# Module 3.0: Data Transmission

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Module 3.0: Data Transmission
Terminology and fundamental concepts Harmonics Bandwidth Transmission impairments Channel Capacity and Nyquist’s Theorem K. Salah

Terminology Medium Guided medium e.g. twisted pair, optical fiber
Unguided medium e.g. air, water, vacuum Simplex One direction e.g. Television Half duplex Either direction, but only one way at a time e.g. police radio Full duplex Both directions at the same time e.g. telephone K. Salah

Fundamental Concepts Time domain concepts Continuous signal (Analog)
Various in a smooth way over time e.g. sound, video Discrete signal (Digital) Maintains a constant level then changes to another constant level e.g., text, integers Periodic signal Pattern repeated over time Aperiodic signal Pattern not repeated over time K. Salah

Sine Wave (Harmonic) Components
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 K. Salah

Wavelength Length of wave or Distance occupied by one cycle
Meaning the distance a simple signal can travel in one period Distance between two points of corresponding phase in two consecutive cycles Assuming signal velocity v  = vT f = v c = 3*108 m/s (speed of light in free space) or  = 3*108 meters. K. Salah

Bandwidth Bandwidth (in analog communications)
The total capacity of communications channel measured in hertz(Hz). It is the difference between the highest and lowest frequencies capable of being carried over a channel. Any transmission system has a limited band of frequencies This limits the data rate that can be carried Bandwidth (in digital communications and networking) The theoretical capacity of communications channel expressed in bits per second (bps), which is called data rate. K. Salah

Propagation Delay Propagation time measures the time required for a signal (or a bit) to travel from one point of the transmission medium to anoterh. K. Salah

Analog and Digital Data Transmission
Signals Electric or electromagnetic or light representations of data. Means of propagating Data. Analog Signal Represent data with continuously varying sine wave. Continuously variable. Various media, e.g. wire, fiber optic, space Speech bandwidth 100Hz to 7kHz, Telephone bandwidth 300Hz to 3400Hz, Video bandwidth 4MHz Digital Signal Represent data with sequence of voltage pulses Use two DC components and vary it over time. Component of zero frequency. K. Salah

Digital Signal In RS-232, voltage that varies between -15 and -5 volts is interpreted as a binary 0, and voltage that varies between +5 and +15 volts is interpreted as a binary 1. Voltage that varies between -5 and +5 volts is ignored; that is, no interpretation is made. K. Salah

Analog Signal A single frequency with fixed A and  has 0 bandwidth. It carries no information. However, when it is combined with other frequencies or its components become variable, the bandwidth gets changed. Amplitude Change Frequency Change K. Salah

Analog Signal Phase Change K. Salah

Modem Modulation Techniques
Usually use digital signals for digital data and analog signals for analog data Can use analog signal to carry digital data Modem Digital signals have a wide spectrum and thus are subject to strong attenuation and delay distortion. DC is good for short distances. Analog signals are used instead with different modulation of components. K. Salah

Analog vs. Digital Transmission
Communication of data by propagation and processing of signals 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 Digital Transmission: Concerned with content Integrity endangered by noise, attenuation etc. Repeaters used Repeater receives signal extracts bit pattern retransmits Attenuation is overcome Noise is not amplified K. Salah

Transmission Impairments
Signal received may differ from signal transmitted Analog - degradation of signal quality Digital - bit errors Caused by Attenuation and attenuation distortion Delay distortion Noise K. Salah

Attenuation Attenuation: Signal strength falls off with distance
Depends on medium Received signal strength: must be enough to be detected must be sufficiently higher than noise to be received without error Attenuation is an increasing function of frequency K. Salah

Delay Distortion Delay Distortion: Only in guided media
Propagation velocity varies with frequency Called also “Intersymbol Interference”. Due to delay distortions, some of the signal components of one bit position will spill over into other bit positions. K. Salah