1、2022-8-6Computer Networks-02-11COMPUTER NETWORKSChapter 02 The Physical Layer Part 12022-8-6Computer Networks-02-12Chapter 2 The Physical Layer Essence:Physical layer defines the mechanical,electrical,and functional timing interfaces to the network,provides the means to transmit bits from sender to
2、receiver,that is,involves a lot on how to use(analog)signals for digital information.What will be talked about?Theoretical analysis of data transmission Transmission media(wires and no wires)Three communication system:telephone,mobile phone,and cable television system2022-8-6Computer Networks-02-132
3、.1 The Theoretical Basis for Data Communication Were living in a digital world,meaning that wed preferably want to send digital(i.e.two-valued)signals through wires.Wires are pretty much physical,meaning that Mother Nature will probably impose a few constraints here and there.Observation:Signals are
4、 not entirely transmitted through a wire as you would expect:2022-8-6Computer Networks-02-142022-8-6Computer Networks-02-15 To understand whats going on,we need Fourier Analysis.A periodic function with period T(and frequency f=1/T),g(t)can be written as:Where f=1/T is the fundamental frequency,an a
5、nd bn are the sine and cosine amplitude of the nth harmonics(terms),and c is a constant.-Fourier SeriesTndtnfttgTa0)2sin()(2nkforTnkfordtnftkftT2/0)2sin()2sin(0TdttgTc0)(2TndtnfttgTb0)2cos()(22022-8-6Computer Networks-02-16Example:(n is the number of harmonics we take into account)2022-8-6Computer N
6、etworks-02-17 Example:01100010,8 bit for ASCII“b”an=1/ncos(n/4)-cos(3n/4)+cos(6n/4)-cos(7n/4)bn=1/nsin(3n/4)-sin(n/4)+sin(7n/4)-sin(6n/4)cn=3/4The root mean square amplitudes is(an2+bn2)1/2.2.1.2 Bandwidth-Limited Signalsbann222022-8-6Computer Networks-02-18Note:root mean squares(on the right)reflec
7、t the dispersedenergy at the given frequency.2022-8-6Computer Networks-02-19Bandwidth What does this all mean?Digital signal transmission can be thought of as being constructed as an infinite number of periodic analog signals.The quality of transmission is frequency dependent,not all parts of the di
8、gital signal get through the wire as you would expect.Digital signal transmission is subject to attenuation,distortion,etc.This is partly caused by disallowing high-frequency components to pass through.So the range of frequency or the number of bits of a transmission medium is called bandwidth.2022-
9、8-6Computer Networks-02-110 Example:(We are trying to transmit a single byte):With a bit rate of b bits/sec,it takes 8/b seconds to send a byte.The frequency f1 of the first harmonic is b/8 Hz Assumption:We are using a simple encoding technique based on the fact that the line supports only two signa
10、l values.bpsT(msec)First harmonic(Hz)#harmonics sent30026.6737.58060013.3375.04012006.67150.02024003.33300.01048001.67600.0596000.831200.02192000.422400.01384000.214800.002022-8-6Computer Networks-02-111Bandwidth Observation:Most telephone carriers cut off the highest frequency at 3000 Hz,we can nev
11、er transmit at a higher speed than 9600 bps(and without special encoding,its much lower)As a matter of fact,telephone lines bandwidth is set at 4000Hz.2022-8-6Computer Networks-02-112Improvement of BandwidthIf there are four signal values available,we could encode 2 bits at a time:00:0 volt;01:2 vol
12、ts;10:4 volts;11:6 voltsThe number changes in a signal per second is called the baud.Example:A 2400 bauds line(modem)can make a bit rate of 9600 bps provided it uses 16(24)signal values:SbitsSbitsSbitsSBits000004010081000121100100015010191001131101200106011010101014111030011701111110111511112022-8-6
13、Computer Networks-02-113 What is baud rate:The number changes in a signal per second is called the baud rate,B=1/T.What is bit rate:The number of bits transmitted per second is called the bit rate,bits per second,b/s or bps.What is the relationship between bit rate and baud rate:s=B*log2 v=1/T*log2
14、v where s is bit rate,B is baud rate,T is frequency period,v is number of signal values.Ex:B=9600,v=2,s=9600 bps;B=2400,v=16,s=9600 bpsBaud-rate and Bit-rate2022-8-6Computer Networks-02-114Example for relationship between baud-rate and bit-rate0100011011 binary 1 0 1 1 0 0 0 12022-8-6Computer Networ
15、ks-02-115 Where do we talk about bit rate and baud rate?DTEDCEDCEDTETransmission mediumBaud rateBit rateinterfaceinterface2022-8-6Computer Networks-02-1162.1.3 The maximum data rate of a channel Nyquist theoryNyquist showed that if the cut-off frequency is H Hz,the filtered signal can be reconstruct
16、ed by making 2H samples.No more,no less.Consequence:Maximum transmission rate=2H log2V bps (where V is the number of signal values)This is for noiseless channels,say the maxim data rate of a 3-kHz noiseless channel is no more than 6000 bps,for binary transmission.2022-8-6Computer Networks-02-117 Sha
