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Data Communication. Lecture # 05 Course Instructor: Engr. Sana Ziafat. Digital Transmission. Digital to Digital Conversion.

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Data communication

Data Communication

Lecture # 05

Course Instructor:

Engr. Sana Ziafat



Digital to digital conversion
Digital to Digital Conversion

  • The conversion involves three techniques:line coding,block coding, andscrambling. Line coding is always needed; block coding and scrambling may or may not be needed.



Signal levels elements vs data levels elements
Signal Levels (Elements) Vs Data Levels (Elements)

  • Data element is defined as smallest entity to represent piece of information

  • Where as signal element is the shortest unit of digital signal.

  • Data element is actually what we need to end and signal element is what we can send




Data rate versus signal rate
Data rate versus signal rate

  • Data rate:

    -Number of data elements sent in one second

    -The unit is bits per second(bps)

  • Signal Rate:

    -Number of signals elements sent in one second.

    -The unit is baud

  • Increase the data rate while decreasing the signal rate.

  • Increasing the data rate increase the speed of data transmission

  • Decreasing the signal rate decrease the bandwidth requirement.


Data rate vs signal rate
Data Rate vs. Signal Rate

  • Increasing the data rate increases the speed of transmission where as increasing the signal rate increases the bandwidth requirement

  • HOW???



Pulse rate vs bit rate
Pulse Rate Vs Bit Rate

Example

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/ 10-3= 1000 pulses/s

Bit Rate = Pulse Rate x log2 L = 1000 x log2 2 = 1000 bps


Example

A signal is carrying data in which one data element is encoded as one signal element ( r = 1). If the bit rate is 100 kbps, what is the average value of the baud rate if c is between 0 and 1?

Solution

We assume that the average value of c is 1/2 . The baud rate is then


Important characteristics of line coding
Important Characteristics of line coding

  • No of signal levels

  • Bit rate verses baud rate

  • DC components

  • Noise immunity

  • Error detection

  • Synchronization

  • Cost of implementation




Example

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?

Solution

At 1 Kbps:

1000 bits sent 1001 bits received1 extra bps

At 1 Mbps:

1,000,000 bits sent 1,001,000 bits received1000 extra bps



Note

In unipolar encoding, we use only one voltage level.


Unipolar nrz encoding
UnipolarNRZ Encoding


Characteristics of unipolar signal
Characteristics of Unipolar Signal

  • It uses only one polarity of voltage level

  • Bit rate same as data rate

  • Dc component present

  • Lack of synchronization for long sequence of 0’s & 1’s.

  • Simple but obsolete


Note

In polar encoding, we use two voltage levels: positive & negative



Note

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.

Both have average signal rate of N/2 baud.


Problems associated with nrz l nrz i
Problems Associated with NRZ-L & NRZ-I

  • Synchronization problem

  • Dc component problem

  • Sudden change of polarity in system (for case of NRZ-L)





Note

In Manchester and differential Manchester encoding, the transition

at the middle of the bit is used for synchronization.


Note

In bipolar encoding, we use three levels: positive, zero, and negative.


Bipolar schemes ami and pseudo ternary
Bipolar schemes: AMI and pseudo ternary

  • AMI: Alternate Mark Inversion

    • It uses three voltage levels

    • Unlike RZ 0 level is used to represent a 0

    • Binary 1’s are represented by alternating positive and negative.

  • Pseudoternary

    • variation of AMI encoding in which 1 bit is encoded as zero voltage level and 0 bit is encoded as alternating positive and negative voltages



Readings
Readings

Chapter 4 (B. A Forouzan)

Section 4.1



Assignment 1
Assignment#1

  • Draw the graphs for all encoding techniques learnt in this lecture, using each of following data stream.

    a.11111111

    b.00000000

    c.00110011

    d.01010101

  • What is the Nyquist sampling rate for each of the following signals?

    a. A low pass signal with bandwidth of 300 KHz?

    b. A band pass signal with bandwidth of 300KHz if the lowest frequency is 100 KHz?


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