2Data CommunicationInformation can be transmitted on wires by varying some physical property such as voltage, current or light.By representing the value of this voltage or current as a single-valued function of time, f(t), we can model the behavior of the signal and analyze it mathematically.computer 2computer 1bitsbitstransmission mediumtransmitterreceiverelectric currentlightelectromagnetic waves
3Signal Analysis Using Fourier Series Any reasonably behaved periodic function, g(t), with period T can be constructed by summing a (possibly infinite) number of sines and cosines:where f=1/T is the fundamental frequency and an and bn are the sine and cosine amplitudes of the nth harmonics.
4Example : Digital Signal Analysis Spectral Analysis
5Digital Signal Synthesis One harmonicTwo harmonics
7Maximum Data Rate or Capacity of a Communication Channel 1. Noiselss Channel Case : Nyquist’s TheoremMaximum capacity ( C ) = 2 H log2 V bits/secbandwidthnumber of signal levels
8Maximum Data Rate or Capacity of a Communication Channel 2. Noisy Channel Case: Shannon’s TheoremIf random noise is present, the situation deteriorates rapidly. The amount of thermal noise present is measured by the ratio of the signal power to the noise power, called the signal-to-noise ratio (S/N).Maximum Capacity ( C ) =H log2(1+S/N)
9High SNR Low SNR Average Signal Power SNR = Average Noise Power signal signal + noiseHighSNRtttsignalnoisesignal + noiseLowSNRtttAverage Signal PowerSNR =Average Noise PowerSNR (dB) = 10 log10 SNR
10Numerical Example 1: 1. Noiseless channel case: Bandwidth H = 3000 Hz Voltage Levels V = 4 ( two binary bits)Then,C = 2H log 2 (V) = 2 * 3000 log 2 (4) bps.= bps.2. Noisy channel case:Bandwidth H = HzVoltage Levels V = 4S/ N = 20 dB 20 = 10 log 10 (S/ N) S/ N = 100Then,C = H log 2 ( 1 + S/N ) == 3000 log 2 ( )= bps.
11Numerical Example 2: 1. Noiseless channel case: Bandwidth H = 3000 Hz Voltage Levels V = 8 ( three binary bits)Then,C = 2H log 2 (V) = 2 * 3000 log 2 (8) bps.= bps.2. Noisy channel case:Bandwidth H = HzS/ N = 20 dBThen,20 = 10 log 10 (S/ N)S/ N = 100C = H log 2 ( 1 + S/N ) == 3000 log 2 ( )= bps.
12Transmission MediaTransmission medium:: the physical path between transmitter and receiver.Guided media :: waves are guided along a physical path (e.g, twisted pair, coaxial cable and optical fiber)Unguided media :: means for transmitting but not guiding electromagnetic waves (e.g., the atmosphere and outer space).
15Guided Transmission Data Magnetic TapesCoaxial CableTwisted PairFiber Optics
16Magnetic Tapes Bandwidth: A tape can hold 7 gigabytes. A box can hold about 1000 tapes.Assume a box can be delivered in 24 hours.The effective bandwidth=7*1000*8/86400=648 MbpsCostCost of 1000 tapes= $5000.If a tape can be reused 10 times and the shipping cost is $ 200, we have a cost of $ 700 to ship 7000 gigabytes.
18Coaxial Cables Types 10Base5 Thick Ethernet :: thick (10 mm) coax 10 Mbps,500 m. max segment length,100 devices/segment,awkward to handle and install.10Base2 Thin Ethernet ::thin (5 mm) coax185 m. max segment length,30 devices/segment, easier to handle,
19Coaxial Cable Applications Television distributionAriel to TVCable TVLong distance telephone transmissionCan carry 10,000 voice calls simultaneouslyBeing replaced by fiber opticShort distance computer systems linksLocal area networks
20Twisted Pair Cables Unshielded Twisted Pair (UTP) Ordinary telephone wireCheapestEasiest to installSuffers from external EM interferenceShielded Twisted Pair (STP)Metal braid or sheathing that reduces interferenceMore expensiveHarder to handle (thick, heavy)
22UTP Categories Cat 3 Cat 5 up to 16MHz Voice grade found in most officesTwist length of 7.5 cm to 10 cmCat 5up to 100MHzCommonly pre-installed in new office buildingsTwist length 0.6 cm to 0.85 cm
23Twisted Pair Applications Most common mediumTelephone networkBetween house and local exchange (subscriber loop)Within buildingsTo private branch exchange (PBX)For local area networks (LAN)10Mbps or 100Mbps
2410BASE-T 10 Mbps baseband transmission over twisted pair. Two Cat 3 cables, Manchester encoding,Maximum distance metersEthernet hub
26Fiber OpticsOptical fiber :a thin flexible medium capable of conducting optical rays. Optical fiber consists of a very fine cylinder of glass (core) surrounded by concentric layers of glass (cladding).a signal-encoded beam of light (a fluctuating beam) is transmitted by total internal reflection.Total internal reflection occurs in the core because it has a higher optical density (index of refraction) than the cladding.
