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Information, Data and Signals. Data - A representation of facts, concepts, or instructions in a formalized manner suitable for communication, interpretation, or processing by human beings or by automatic means
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Information, Data and Signals • Data - A representation of facts, concepts, or instructions in a formalized manner suitable for communication, interpretation, or processing by human beings or by automatic means • Information - The meaning that is currently assigned to data by means of the conventions applied to those data
Signal = Function of Time • The signal is a function of time. Horizontal axis represents time and the vertical axis represents the voltage level. • Signal represents data OR Data is encoded by means of a signal • Signal is what travels on a communication medium • An understanding of signals is required so that suitable signal may be chosen to represent data
Periodic Signals • Some signals repeat themselves over fixed intervals of time. Such signals are said to be periodic • A signal s(t) is periodic if and only if: s(t+T) = s(t) -∞ < t < +∞ where the constant T is the periodic of the signal, otherwise a signal is aperiodic (or non- periodic).
Periodic Signal Properties • Three important characteristics of a periodic signal are : • Amplitude (A): the instantaneous value of a signal at any time measured in volts. • Frequency (f): the number of repetitions of the period per second or the inverse of the period; it is expressed in cycles per second or Hertz (Hz). T=1/ f • Phase (φ): a measure of the relative position in time within a single period of a signal, measured in degrees • Wavelength (λ): distance occupied by a signal in one period
Fourier Transform • Even non-periodic signals can be represented in the frequency domain by using the Fourier transform • The Fourier transform of a periodic signal gives us the frequency-domain representation of a signal • Frequency domain is useful because it provides us another view of the signal
Frequency Domain Concepts • Spectrum: range of frequencies contained in a signal (is a plot the amplitude of the individual frequencies in a signal on a frequency-domain plot) • Bandwidth: Width of a spectrum (normally measured in Hertz) BW = fmax - fmin • Fundamental Frequency: When all frequency components of a signal are integer multiples of one frequency, the fundamental frequency. The other frequencies are harmonics
Spectrum of a Digital Pulse • Consider the case binary data is encoded into digital signal, and to be transmitted by a transmission medium • Digital signal contains an infinite bandwidth, but a real transmission medium has a finite bandwidth, which can limit the data rate that can be carried on the transmission medium • Limited bandwidth creates distortions of the input signal, which makes the task of interpreting the received signal more difficult • The more limited bandwidth, the greater the distortion, and the greater the potential for error by the receiver • The high the data rate of a signal, the greater is its effective bandwidth • The grater the bandwidth of a transmission system, the higher is the data rate that can be transmitted
General Observations aboutSignals • For a signal with multiple frequencies, energy is in the first few frequency components • Increasing the bandwidth increases data rate • The transmission medium limits the bandwidth • Greater the bandwidth, the greater the cost • For a given data rate, limiting the bandwidth, increases distortion, and hence the error rate
Broadband Signals • A broadband transmission is an analog communication strategy in which multiple communication channels are used simultaneously. The data in a broadband transmissions modulated into frequency bands, or channels, and is transmitted in these channels. • Guards bands which are small bands of unused frequencies, are allocated between data channels. These provide a buffer against interference due to signals from one channel drifting into a neighboring one.
Baseband Signals • A baseband connection is one that uses digital signals, which are sent over wires without modulation. Binary values are sent directly as pulses of different voltages levels rather than being transmitted with a carrier signal. • It is possible to transmit multiple signals using multiplexing scheme.
Transmission Impairments • Attenuation • Delay • Noise
Attenuation • Loss of signal strength over distance. The strength of a signal falls off with distance. • Use of amplifiers to boost analog signals; entire signal (including noise or distortion) is amplified • Use of repeaters for digital data; data recovered and then transmitted
Attenuation Distortion • Attenuation Distortion – attenuation varies as a function of frequency • Analog signal is made up of several frequencies • Attenuation is different for different frequencies; Different losses at different frequencies • More of a problem for analog signals than digital
Attenuation • Signal strength falls off with distance
dB Calculation • Example • Input power is 1 Watt • Output power is 1 mW • dB Attenuation is 10 x log (1 W/1 mW) = 10 x (3) = 30 dB
Delay Distortion • Only in guided media • The velocity of propagation of a signal through a guided medium varies with frequency. • Different frequency components travel at different speeds therefore arrive at a destination at different times. • Particularly critical for digital data because bits may spill over into other bit position causing Inter-Symbol Interference.
Noise • What is Noise? Any unwanted signal • Types of Noise - Thermal - Intermodulation - Impulse - Crosstalk
Thermal Noise • Thermal noise, white noise - Due to random motion of atoms N = kT (W/Hz) k = Boltzman Constant (1.381 X 10-23 J/K) T = Absolute Temperature (Kelvin) W = Bandwidth (Hz) • At room temperature, T = 17 oC, or 290 K, and the thermal noise power density is N = 1.3803 x 10-23 x 290 = 4x10-21W/Hz • Thermal noise in a bandwidth of B Hz can be expressed as N = kTB
Inter-modulation Noise • Inter-modulation noise When two signals at different frequencies are mixed in the same medium, sum or difference of original frequencies or multiples of those frequencies can be produced, which can interfere with the intended signal. – Occurs when there is some non-linearity in the system
Noise • Crosstalk When there is an unwanted coupling between signal paths. For example some times talking on the telephone you can hear another conversation. • Impulse noise • Due to lightning or some other random transient phenomenon (Sudden bursts of irregular pulses or spikes, may caused by atmospheric noises)
