# Transmission Impairments-Data Communication-Lecture Notes, Study notes for Data Communication Systems and Computer Networks

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Data Communication is exchange of data between two devices. In computers data exchange is in form of 0 and 1. This course discuss how computer communicate, what is medium and what are expenses. This handout includes: Tra...
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LECTURE #1

LECTURE #28

Transmission Impairments TX Media are not perfect Imperfections cause impairments in the signal through the medium This means that the signal at the beginning and the end of the medium are not the same What is sent is not what is received

 o o

o o o l to

  Measures the relative strength of the two signals or a signal at 2

al is attenuated  dB is positive if a signal is Amplified

Relative strengths of two signals or a signal at two points = 10 log10 (P2/P1)

Negative dB means attenuation ation

Attenuation Loss of Energy When a signal travels through a medium, it looses some of its energy in order to overcome the resistance of the medium That is why wire carrying electric signals get hot Some of the electrical energy in the signal is converted to heat To compensate for this loss Amplifiers are used to amplify the signa heat

Decibel (dB)

different points  dB is negative if a sign

dB P2 and P1 are signal powers

Positive dB means amplific

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Examp Ima wer is reduced to half. This m Sol

le 7.1 gine a signal travels through a transmission medium and its po

eans P2=(1/2)P1. Calculate Attenuation? ution:

–10log

 o o o o o

motion of electrons in a wire that creates an extra signal not

o

) that comes from

10(P / P )= 10log (0.5 P / P )2 1 10 1 1 =10(-0.3)= -3 dB

Distortion Distortion means that the signal changes its form or shape Distortion occurs in a composite signal Signal changes its form or shape Occurs in a composite signal, made of different frequencies Each signal component has its own speed

 NOISE o Thermal Noise

–Due to random originally sent by TX

Induced Noise –Comes from sources like Motors and Appliances

o Crosstalk: –Effect of one wire on another

o Impulse Noise –Spike (A signal with high energy in a very short period of time power lines, lightening etc,.

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 Thr c



als depend on the medium and the

 o nt of

o

 WAVELENGTH o Another characteristic of a signal traveling through the TX Medium o This concept binds the frequency of the signal to the propagation speed of

medium o It is customary to talk about wavelength when talking about TX of light in

Optical Fiber

Performance of Medium ee oncepts are used to measure the performance of TX Media:

–Throughput –Propagation Speed –Propagation Time

Throughput o Measurement of how fast data can pass through a point o In other words, if we consider any point in the TX Medium as a wall through

which bits pass, then throughput is the number of bits that can pass this wall in second

Propagation Speed o Propagation speed measures the distance a signal or a bit can travel through a



medium in one second ation speed of EM signo The propag

frequency of the signal

Propagation Time Measures the time required for a signal (or a bit) to travel from one poi the TX medium to another The propagation time is calculated as:

Propagation time = peedopagationS ceDis

Pr tan

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o The wavelength is the distance a simple signal can travel in one period o The wavelength depends on both the frequency and the medium

–Wavelength = Propagation speed * period –Wavelength = Propagation speed / frequency

 Shannon Capacity In 1944, Claude Shannon introduced a formula to determine the theoretical highest data rate for a channel:

C = B log2 (1 + S/N) in bps –B: bandwidth of the channel –S/N: signal to noise ratio

Examples

 Extremely noisy channel

– S/N  0 – C = B log2 (1+0) = 0

 Telephone line – Bandwidth is 3000 Hz, S/N ratio is 3162 (35 dB) – C = 3000 log2 (1+3162) = 34,860 bps

Media Comparison When evaluating the suitability of a particular medium to a specific application, 5 factors should be kept in mind:

COST: This is the cost of materials plus installation SPEED: Speed is the max no. of bits per second that a medium can transmit reliably

–Speed varies with frequency, with physical size of medium and Transmission Equipment

ATTENUATION: Tendency of EM signal to become weak or distorted over signal EM Interference: EMI is the susceptibility of the medium to external EM energy introduced into the link that interferes with the intelligibility of a signal

–Familiar effects of EMI are static(audio) and snow (visual)

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SECURITY: How easy it is for an unauthorized device to listen on the link? –Twisted pair is intercept able –Optical Fiber is more secure

Summary  Transmission Impairments  Performance of Transmission Medium  Wavelength  Shannon Capacity  Media Comparison

Reading Sections  Section 7.4, 7.5, 7.6. 7.7, “Data Communications and Networking” 4th Edition by

Behrouz A. Forouzan

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