1 computer communication & networks lecture 6 physical layer: digital transmission waleed ejaz...
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Computer Communication & Networks
Lecture 6
Physical Layer: Digital Transmission
http://web.uettaxila.edu.pk/CMS/coeCCNbsSp09/index.asp
Waleed [email protected]
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Physical Layer
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Physical Layer Topics to CoverSignals
Digital Transmission
Analog Transmission
Multiplexing
Transmission Media
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Digital to Digital Conversion
The conversion involves three techniques: line line codingcoding, block codingblock coding, and scramblingscrambling. . Line coding is always needed; block coding and scrambling may or may not be needed.
Line CodingLine Coding Line Coding SchemesLine Coding Schemes Block CodingBlock Coding ScramblingScrambling
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Line Coding & Decoding
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Signal Levels (Elements) Vs Data Levels (Elements)
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Pulse Rate Vs Bit Rate
ExampleExample
A signal has two data levels with a pulse duration of 1 ms. We calculate the pulse rate and bit rate as follows:
Pulse Rate = 1/ 10Pulse Rate = 1/ 10-3-3= 1000 pulses/s= 1000 pulses/s
Bit Rate = Pulse Rate x logBit Rate = Pulse Rate x log22 L = 1000 x log L = 1000 x log22 2 = 1000 bps 2 = 1000 bps
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DC Component
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Lack of Synchronization
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Example 3Example 3
In a digital transmission, the receiver clock is 0.1 percent faster than the sender clock. How many extra bits per second does the receiver receive if the data rate is 1 Kbps? How many if the data rate is 1 Mbps?
SolutionSolution
At 1 Kbps:1000 bits sent 1001 bits received1 extra bpsAt 1 Mbps: 1,000,000 bits sent 1,001,000 bits received1000 extra bps
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Line Coding Schemes
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In unipolar encoding, we use only one voltage level.
Note
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Unipolar Encoding
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In polar encoding, we use two voltage levels: positive & negative
Note
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Polar: NRZ-L and NRZ-I Encoding
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In NRZ-L the level of the voltage determines the value of the bit.
In NRZ-I the inversion or the lack of inversion
determines the value of the bit.
Note
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Polar: RZ Encoding
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Polar: Manchester Encoding
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Polar: Differential Manchester Encoding
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In Manchester and differential Manchester encoding, the transition
at the middle of the bit is used for synchronization.
Note
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In bipolar encoding, we use three levels: positive, zero, and negative.
Note
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Bipolar: AMI (Alternative Mark Inversion) Encoding
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Summary
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Sampling
Pulse Code ModulationPulse Code ModulationSampling Rate: Nyquist TheoremSampling Rate: Nyquist Theorem
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PCM
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Quantization & Encoding Samples
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According to the Nyquist theorem, the sampling rate must be
at least 2 times the highest frequency contained in the signal.
Note
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Transmission Modes
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Transmission Modes
The transmission of binary data across a link can be accomplished in either parallel or serial mode. In parallel mode, multiple bits are sent with each clock tick. In serial mode, 1 bit is sent with each clock tick. While there is only one way to send parallel data, there are two subclasses of serial transmission: asynchronous, synchronous.
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Parallel Transmission
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Serial Transmission
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In asynchronous transmission, we send 1 start bit (0) at the beginning and 1 or more stop bits (1s) at the end of each
byte. There may be a gap between each byte.
Note
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Asynchronous here means “asynchronous at the byte level,”
but the bits are still synchronized; their durations are the same.
Note
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Asynchronous Transmission
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In synchronous transmission, we send bits one after another without start or
stop bits or gaps. It is the responsibility of the receiver to group the bits.
Note
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Synchronous Transmission
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Readings
Chapter 4 (B.A Forouzan) Section 4.1, 4.2, 4.3
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