27Fiber Cables (a). Side view of a single fiber. (b). End view of a sheath with three fibers.
28Total Internal Reflection (a). Three examples of a light ray from inside a silica fiber impinging on the air/silica boundary at different angles.(b). Light trapped by total internal reflection.
29A fiber optic ring with active repeaters. Fiber Optic NetworksA fiber optic ring with active repeaters.
30Optical Fiber - Benefits Greater capacity (Gbps)Smaller size & weightLower attenuationElectromagnetic isolationGreater repeater spacing ( 10s of Km)
37ISM (Industrial/Scientific/Medical) Band Wireless Transmission Frequencies2GHz to 40GHz ( Microwave, Satellite)30MHz to 1GHz ( Broadcast radio )3 x 1011 to 2 x 1014 ( Infrared)ISM (Industrial/Scientific/Medical) BandTransmitters using these bands do not require government licensing. One band is allocated worldwide: GHz. In addition, in the US and Canada, bands also exist from MHz and from GHz. These bands are used for cordless telephones, garage door openers, wireless hi-fi speakers, security gates, etc.
38Antennas Radio frequency energy from transmitter Electrical conductor used to radiate or collect electromagnetic energy. Same antenna often used for both transmission and receptionTransmissionRadio frequency energy from transmitterConverted to electromagnetic energy by antennaRadiated into surrounding environmentReceptionElectromagnetic energy impinging on antennaConverted to radio frequency electrical energyFed to receiver
39Radio TransmissionRadio waves are easy to generate, can travel long distance, and penetrate buildings easily, so they are widely used for communication, both indoors and outdoors.Radio waves are also omnidirectional, meaning that they travel in all directions from the source, so that the transmitter and receiver do not have to be carefully aligned physically.
40Radio Transmission(a). In the VLF, LF, and MF bands, radio waves follow the curvature of the earth.(b). In the HF band, they bounce off the ionosphere.
41Microwave Transmission Above 100 MHz, the waves travel in straight lines and can therefore be narrowly focused. Concentrating all the energy into a small beam using a parabolic antenna gives a much higher signal to noise ratio.Since the microwaves travel in a straight line, if the towers are too far apart, the earth will get in the way. Consequently, repeaters are needed periodically.
42Disadvantages:do not pass through buildings wellmultipath fading problem (the delayed waves cancel the signal)absorption by rain above 8 GHzsevere shortage of spectrumAdvantages:no right way is needed (compared to wired media)relatively inexpensivesimple to install
43Infrared and Millimeter Transmission . Unguided infrared and millimeter waves are widely used for short-range communication. The remote controls used on televisions, VCRs, and stereos all use infrared communication.. They are relatively directional, cheap, and easy to build, but have a major drawback: they do not pass through solid objects.. This property is also a plus. It means that an infrared system in one room will not interfere with a similar system in adjacent room. It is more secure against eavesdropping.
44Convection currents can interfere with laser communication systems. A bidirectional system with two lasers is pictured here.
45Communication Satellites Satellite is relay stationSatellite receives on one frequency, amplifies or repeats signal and transmits on another frequencyTypes based on orbital altitude:Geostationary Orbit Satellites (GEO)Medium-Earth Orbit Satellites (MEO)Low-Earth Orbit Satellites (LEO)Applications : Television, Long distance telephone, Private business networks
48Satellites TypesCommunication satellites and some of their properties,including altitude above the earth, round trip delay time andnumber of satellites needed for global coverage.
49Satellites versus fiber cables High bandwidth available for individual users.More suitable for mobile communicationNaturally suited for broadcast applicationsBetter suited for connecting remote areas.
51Wired LAN Digital Signal Encoding The following Schemes to encode frame bitsinto voltage or light signals for transmissionThrough guided media:Nonreturn to Zero-Level (NRZ-L)Nonreturn to Zero Inverted (NRZI)ManchesterDifferential ManchesterBipolar -AMIPseudo ternary
52Binary Encoding Schemes Non-return to Zero-Level (NRZ-L)Non-return to Zero Inverted (NRZI) negative voltage0 positive voltage1 existence of a signal transition at the beginning of thebit time (either a low-to-high or a high-to-low transition) 0 no signal transition at the beginning of the bit time
54More Encoding Schemes Manchester Differential Manchester 0 low-to-high transition1 high-to-low transition1 absence of transition at the beginning of the bit interval0 presence of transition at the beginning of the bit interval
56More Encoding Schemes Bipolar-AMI Pseudo-ternary zero represented by no line signalone represented by positive or negative pulseone pulses alternate in polarityPseudo-ternaryOne represented by absence of line signalZero represented by alternating positive and negativeNo advantage or disadvantage over bipolar-AMI
58Communication Network Example: The Public Telephone Network WAN
59WAN Communication Networks Example: Public Telephone Network (a). Fully-interconnected network.(b). Centralized switch.(c). Two-level hierarchy.