17、nnon showed that a noisy channel with a signal-to-noise ratio S/N,has a limit with respect to the bit rate:Maximum transmission rate=H log2(1+S/N)In practice,we use dB(decibel)to represent signal-to-noise ratio in stead of S/N:10log10(S/N)=?dBExample:A telephone line with H=3000 and 10log10(S/N)=30d
18、B,can do no better than 30 kbps,no matter how you do your encoding(excluding compression).Here S/N is 1000,log2(1+S/N)is about 10.2022-8-6Computer Networks-02-1182.2 Guided Transmission Media Magnetic MediaNever underestimate the bandwidth of a station wagon full of tapes hurtling down the highway T
19、ake a standard videotape that can carry about 200 gigabytes of data.A box of 60 x 60 x 60 cm can hold about 1000 tapes,which corresponds to 200TB,or 1600Tb.Sending such a box can be done within 24 hours,worldwide,19Gbps;or 400Gbps within one hour.Costs:roughly$5000 to ship 200TB,3 cents for 1GB Ques
20、tion:What is overlooked in this reasoning?Answer:Delay2022-8-6Computer Networks-02-119Copper Wires Twisted pair Two insulated copper wires,twisted like a DNA string(reduces electrical interference).Often,twisted pairs go by the bundle.Comparable to telephone wiring at home.Further distinction betwee
21、n shielded(STP)and unshielded(UTP)versions.10Base-T:10Mbps,100 meters100Base-T:100Mbps,100 meters 1000Base-T:1Gbps,100 meters Category 3:not to use anymore Category 5/Super Category 5 Category 6/7:for Gigabit transmission2022-8-6Computer Networks-02-120 Connection standard:568A/568B 568B:棕白棕白兰白橙白绿绿兰
22、橙8765432187654321BrownWhite/BrownGreenWhite/BlueBlueWhite/GreenOrangeWhite/Orange2022-8-6Computer Networks-02-121Link Terminology Full-duplex link Used for transmission in both directions at once e.g.,use different twisted pairs for each direction Half-duplex link Both directions,but not at the same
23、 time e.g.,senders take turns on a wireless channel Simplex link Only one fixed direction at all times;not common2022-8-6Computer Networks-02-122 Coaxial CableExactly like the one you use for your TV Set:Baseband Coax50-ohm cable for digital transmission10Base-2,BNC,Thin-LAN,185m/per segment10Base-5
24、,AUI,Thick-LAN,500m/per segmentAt most 5 segments,up to 945m/2500m.Broadband Coax75-ohm cable for analog transmission,like cable TV.2022-8-6Computer Networks-02-1232022-8-6Computer Networks-02-124Power Lines Household electrical wiring is another example of wires Convenient to use,but horrible for s
25、ending data2022-8-6Computer Networks-02-125 Principle:Rather than using electrical signals,we use optical ones that are passed through optical fiber.Principal working is based on the refraction property of light:(a)Three examples of a light ray from inside a silica fiber impinging on the air/silica
26、boundary at different angles.(b)Light trapped by total internal reflection.Fiber Optics2022-8-6Computer Networks-02-126As it turns out,attenuation is extremely well in optical fiber.This means that they can be used for long distances.In addition,the bandwidth is enormous.ItemLEDSemiconductor LaserDa
27、ta rateLowHighModeMultimodeMultimode or single modeDistanceShortLongLifetimeLong lifeShort lifeTemperature sensitivityMinorSubstantialCostLow costExpensive2022-8-6Computer Networks-02-127ST：插入锁定光耦合器（ST）SC：568A标准，方形，插入锁定2022-8-6Computer Networks-02-128Fig.2-6 Attenuation of light through fiber in the
28、 infrared region.Attenuation in decibels=10log10(transmitted power/received power)2022-8-6Computer Networks-02-129Fiber Connections Observation:An interface consists of a receiver(photodiode)which transforms light into electrical signals,and/or a transmitter(LED or laser-diode)Passive interface:A co
29、mputer is directly connected to the optical fiber Active interface:Theres an ordinary electrical repeater connected to two fiber segments and the computer:2022-8-6Computer Networks-02-1302022-8-6Computer Networks-02-131Optical Fiber vs.Copper Wire(1)Bandwidth:Fiber can support enormous bandwidths,ex
30、actly what we need with upcoming image-based applications(video-on-demand).Attenuation:Because the attenuation in fiber is less than in copper(can you imagine why?),we dont need to boost the signal as often.In practice,fiber requires an active repeater every 30 km,copper every 5 km.2022-8-6Computer
31、Networks-02-132 External influences:Thats right,no more interference from other cables,radios,power failures,etc.Cross-talk(you hearing another conversation)is out of the question.Weight:Fiber simply doesnt weigh as much.Good for backbones,and the use of heavy maintenance equipment.Optical Fiber vs.
