DATA ENCODING SIMULATION

SUBMITTED BY Shashikala.N

CONTENTS1.What is data encoding?2.Why data encoding?3.When data encoding required?4.Advantages of data encoding 5.Data encoding methods6.Unipolar NRZ7.Unipolar RZ8.Polar NRZ

9.AMI NRZ10.AMI RZ11.Manchester Encoding Reference

CONTENTS

What Is Data Encoding?

Encoding is the process of transforming information from one format into another. Data compression and data encryption are both forms of data encoding . Both analog and digital data can be encoded as either analog or digital.

Why Data Encoding?

An increase in data rate increases bit error rate.

An increase S/N decrease bit error rate.

An increase in bandwidth allows an increase in data rate.

When Data Encoding is Required?

It required in the physical layer of OSI reference model in network .

Physical Layer Interfaces network devices with transmission medium and provides the hardware a means of sending and receiving data. Responsible for data coding (converting 0s and 1s into electrical or light pulses). Defines physical and electrical specifications for transmission.

Advantages Of Data Encoding

Encoding is done to reduce the number of bit to be transmitted and save bandwidth . Less storage space required Comparisons are shortened and can therefore be made quicker, thus speeding up searches A limited number of codes makes data input faster and simplifies validation.

Data Encoding Methods

Data encoding deals with how best to transmit data (0s and 1s)across various media.

Popular Data Encoding Methods Unipolar NRZ (Non-Return to Zero) Unipolar RZ Manchester Encoding NRZ AMI NRZ AMI RZ Polar NRZ Polar RZ

1 .Unipolar

Unipolar signalling (also called on-off keying, OOK) is the type of line coding in which one binary symbol (representing a 0 for example) is represented by the absence of a pulse (i.e. a SPACE) and the other binary symbol (denoting a 1) is represented by the presence of a pulse (i.e. a MARK).

1.Unipolar Non-Return to Zero (NRZ)

In unipolar NRZ the duration of the MARK pulse (Ƭ ) is equal to the duration

(To) of the symbol slot.

1 0 1 0 1 1 1 1 1 0

V

0

Advantage and disadvantage of Unipolar NRZ

Unipolar Non-Return to Zero (NRZ):

In unipolar NRZ the duration of the MARK pulse (Ƭ ) is equal to the duration (To) of the symbol slot. (put figure here).

Advantages:– Simplicity in implementation.– Doesn’t require a lot of bandwidth for transmission.

Disadvantages:– Presence of DC level (indicated by spectral line at 0 Hz).– Contains low frequency components. Causes “Signal Droop” (explained later).– Does not have any error correction capability.– Does not posses any clocking component for ease of synchronisation.– Is not Transparent. Long string of zeros causes loss of synchronisation.

Unipolar Signalling

Figure. PSD of Unipolar NRZ

2.Unipolar Return to Zero (RZ)

In unipolar RZ the duration of the MARK pulse (Ƭ ) is less than the duration (To) of the symbol slot. Typically RZ pulses fill only the first half of the time slot, returning to zero for the second half.

1 0 1 0 1 1 1 0 0 0

V

0

To

Ƭ

Advantage and disadvantage of Unipolar

RZUnipolar Return to Zero (RZ):

Advantages:– Simplicity in implementation.– Presence of a spectral line at symbol rate which can be used as symbol timing clock signal.

Disadvantages:– Presence of DC level (indicated by spectral line at 0 Hz).– Continuous part is non-zero at 0 Hz. Causes “Signal Droop”.– Does not have any error correction capability.– Occupies twice as much bandwidth as Unipolar NRZ.– Is not Transparent

Unipolar Return to Zero (RZ)

Figure. PSD of Unipolar RZ

3. Polar NRZ Polar signalling also has NRZ and RZ forms.

In polar signalling a binary 1 is represented by a pulse g1(t) and a binary 0 by the opposite (or antipodal) pulse g0(t) = -g1(t).

1 0 1 0 1 1 1 1 1 0

+V

-V

0

Figure. Polar NRZ

Advantage and disadvantage of polar

NRZPolar Non-Return to Zero (NRZ):

Advantages:– Simplicity in implementation.– No DC component.

Disadvantages:– Continuous part is non-zero at 0 Hz. Causes “Signal Droop”.– Does not have any error correction capability.– Does not posses any clocking component for ease of synchronisation.– Is not transparent.

4. Polar RZIn polar signalling a binary 1 is represented by a pulse g1(t) and a binary 0 by the

opposite (or antipodal) pulse g0(t) = -g1(t).

