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Computer Communication & Networks

Lecture 6

Physical Layer: Digital Transmission

http://web.uettaxila.edu.pk/CMS/coeCCNbsSp09/index.asp

Waleed Ejazwaleed.ejaz@uettaxila.edu.pk

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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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