60Hierarchical Network Structure TandemCOTollCO = central officeTelephone subscribers connected to local CO (central office)Tandem & Toll switches connect CO’s
61Major Components of the Telephone System I. Local loopsAnalog twisted pairs going to houses and businessesII. TrunksDigital fiber optics connecting the switching officesIII. Switching officesWhere calls are moved from one trunk to another
62Structure of the Telephone System A typical circuit route for a medium-distance call.
64I. The Local Loop Transmission Problems: Modulation This is the connection from the local switching station to houses.This is ultimately what controls the transmission speed to houses.Transmission Problems:Attenuation - the loss of energy as the signal propagates.Delay Distortion - different frequencies travel at different speeds so the wave form spreads out.Noise - unwanted energy that combines with the signal - difficult to tell the signal from the noise.ModulationTo get around the problems associated with digital signaling, analog signaling is used. A continuous tone in the 1000 to 2000 Hz range, called a sine wave carrier is introduced. We vary the carrier to represent different signal (data).
68Modulation Techniques (a). A binary signal(b). Amplitude modulation(c). Frequency modulation(d). Phase modulation
69II. Trunks And Multiplexing The cost of a wire is pretty much constant, independent of the bandwidth of that wire - costs come from installation and maintenance of the physical space (digging, etc.). So, how can we stuff more through that medium? The answer is :Multiplexing(a)(b)AAATrunkgroupABBBMUXMUXBCCCC
70Time Division Multiplexing: TDMA Time sharing multiplexing4 usersExample:Frequencytime
71Example on TDMA TDMA: time division multiple access access to channel in "rounds"each station gets fixed length slot (length = pkt trans time) in each roundunused slots go idleexample: 6-station LAN, 1,3,4 have pkt, slots 2,5,6 idleTDM (Time Division Multiplexing): channel divided into N time slots, one per user; inefficient with low duty cycle users and at light load.FDM (Frequency Division Multiplexing): frequency subdivided.
72Multiplexing T1 streams into higher carriers. The T linksMultiplexing T1 streams into higher carriers.
73Frequency Division Multiplexing: FDMA Channel spectrum divided into frequency bands4 usersExample:frequencytime
74Example on FDMA each station assigned fixed frequency band unused transmission time in frequency bands go idleexample: 6-station LAN, 1,3,4 have pkt, frequency bands 2,5,6 idleTDM (Time Division Multiplexing): channel divided into N time slots, one per user; inefficient with low duty cycle users and at light load.FDM (Frequency Division Multiplexing): frequency subdivided.timefrequency bands5: DataLink Layer
75Example on FDMA (a) Individual signals occupy H Hz H f BAH(b) Combined signal fits into channel bandwidthACBf
76Example on FDMA (a). The original bandwidths. (b). The bandwidths raised in frequency.(b). The multiplexed channel.
77Wavelength Division Multiplexing (Used with Fiber)
78III. SwitchingThis is what happens inside the phone company - the various wires or fibers interconnect the switching centers. Methods of switching include:Circuit Switching: A connection (electrical, optical, radio) is established from the caller phone to the callee phone. This happens BEFORE any data is sent.Packet Switching: Divides the message up into blocks (packets). Therefore packets use the transmission lines for only a short time period - allows for interactive traffic.Message Switching: The connection is determined only when there is actual data (a message) ready to be sent. The whole message is re-collected at each switch and then forwarded on to the next switch. This method is called store-and-forward. This method may tie up routers for long periods of time - not good for interactive traffic.
79Fully Interconnected Network ( No Switching Case) For N users to be fully connected directlyRequires N(N – 1)/2 connectionsRequires too much space for cablesInefficient & costly since connections not always on1234N. . .N = 1000N(N – 1)/2 =
80Circuit Switching . Example: Telephones A connection (electrical, optical, radio) is establishedfrom the caller phone to the callee phone. Thishappens BEFORE any data is sent.fixed bandwidthroute fixed at setupidle capacity wasted. Example:Telephones
81Manual Circuit Switching Patchcord panel switch invented in 1877Operators connect users on demandEstablish circuit to allow electrical current to flow from inlet to outletOnly N connections required to central office1NN – 123
83Packet Switching . Divides the message up into blocks (packets). The connection is determined only when there is actual packet ready to be sent.The packet is re-collected at each switch and then forwarded on to the next switch.. Packets use the transmission lines for onlya short time period.. Example:Postal Service
84Circuit Switching vs. Packet Switching Dedicatedfixed bandwidthroute fixed at setupidle capacity wastednetwork stateBest Effortend-to-end controlmultiplexing techniquere-route capabilitycongestion problems
85Circuit Switching vs. Packet Switching (a). Circuit switching.(b). Packet switching.
87Message SwitchingThe connection is determined only when there is actual data (a message) ready to be sent.The whole message is re-collected at each switch and then forwarded on to the next switch.This method is called store-and-forward.This method may tie up routers for long periods of time - not good for interactive traffic.