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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


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



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?


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


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?


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



In unipolar encoding, we use only one voltage level.

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


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



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)


In Manchester and differential Manchester encoding, the transition

at the middle of the bit is used for synchronization.



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

Chapter 4 (B. A Forouzan)

Section 4.1

assignment 1
  • Draw the graphs for all encoding techniques learnt in this lecture, using each of following data stream.





  • 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?