Computer EngineeringCarnegie Mellon University15-441 Networking, Spring 2008http://www.cs.cmu.edu/~dga/15-441/S082Physical 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 LectureModulation.Bandwidth limitations.Frequency spectrum and its use.Multiplexing.Media: Copper, Fiber, Optical, Wireless.Coding.Framing.5Why Do We Care?I am not an electrical engineer?Physical layer places constraints on what the hardware network infrastructure can deliver= reality checkNeed to understand impact of the physical layer on the higher protocol layers»Fiber fiber copper?»Why do we need wires at all?»Error characteristic and failure modes»Effects of distance6ModulationSender changes the nature of the signal in a way that the receiver can recognize.»Similar to radio: AM or FMDigital transmission: encodes the values 0 or 1 in the signal.»It is also possible to encode multi-valued symbolsAmplitude modulation: change the strength of the signal, typically between on and off.»Sender and receiver agree on a “rate”»On means 1, Off means 0Similar: frequency or phase modulation.Can also combine method modulation types.7Amplitude 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 18The Frequency DomainA (periodic) signal can be viewed as a sum of sine waves of different strengths.»Corresponds to energy at a certain frequencyEvery signal has an equivalent representation in the frequency domain.»What frequencies are present and what is their strength (energy)Again: Similar to radio and TV signals.TimeFrequencyAmplitude9Signal = Sum of Waves=+ 1.3 X+ 0.56 X+ 1.15 X10Why Do We Care?How much bandwidth can I get out of a specific wire (transmission medium)?What limits the physical size of the network?How can multiple hosts communicate over the same wire at the same time?How can I manage bandwidth on a transmission medium?How do the properties of copper, fiber, and wireless compare?11Transmission Channel ConsiderationsEvery medium supports transmission in a certain frequency range.»Outside this range, effects such as attenuation, .. degrade the signal too muchTransmission and receive hardware will try to maximize the useful bandwidth in this frequency band.»Tradeoffs between cost, distance, bit rateAs technology improves, these parameters change, even for the same wire.»Thanks to our EE friendsFrequencyGood BadSignal12The Nyquist LimitA 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 encoding13Past the Nyquist LimitMore aggressive encoding can increase the channel bandwidth.»Example: modems–Same f requency - number of symbols per second–Symbols have more possible valuesEvery 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+ AM14Capacity of a Noisy ChannelCan’t add infinite symbols - you have to be able to tell them apart. This is where noise comes in.Shannon’s theorem: C = B x log2(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)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.15Example: Modem Rates1001000100001000001975 1980 1985 1990 1995 2000YearModem rate16Limits to Speed and DistanceNoise: “random” energy is added to the signal.Attenuation: some of the energy in the signal leaks away.Dispersion: attenuation and propagation speed are frequency dependent.»Changes the shape of the signalEffects limit the data rate that a channel can sustain.»But affects different technologies in different waysEffects become worse with distance.»Tradeoff between data rate and distance17Supporting Multiple ChannelsMultiple 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, timeSpace can be limited using wires or using transmit power of wireless transmitters.Frequency multiplexing means that different users use a different part of the spectrum.»Again, similar to radio: 95.5 versus 102.5 stationControlling time is a datalink protocol issue.»Media Access Control (MAC): who gets to send when?18Time Division MultiplexingDifferent users use the wire at different points in time.Aggregate bandwidth also requires more spectrum.FrequencyFrequency19Baseband versus Carrier
View Full Document
Unlocking...