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Thursday, August 8, 2013

The Physical Layer and Data Communication Fundamentals PPT


The Physical Layer and Data Communication Fundamentals Presentation Transcript:
1.The Physical Layer and Data Communication Fundamentals

2.Transmission Terminology
Data transmission occurs between transmitter and receiver over some transmission medium.
Transmission media may be classified as guided or unguided. In both cases, communication is in the form of electromagnetic waves.
Guided media: the waves are guided along a physical path. e.g.
Twisted pair, coaxial cable, and optical fiber
Unguided media: provide a means for transmitting electromagnetic waves but do not guide them. e.g
Air, vacuum, and sea water

3.Direct link: transmission path between two devices in which signals propagate directly from transmitter to receiver with no intermediate devices, other than amplifiers or repeaters used to increase signal strength.
Point-to-point: first, it provides a direct link between two devices and second, those are the only two devices sharing the medium.

4.Multipoint guided configuration: more than two devices share the same medium.
Frequency-domain view of a signal is far more important to an understanding of data transmission than a time-domain view.

5.In transmitting data from a source to a destination, one must be concerned with the nature of the data, the actual physical means used to propagate the data, and what processing may be required.
Analog and digital terms are used frequently in data communications in at least three contexts

6.Data and Signaling


8.Fundamental Limits of Transmission Media
Background Noise
Crosstalk: a signal on one line is picked up by adjacent lines as a small noise signal. It caused when a strong transmitter output signal interferes with a much weaker incoming receiver signal
Impulse noise: caused by external activity or equipment which generates electrical impulses on the line which cause large signal distortion for their duration.
Thermal noise(white noise): caused by the thermal agitation of electrons associated with each atom in the device or transmission line material. It consists of random frequency components of continuously varying amplitude.

9.Fundamental Limits of Transmission Media

10.Shannon Channel Capacity
The maximum bit rate of a channel depends on the bandwidth of the channel (W) and the Signal to Noise Ratio (SNR)
C=W log2(1+SNR) bits/sec
SNR=Avg. Signal Power/Avg. Noise Power

11.Signal Attenuation is the phenomenon whereby the Amplitude of a signal decreases as it propagates along a transmission line.
Attenuation is a function of distance and frequency of signal
Repeaters are used to increase the power of the signal at appropriate intervals
Signal Distortion involves the Shape of the signal becoming altered as it propagates along the line.
One cause of distortion is the different attenuation rates for different frequency components of the signal.
This can be addressed using an equalizer

12.Delay distortion is caused by different frequency components of the signal propagating at slightly different speeds.
If the frequency components of one bit are delayed sufficiently they will overlap with the component of the next bit resulting in Inter Symbol Interference (ISI).

13.Two fundamentally different ways to produce digital signals
DC or lowpass where bits are represented using square waves.
Common encoding schemes include:
Non-return-to-zero (NRZ)
Bandpass or modulated where bits are represented using fixed frequency sinusoids. This is used when the channel does not pass low frequency signals.
Common modulation techniques include:
Amplitude shift keying (ASK)
Frequency shift keying (FSK)
Phase shift keying (PSK)

14.Encoding schemes: Polar Schemes
Polar encoding schemes rely on the voltage level to make a determination of whether a binary 1 or a 0 was sent.
Common Schemes:
Unipolar NRZ: binary 1: +A volts, binary 0: 0 volts.
Polar NRZ: binary 1: +A/2 volts, binary 0: –A/2 volts.
Half the power requirement of unipolar NRZ
Bipolar: Consecutive binary 1s: +/-A/2 volts, binary 0: 0 volts
Produces a frequency spectrum with less low frequency components.
Drawbacks to polar encoding schemes are:
Long strings of either 1s or 0s can cause loss of timing information
Systematic errors in polarity can cause all 1s to be read as 0s and vice versa

15.Manchester encoding: binary 1: transition from A/2 to –A/2 in middle of bit time interval, binary 0: transition from –A/2 to +A/2 in middle of bit time interval.
Differential Manchester encoding: There is a transition at the centre of each bit, but there is only a transition at the start of a 0 bit.
NRZ inverted (NZRI):starting at a fixed signal level, binary 1:denoted by a transition and binary 0: by no transition.
The maximum frequency of a NZRI signal is half that of Bipolar and Manchester encoded signals so it only requires half the transmission bandwidth
Use NRZI in Wide Area Networks (WANs) while the other methods are generally used in LANs.

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