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Chapter 4

DigitalTransmission

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Example 1Example 1

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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Example 2Example 2

A signal has four data levels with a pulse duration of 1 ms. We calculate the pulse rate and bit rate as follows:

Pulse Rate = = 1000 pulses/sPulse Rate = = 1000 pulses/s

Bit Rate = PulseRate x logBit Rate = PulseRate x log22 L = 1000 x log L = 1000 x log22 4 = 2000 bps 4 = 2000 bps

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Figure 4.3 DC component

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Figure 4.4 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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Figure 4.4 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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Figure 4.5 Line coding schemes

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Unipolar encoding uses only one voltage level.

Note:Note:

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Figure 4.6 Unipolar encoding

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Polar encoding uses two voltage levels Polar encoding uses two voltage levels (positive and negative).(positive and negative).

Note:Note:

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Figure 4.7 Types of polar encoding

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In NRZ-L the level of the signal is In NRZ-L the level of the signal is dependent upon the state of the bit.dependent upon the state of the bit.

Note:Note:

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In NRZ-I the signal is inverted if a 1 is In NRZ-I the signal is inverted if a 1 is encountered.encountered.

Note:Note:

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Figure 4.8 NRZ-L and NRZ-I encoding

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Figure 4.9 RZ encoding

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A good encoded digital signal must A good encoded digital signal must contain a provision for contain a provision for

synchronization.synchronization.

Note:Note:

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Figure 4.10 Manchester encoding

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In Manchester encoding, the In Manchester encoding, the transition at the middle of the bit is transition at the middle of the bit is

used for both synchronization and bit used for both synchronization and bit representation.representation.

Note:Note:

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Figure 4.11 Differential Manchester encoding

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In differential Manchester encoding, In differential Manchester encoding, the transition at the middle of the bit is the transition at the middle of the bit is

used only for synchronization. used only for synchronization. The bit representation is defined by the The bit representation is defined by the

inversion or noninversion at the inversion or noninversion at the beginning of the bit.beginning of the bit.

Note:Note:

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In bipolar encoding, we use three In bipolar encoding, we use three levels: positive, zero, levels: positive, zero,

and negative.and negative.

Note:Note:

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Figure 4.12 Bipolar AMI encoding

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Figure 4.31 Data transmission and modes

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Figure 4.32 Parallel transmission

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Figure 4.33 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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Figure 4.34 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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Figure 4.35 Synchronous transmission

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Exempel på asynkron serie-kommunikation: RS232C (”com-

porten”)