1Lecture 5TransmissionDavid AndersenDepartment of Computer ScienceCarnegie Mellon University15-441 Networking, Spring 2005http://www.cs.cmu.edu/~srini/15-441/S052Physical and Datalink Layers:3 Lectures Physical layer. Datalink layerintroduction, framing,error coding, switchednetworks. Broadcast-networks,home networking.ApplicationPresentationSessionTransportNetworkDatalinkPhysical3From Signals to PacketsAnalog Signal“Digital” SignalBit Stream0 0 1 0 1 1 1 0 0 0 1Packets0100010101011100101010101011101110000001111010101110101010101101011010111001Header/BodyHeader/BodyHeader/BodyReceiverSenderPacketTransmission4Today’s Lecture Modulation. Bandwidth limitations. Frequency spectrum and its use. Multiplexing. Media: Copper, Fiber, Optical, Wireless. Coding. Framing.5Modulation Sender changes the nature of the signal in away that the receiver can recognize.» Similar to radio: AM or FM Digital transmission: encodes the values 0 or1 in the signal.» It is also possible to encode multi-valued symbols Amplitude modulation: change the strengthof the signal, typically between on and off.» Sender and receiver agree on a “rate”» On means 1, Off means 0 Similar: frequency or phase modulation. 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 Domain A (periodic) signal can be viewed as a sum of sinewaves of different strengths.» Corresponds to energy at a certain frequency Every signal has an equivalent representation in thefrequency domain.» What frequencies are present and what is their strength (energy) Again: Similar to radio and TV signals.TimeFrequencyAmplitude8Signal = Sum of Waves=+ 1.3 X+ 0.56 X+ 1.15 X9Why Do We Care? How much bandwidth can I get out of aspecific wire (transmission medium)? What limits the physical size of the network? How can multiple hosts communicate over thesame wire at the same time? How can I manage bandwidth on atransmission medium? How do the properties of copper, fiber, andwireless compare?10Transmission ChannelConsiderations Every medium supportstransmission in a certainfrequency range.» Outside this range, effectssuch as attenuation, .. degradethe signal too much Transmission and receivehardware will try tomaximize the usefulbandwidth in this frequencyband.» Tradeoffs between cost,distance, bit rate As technology improves,these parameters change,even for the same wire.» Thanks to our EE friendsFrequencyGood BadSignal11The Nyquist Limit A noiseless channel of width H can at mosttransmit a binary signal at a rate 2 x H.» E.g. a 3000 Hz channel can transmit data at a rate of atmost 6000 bits/second» Assumes binary amplitude encoding12Past the Nyquist Limit More aggressive encoding can increase the channelbandwidth.» Example: modems– Same frequency - number of symbols per second– Symbols have more possible values Every transmission medium supports transmission ina certain frequency range.» The channel bandwidth is determined by the transmissionmedium and the quality of the transmitter and receivers» Channel capacity increases over timepskPsk+AM13Capacity of a Noisy Channel Can’t add infinite symbols - you have to be able to tellthem apart. This is where noise comes in. 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). 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 digitalsignal, which sets a limit on B and the S/N.14Example: Modem Rates1001000100001000001975 1980 1985 1990 1995 2000YearModem rate15Limits to Speed and Distance Noise: “random” energy isadded to the signal. Attenuation: some of theenergy in the signal leaks away. Dispersion: attenuation andpropagation speed arefrequency dependent.» Changes the shape of the signal Effects limit the data rate that a channel can sustain.» But affects different technologies in different ways Effects become worse with distance.» Tradeoff between data rate and distance16Supporting Multiple Channels Multiple channels can coexist if they transmitat a different frequency, or at a different time,or in a different part of the space.» Three dimensional space: frequency, space, time Space can be limited using wires or usingtransmit power of wireless transmitters. Frequency multiplexing means that differentusers use a different part of the spectrum.» Again, similar to radio: 95.5 versus 102.5 station Controlling time is a datalink protocol issue.» Media Access Control (MAC): who gets to send when?17Time Division Multiplexing Different users use the wire at different points in time. Aggregate bandwidth also requires more spectrum.FrequencyFrequency18Baseband versus CarrierModulation Baseband modulation: send the “bare”signal. Carrier modulation: use the signal tomodulate a higher frequency signal (carrier).» Can be viewed as the product of the two signals» Corresponds to a shift in the frequency domain Same idea applies to frequency and phasemodulation.» E.g. change frequency of the carrier instead of itsamplitude19Amplitude Carrier ModulationAmplitudeSignalCarrierFrequencyAmplitudeModulatedCarrier20Frequency Division Multiplexing:Multiple ChannelsAmplitudeDifferent CarrierFrequenciesDeterminesBandwidthof ChannelDetermines Bandwidth of Link21Frequency versusTime-division Multiplexing With frequency-divisionmultiplexing different usersuse different parts of thefrequency spectrum.» I.e. each user can send all thetime at reduced rate» Example: roommates With time-divisionmultiplexing different userssend at different times.» I.e. each user can sent at fullspeed some of the time» Example: a time-share condo The two solutions can becombined.FrequencyTimeFrequencyBandsSlotFrame22Copper Wire Unshielded twisted pair» Two copper wires twisted - avoid antenna effect» Grouped into cables: multiple pairs with common sheath» Category 3 (voice grade) versus category 5» 100 Mbit/s up to 100 m, 1 Mbit/s up to a few km» Cost: ~ 10cents/foot Coax cables.» One connector is placed inside the other connector»
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