+V

-V

0

1 0 1 0 1 1 1 0 0 0

Figure. Polar RZ

Advantage and disadvantage of polar RZ

Polar Return to Zero (RZ):

Advantages:– Simplicity in implementation.– No DC component.

Disadvantages:– Continuous part is non-zero at 0 Hz. Causes “Signal Droop”.– Does not have any error correction capability.– Does not posses any clocking component for easy synchronisation. However, clock can be extracted by rectifying the received signal.– Occupies twice as much bandwidth as Polar NRZ.

Polar Signalling:

Polar NRZ and RZ have almost identical spectra to the Unipolar NRZ and RZ. However, due to the opposite polarity of the 1 and 0 symbols, neither contain any spectral lines.

Figure. PSD of Polar NRZ

5.Bipolar NRZ/AMI NRZ

Bipolar Signalling is also called “alternate mark inversion” (AMI) uses three voltage levels (+V, 0, -V) to represent two binary symbols. Zeros, as in unipolar, are represented by the absence of a pulse and ones (or marks) are represented byalternating voltage levels of +V and –V.

1 0 1 0 1 1 1 1 1 0

+V

-V

0

Figure. BiPolar NRZ

Has the same issues as NRZI for a long string of 0’s. A systemic problem with polar is the polarity can be backwards Alternating the mark level voltage ensures that the bipolar spectrum has a null at DC.And that signal droop on AC coupled lines is avoided. The alternating mark voltage also gives bipolar signalling a single error detection capability.

1 alternating +1/2 , -1/2 voltage 0 0 voltage

Advantage and disadvantage of Bipolar

NRZBiPolar / AMI NRZ:

Advantages:– No DC component.– Occupies less bandwidth than unipolar and polar NRZ schemes.– Does not suffer from signal droop (suitable for transmission over AC coupled lines).– Possesses single error detection capability.

Disadvantages:– Does not posses any clocking component for ease of synchronisation.– Is not Transparent.

BiPolar/ AMI NRZ Signalling

Figure. PSD of BiPolar RZ

6.Bipolar RZ/AMI RZ1 is represented by positive for the first half of T and zero for the second half.

0 is represented by negative for the first half of T and zero for the second half.

1 0 1 0 1 1 1 1 1 0

+V

-V

0

Figure. BiPolar RZ

Advantage and disadvantage of Bipolar

RZBiPolar / AMI RZ:

Advantages:– No DC component.– Occupies less bandwidth than unipolar and polar RZ schemes.– Does not suffer from signal droop (suitable for transmission over AC coupled lines).– Possesses single error detection capability.– Clock can be extracted by rectifying (a copy of) the received signal.

Disadvantages:–Is not Transparent.

BiPolar/ AMI RZ Signalling

Figure. PSD of BiPolar RZ

7. Manchester

There is always a mid-bit transition {which is used as a clocking mechanism}.The direction of the mid-bit transition represents the digital data.

Consequently, there may be a second transition at the beginning of the bit interval.Used in 802.3 baseband coaxial cable and CSMA/CD twisted pair.

1 low-to-high transition

0 high-to-low transition

Manchester Encoding

1 0 1 0 1 1 1 1 1 0

+V

-V

0

Note: There is always a transition at the centre of bit duration.

Figure. Manchester Encoding.

The transition at the centre of every bit interval is used for synchronization at the receiver.

Manchester encoding is called self-synchronizing. Synchronization at the receiving end can be achieved by locking on to the the transitions, which indicate the middle of the bits.

It is worth highlighting that the traditional synchronization technique used for unipolar, polar and bipolar schemes, which employs a narrow BPF to extract the clock signal cannot be used for synchronization in Manchester encoding.

This is because the PSD of Manchester encoding does not include a spectral line/ impulse at symbol rate (1/To). Even rectification does not help.

Advantage and disadvantage of

Manchester EncodingManchester Signalling:

Advantages:– No DC component.– Does not suffer from signal droop (suitable for transmission over AC coupled

lines).– Easy to synchronise with.– Is Transparent.

Disadvantages: – Because of the greater number of transitions it occupies a significantly large

bandwidth.– Does not have error detection capability.

These characteristic make this scheme unsuitable for use in Wide Area Networks. However, it is widely used in Local Area Networks such as Ethernet and Token Ring.

Manchester Signalling

Figure. PSD of Manchester

Reference

1.“Data Communication & Networking” 4th Edition by Behrouz A. Forouzan

2. “Communication Systems” 4th Edition by Simon Haykin