Data Communication (ch3)

transmission

communication of data by propagation and processing of signals

DC component

component of zero frequency

types of impairments

attenuation, delay distortion

full duplex

both directions at the same time. both transmit simultaneously. e.g., telephone

analog

continuous values within some interval

bandwidth of the signal

cycles per second or Hertz

analog and digital signals

data propagated by means of electromagnetic signals Analog: - continuously variable electromagnetic wave - various media (wire, fibre optic, space) - speech bandwidth 100Hz to 7kHz - telephone bandwidth 300Hz to 3400Hz - video bandwidth 4MHz Digital: - sequence of voltage pulses. Use two DC components

digital

discrete values

wavelength

distance occupied by one cycle OR distance between two points of corresponding phase in two consecutive cycles

half duplex

either direction, but only one way at a time. both stations may transmit, but one at a time. e.g., police radio

signals

electric or electromagnetic representations of data

data

entities that convey meaning or information

Channel Capacity in Nyquist Theorem (noiseless channel)

essentially, double the bandwidth: C = 2Blog2M

medium

guided medium e.g., twisted pair, coaxial cables, optical fiber unguided medium or wireless e.g., air, water, vacuum

data rate & bandwidth

- any transmission system has a limited band of frequencies - this limits the data rate that can be carried - higher data rate means higher bandwidth and vice versa

three causes of impairments

- attenuation - delay distortion - noise

noise definition

addition signals inserted between transmitter and receiver

crosstalk

- a signal from one line is picked up by another - it can occur by electrical coupling between nearby twisted paris or, rarely, coax cables carrying multiple signals

analog transmission

- analog signal transmitted without regard to content - signals may represent analog or digital data - attenuated over distance - use amplifiers to boost signal - also amplifies noise

notes about signals

- change in a short span of time means high frequency - change over a long span of time means low frequency - if a signal does not change at all, its frequency is zero - if a signal changes instantaneously, its frequency is infinite - phase describes the position of the waveform relative to time 0

advantages & disadvantages of digital signal

- cheaper than analog - less susceptible to noise interference - greater attenuation - pulses become rounded and smaller - leads to loss of information

digital transmission

- concerned with content - integrity endangered by noise, attenuation, etc - repeaters used for greater distance - repeater - receives digital signal - extracts bit pattern - retransmits - attenuation is overcame - noise is not amplified

attenuation in detail

- depends on medium -> un/guided - more complex in unguided medium - measure in decibels/unit distance - 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

advantages of digital transmission

- digital technology - LSI/VLSI -> low cost and size of circuits - data integrity - repeaters rather than amplifiers -> 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

point-to-point

- direct link - only 2 devices share the link

thermal noise(white noise)

- due to moving of electrons in a conductor - uniformly distributed across the frequency spectrum - it's function of temperature - can not be eliminated and therefore put an upper bound on communications system performance

Channel Capacity in Shannon Capacity (Noisy Channel)

- faster data rate shortens each bit so burst of noise affects more bits - at given noise level, high data rate means higher error rate - consider white noise only: SNRdb = 10 log10(signal/noise) Capacity C = B log2(1+SNR) - this is an error free capacity

impulse noise

- irregular pulses or spikes - e.g. electromagnetic disturbances, such as lightening - short duration with relatively high amplitude

channel capacity definition

- max possible data rate on communication channel

direct link

- no intermediate devices - signal strength using repeaters or amplifiers - this term can be applied to un/guided signals

delay distortion

- occurs only in guided media - because propagation velocity varies with frequency - for band limited signal, the velocity tends to be higher near the centre frequency and fall off toward the two edges of the band - a phase shift between different frequencies because, various frequency components arrive at different times

effective bandwidth

- often just bandwidth - narrow band of frequencies containing most of the energy

frequency domain concepts

- signal usually made up of many frequencies - any signal is made up of components at various frequencies, each is a sine wave - can be shown (Fourier analysis) that any signal is made up of component sine waves - can plot frequency domain functions

noise types

- thermal noise(white noise) - intermodulation noise - crosstalk - impulse noise

Decibel definition

A measure of the ration between two different power levels

data rate

Bits per second (bps)

attenuation definition

loss of a signal strength over long distance

peak amplitutde

maximum value or strength f signal over time measured in volts

multi-point

more than two devices share the link

simplex

one direction. one station transmit and the other receive. e.g., television

spectrum

range of frequencies contained in signal

frequency

rate of change of signal repeats; or the number of periods in one second - Hertz (Hz) or cycles per second period = time required for one repetition (T), T = 1 / f

digital data

refers to information that has discrete states. it takes on discrete values

analog data

refers to information that is continuous. it takes on continuous values

phase

relative position in time within a single period of a signal

sine wave function

s(t) = A sin(2 pi f t + phase)

intermodulation

signals that are the sum and difference of original frequencies sharing a medium

sine wave

simple and composite sine wave represented by three parameters: - Peak Amplitude (A) - Frequency (f) - Phase (/0)

direct relationship between data rate and bandwidth

the greater the bandwidth of a transmission system, the higher is the data rate that can be transmitted

data transmission successful factors

the quality of signal being transmitted the characteristic of the transmission medium

Signal to Noise Ratio (SNR)

the ratio of the power in a signal to the power contained in the noise. this expresses the amount, in decibels, that the signal exceeds the noise level - high S/N ratio means high quality signal - therefore, less repeaters

forms of noise

thermal noise, inter-modulation, cross talk, impulse noise

codec

translate analog signals into digital signals

absolute bandwidth

width of spectrum