IRAN UNIVERSITY OF SCIENCE AND TECHNOLOGY Computer department Chapter 3 Data Transmission Ahmad AKBARI Assistant Prof Spring 2002

Terminology (1) zTransmitter zReceiver zMedium yGuided medium xe.g. twisted pair, optical fiber yUnguided medium xe.g. air, water, vacuum

Terminology (2) zDirect link yNo intermediate devices zPoint-to-point yDirect link yOnly 2 devices share link zMulti-point yMore than two devices share the link

Terminology (3) zSimplex yOne direction xe.g. Television zHalf duplex yEither direction, but only one way at a time xe.g. police radio zFull duplex yBoth directions at the same time xe.g. telephone

Frequency, Spectrum and Bandwidth zTime domain concepts yContinuous signal xVarious in a smooth way over time yDiscrete signal xMaintains a constant level then changes to another constant level yPeriodic signal xPattern repeated over time yAperiodic signal xPattern not repeated over time

Continuous & Discrete Signals

Periodic Signals

Sine Wave zPeak Amplitude (A) ymaximum strength of signal yvolts zFrequency (f) yRate of change of signal yHertz (Hz) or cycles per second yPeriod = time for one repetition (T) yT = 1/f zPhase (  ) yRelative position in time

Varying Sine Waves

Wavelength zDistance occupied by one cycle zDistance between two points of corresponding phase in two consecutive cycles z zAssuming signal velocity v y = vT y f = v yc = 3*10 8 ms -1 (speed of light in free space)

Frequency Domain Concepts zSignal usually made up of many frequencies zComponents are sine waves zCan be shown (Fourier analysis) that any signal is made up of component sine waves zCan plot frequency domain functions

Addition of Frequency Components

Frequency Domain

Spectrum & Bandwidth zSpectrum yrange of frequencies contained in signal zAbsolute bandwidth ywidth of spectrum zEffective bandwidth yOften just bandwidth yNarrow band of frequencies containing most of the energy zDC Component yComponent of zero frequency

Signal with DC Component

Data Rate and Bandwidth zAny transmission system has a limited band of frequencies zThis limits the data rate that can be carried

Analog and Digital Data Transmission zData yEntities that convey meaning zSignals yElectric or electromagnetic representations of data zTransmission yCommunication of data by propagation and processing of signals

Data zAnalog yContinuous values within some interval ye.g. sound, video zDigital yDiscrete values ye.g. text, integers

Acoustic Spectrum (Analog)

Signals zMeans by which data are propagated zAnalog yContinuously variable yVarious media xwire, fiber optic, space ySpeech bandwidth 100Hz to 7kHz yTelephone bandwidth 300Hz to 3400Hz yVideo bandwidth 4MHz zDigital yUse two DC components

Data and Signals zUsually use digital signals for digital data and analog signals for analog data zCan use analog signal to carry digital data yModem zCan use digital signal to carry analog data yCompact Disc audio

Analog Signals Carrying Analog and Digital Data

Digital Signals Carrying Analog and Digital Data

Analog Transmission zAnalog signal transmitted without regard to content zMay be analog or digital data zAttenuated over distance zUse amplifiers to boost signal zAlso amplifies noise

Digital Transmission zConcerned with content zIntegrity endangered by noise, attenuation etc. zRepeaters used zRepeater receives signal zExtracts bit pattern zRetransmits zAttenuation is overcome zNoise is not amplified

Advantages of Digital Transmission zDigital technology yLow cost LSI/VLSI technology zData integrity yLonger distances over lower quality lines zCapacity utilization yHigh bandwidth links economical yHigh degree of multiplexing easier with digital techniques zSecurity & Privacy yEncryption zIntegration yCan treat analog and digital data similarly

Some definitions zSpectrum. Range of frequencies contained in a signal zAbsolute bandwidth. Width of the spectrum zEective bandwidth. Narrow band of frequencies containing most of the energy of the signal zDC component. Component of zero frequency; changes the average amplitude of the signal to non-zero

