1 3. Data Transmission

Prof. Sang-Jo Yoo 2 Contents  Concept and Terminology  Analog and Digital Data Transmission  Transmission Impairments  Asynchronous & Synchronous Transmission

Prof. Sang-Jo Yoo 3 Concept and Terminology  Transmission medium  Guided - twisted pair, coaxial cable, optical fiber  Unguided - air, vacuum, sea water  Direct link  the transmission path in which signals propagate directly from the transmitter to the receiver with no intermediate devices  Point-to-Point vs Multipoint transmission  Point-to-point - Only two devices share the medium  Multipoint - more than two devices share the same medium

Prof. Sang-Jo Yoo 4 Concept and Terminology  Communication Modes  Simplex  Data is transmitted in one direction only.  Ex) pager, TV  Half-duplex  Data is exchanged alternatively.  Ex) Walkie-Talkie  Duplex(or Full-duplex)  data is exchanged in both directions simultaneously  Ex) telephone AB AB Send and receive Only one way at a time AB Send receive simultaneously

Prof. Sang-Jo Yoo 5 Frequency, Spectrum and Bandwidth  Time domain concepts  Continuous signal  Various in a smooth way over time  Discrete signal  Maintains a constant level then changes to another constant level  Periodic signal  Pattern repeated over time  Aperiodic signal  Pattern not repeated over time

Prof. Sang-Jo Yoo 6 Continuous & Discrete Signals

Prof. Sang-Jo Yoo 7 Periodic Signals

Prof. Sang-Jo Yoo 8 Sine Wave  Peak Amplitude (A)  maximum strength of signal  volts  Frequency (f)  Rate of change of signal  Hertz (Hz) or cycles per second  Period = time for one repetition (T)  T = 1/f  Phase (  )  Relative position in time

Prof. Sang-Jo Yoo 9 Varying Sine Waves

Prof. Sang-Jo Yoo 10 Wavelength  Distance occupied by one cycle  Distance between two points of corresponding phase in two consecutive cycles   Assuming signal velocity v  = vT  f = v  c = 3*10 8 ms -1 (speed of light in free space)

Prof. Sang-Jo Yoo 11 Frequency Domain Concepts  Signal usually made up of many frequencies  Components are sine waves  Can be shown (Fourier analysis) that any signal is made up of component sine waves  Can plot frequency domain functions

Prof. Sang-Jo Yoo 12 All of the frequency components are integer multiple of one frequency (fundamental frequency) The period of the total signal is equal to he period of the fundamental frequency.

Prof. Sang-Jo Yoo 13 Frequency Domain X/2 -X/2 1.0

Prof. Sang-Jo Yoo 14 Spectrum & Bandwidth  Spectrum  range of frequencies contained in signal  Absolute bandwidth  width of spectrum  Effective bandwidth  Often just bandwidth  Narrow band of frequencies containing most of the energy  DC Component  Component of zero frequency

Prof. Sang-Jo Yoo 15 Signal with DC Component

Prof. Sang-Jo Yoo 16 Data Rate and Bandwidth  Signal bandwidth is limited by the channel bandwidth  Effect of channel bandwidth on a digital signal

Prof. Sang-Jo Yoo 17 Analog and Digital Data Transmission  The terms analog and digital are used in three contexts  Data  entities that convey meaning  Signal  electric or electromagnetic encoding of data  Transmission  the communication of data by the propagation and processing of signals

Prof. Sang-Jo Yoo 18 Analog and Digital Transmission  Data  Analog Data take on continuous values  Ex) acoustic data, video data  Digital Data take on discrete values  Ex) text or character string  Signal  Analog Signal is a continuously varying electromagnetic wave  Ex) acoustic signal, video signal  Digital Signal is a sequence of voltage pulses  Ex) horizontal sync/blanking pulse in video signal, binary digital signal

Prof. Sang-Jo Yoo 19 Analog Signals Carrying Analog and Digital Data

Prof. Sang-Jo Yoo 20 Digital Signals Carrying Analog and Digital Data

Prof. Sang-Jo Yoo 21 Analog and Digital Transmission  Analog and Digital Data vs Signal

Prof. Sang-Jo Yoo 22 Analog and Digital Transmission  Analog and Digital Signal vs Transmission

Prof. Sang-Jo Yoo 23 Analog Transmission  Analog signal transmitted without regard to content  May be analog or digital data  Attenuated over distance  Use amplifiers to boost signal  Also amplifies noise

Prof. Sang-Jo Yoo 24 Digital Transmission  Concerned with content  Repeaters used  Repeater receives signal  Extracts bit pattern  Retransmits  Attenuation is overcome  Noise is not amplified

Prof. Sang-Jo Yoo 25 Advantages of Digital Transmission  Digital technology  Low cost LSI/VLSI technology  Data integrity  Longer distances over lower quality lines  Capacity utilization  High bandwidth links economical  High degree of multiplexing easier with digital techniques  Security & Privacy  Encryption  Integration  Can treat analog and digital data similarly

Prof. Sang-Jo Yoo 26 Transmission Impairments  Generally, the received signal differs from the transmitted signal due to various transmission impairments  Kinds of impairments  Attenuation and attenuation distortion  Delay distortion  Noise

Prof. Sang-Jo Yoo 27 Attenuation  The strength of a signal falls off with distance and this reduction is generally logarithmic.  Attenuation introduces three considerations for transmission  a received signal must have sufficient strength so that the receiver can detect and interpret the signal  the signal must maintain a level sufficiently higher than noise  Attenuation is an increasing function of frequency

Prof. Sang-Jo Yoo 28 Delay Distortion  It is caused by the fact that the velocity of propagation of a signal through a guided medium varies with frequency.  Various frequency components of a signal will arrive at the receiver at different times.  Delay distortion particularly corrupts digital signal by causing intersymbol interference.

Prof. Sang-Jo Yoo 29 Noise  Four categories of noise  Thermal noise  caused by thermal agitation of electrons in a conductor  cannot be eliminated  Intermodulation noise  caused by nonlinearity in the transmitter, receiver, or by intervening transmission system  Crosstalk  caused by coupling between signal paths  Impulse noise  generated from various sources including external electromagnetic disturbances, such as lightning and faults and flaws in the communications system.

Prof. Sang-Jo Yoo 30 Noise  Effect of noise on a digital signal

Prof. Sang-Jo Yoo 31 Channel Capacity  Various impairments limit maximum data rate.  Channel capacity - the rate at which data can be transmitted over a given communication path under given condition  Shannon’s formula, where C : the capacity of the channel (bps) W : the bandwidth of the channel (Hz) S : the signal power N : the noise power