32、Copper Wire(2)2022-8-6Computer Networks-02-1332.3 Wireless Transmission The electromagnetic spectrum Wireless Transmission is really great for all of us who cant sit still,or feel they have to be on-line all the time(watch it you may miss something).Its also convenient when wiring is needed where it
33、 cant be done,or isnt really worth the trouble(jungles,islands,mountains).2022-8-6Computer Networks-02-134 Wireless transmissions travel at the speed of light(c),uses a frequency(f)which has a wavelength(l).The relation is that:c=l*f c is a constant,approximately 3 x 108m/sec,that is theoretical in
34、vacuum.In practice,c is about 2 x 108m/sec either in fiber or copper.That is 200m/sec.(IMPORTANT!)The electromagnetic spectrum2022-8-6Computer Networks-02-1352022-8-6Computer Networks-02-136 Note:We can encode only a few bits per Hertz in the low frequency range,but much more in the higher ranges.Th
35、is means that wireless transmission will generally have a much lower bandwidth(in practice:1-2 Mbps).Observation:Fiber optics operate in the high frequency range,which explains the transmission rates of gigabits per second.If given the width of a wavelength band,we can compute the corresponding freq
36、uency band f,and from that the data rate the band can produce.2022-8-6Computer Networks-02-137 Conclusion:the wider the range,and the shorter the wavelength,the higher the bandwidth.Example:Fiber optics often work at l=1.3*10-6 with Dl 0.16*10-6 leading to 30THz bandwidth!In frequency hopping spread
37、 spectrum,the transmitter hops from frequency to frequency hundreds of times per second.In recent years,this technique has also been applied commerciallyboth 802.11 and Bluetooth use it.2022-8-6Computer Networks-02-138 The other form of spread spectrum,direct sequence spread spectrum,which spreads t
38、he signal over a wide frequency band,is also gaining popularity in the commercial world.Observation:Radio transmission(VLFVHF)is extremely popular for its cheapness and range.Also,waves just go all over the place.2022-8-6Computer Networks-02-139Fig.2-12.(a)In the VLF,VF,and MF bands,radio waves foll
39、ow the curvature of the earth.(b)In the HF they bounce off the ionosphere.Observation:Microwave transmission is also popular and is good for long distances,as long as its directed.Problem is the density in the spectrum,requiring higher frequency ranges(which are hard for unguided transmissions)2022-
40、8-6Computer Networks-02-140Microwave Transmission Waves travel in straight lines and can be narrowly focused above 100MHz,concentrating energy into a small beam using dish-like antenna.Repeaters are needed periodically,for 100m high towers,repeaters can be spaced 80km apart.Multipath-fading is weath
41、er and frequency dependent.Frequency range:2.4GHz-2.484GHz,no FCC(federal communication commission)licensing needed.ISM(Industrial,Scientific,Medical)bands for unlicensed usage.use spread spectrum techniques.2022-8-6Computer Networks-02-141Infrared and Millimeter Waves Light Wave TransmissionFig.2-1
42、4.Convection currents can interfere with laser communication systems.A bidirectional system,with two lasers,is pictured here.2022-8-6Computer Networks-02-1422.4 Communication Satellites Geostationary Satellites Medium-Earth Orbit Satellites Low-Earth Orbit Satellites Satellites versus Fiber2022-8-6C
43、omputer Networks-02-143Satellites Essence:You direct a signal to something way up in the sky,and that reflects it back to earth.Observation:Reflection=receipt+amplification+sending back,possibly using a large dispersion so it can be received by many receivers on earth.Issue:Where to put the satellit
44、es(you should be able to direct your signal to them):At 36,000 km height:they will move in sync with the rotation speed of the earth At low-orbit,but then you will need to build a network of satellites for wide-area communication2022-8-6Computer Networks-02-144Communication Satellites Communication
45、satellites and some of their properties,including altitude above the earth,round-trip delay time and number of satellites needed for global coverage.2022-8-6Computer Networks-02-145The principal satellite bandsVSATs using a hubVSAT:very small aperture terminals2022-8-6Computer Networks-02-146Low-orb
46、it Satellites Essence:We throw in a relatively large number of low-orbit satellites which jointly cover the surface of the earth;when you are out of your current satellites spot beam,you should be in that of the next satellite:(a)The Iridium satellites from six necklaces around the earth.(b)1628 mov
47、ing cells cover the earth.2022-8-6Computer Networks-02-147Globalstar(a)Relaying in space.(b)Relaying on the ground.2022-8-6Computer Networks-02-148Where(not)to use Satellites Bandwidth:Fiber wins,but not everyone has access to all the available bandwidth.Satellites may make it easier to transfer dat
48、a anyway Mobility and remote locations:Satellites win,although it isnt clear whether simple cellular techniques may do just fine Broadcasting:Satellites win easily:broadcasting essentially comes for free Fast and reliable:Give credits to fiber:satellites are pretty bad due to inherent high latency(230 ms round-trip for geostationary satellites),and too much Mother Nature(rain!)