Relationship between data rate and bandwidth z Any transmitter/receiver system can accommodate only a limited range of frequencies z The range for fm radio transmission is mhz z This limits the data rate that can be carried over the transmission medium z Consider a sine wave of period f z Consider the positive pulse to be binary 1 and the negative pulse to be binary 0 z Add to it sine waves of period 3f, 5f, 7f, : : : z The resultant waveform starts to approximate a square wave

Relationship between data rate and bandwidth (continued) z This waveform has innite number of frequency components and innite bandwidth z Peak amplitude of kth frequency component is 1 zk so most of the energy is concentrated in the first few frequencies z Limiting the bandwidth to only the rst few frequencies gives a shape that is reasonably close to square wave

Transmission Impairments zSignal received may differ from signal transmitted zAnalog - degradation of signal quality zDigital - bit errors zCaused by yAttenuation and attenuation distortion yDelay distortion yNoise

Transmission Impairments z Attenuation z To reduce the amplitude of an electrical signal with little or no distortion zLogarithmic in nature for guided media; expressed as a constant number of decibels per unit distance z For unguided media, complex function of distance and atmospheric conditions

Transmission Impairments z Three considerations for transmission engineer z1. Received signal must have sufficient strength to enable detection z2. Signal must maintain a level sufficiently higher than noise to be received without error z3. Attenuation is an increasing function of frequency zSignal strength must be strong but not too strong to overload the circuitry of transmitter or receiver,which will cause distortion

Transmission Impairments z Beyond a certain distance, attenuation becomes large to require the use of repeaters or ampliers to boost the signal z Attenuation distorts the received signal, reducing intelligibility z Attenuation can be equalized over a band of frequencies z Use amplifiers than can amplify higher frequencies more than low frequencies

Attenuation zSignal strength falls off with distance zDepends on medium zReceived signal strength: ymust be enough to be detected ymust be sufficiently higher than noise to be received without error zAttenuation is an increasing function of frequency

Delay Distortion zOnly in guided media zPropagation velocity varies with frequency z Peculiar to guided transmission media z Caused by the fact that the velocity of signal propagation through a guided medium varies with frequency z In band-limited signal, velocity tends to be highest near the center frequency and falls towards the two edges of band

Delay distortion zVarying frequency components arrive at the receiver at different times, resulting in phase shifts between different frequencies zIn digital data transmission, some signal components of one bit position will spill over into other bit positions, causing inter symbol interference (ISI) z May be reduced by using equalization techniques

Noise (1) zAdditional signals inserted between transmitter and receiver zThermal yDue to thermal agitation of electrons yUniformly distributed yWhite noise zIntermodulation ySignals that are the sum and difference of original frequencies sharing a medium

Noise (2) zCrosstalk yA signal from one line is picked up by another zImpulse yIrregular pulses or spikes ye.g. External electromagnetic interference yShort duration yHigh amplitude

Channel Capacity zData rate yIn bits per second yRate at which data can be communicated zBandwidth yIn cycles per second of Hertz yConstrained by transmitter and medium y Bandwidth is proportional to cost y For digital data, we'll like to get as high a data rate as possible within a limit of error rate for a given bandwidth

Channel capacity zMaximum rate at which data can be transmitted over a communication path or channel Depends on four factors z1. Data rate in bps z2. Bandwidth constrained by transmitter and nature of transmission medium, expressed in cycles per second, z3. Noise, Average noise level over channel z4. Error rate

Nyquist bandwidth z Limitation on data rate for a noise free channel; equals that of channel bandwidth z If the rate of signal transmission is 2B, then a signal with frequencies no greater than B is sufficient to carry the signal rate z Given a bandwidth B, the highest possible signal rate is 2B z The above is true for signals with two voltage levels z With multilevel signaling, Nyquist formulation is C = 2B log 2 M z For a given bandwidth, data rate can be increased by increasing the number of different signal elements z Value of M is practically limited by noise and other impairments on transmission line

Required Reading zStallings chapter 3