© N. Ganesan, Ph.D., All rights reserved. Chapter Error Causes and Detection

© N. Ganesan, Ph.D. , All rights reserved.

Chapter

Error Causes and Detection

Chapter Objectives

• List the different types of errors affecting transmission

• Provide an understanding of electromagnetic interference in terms of propagation, data corruption etc.

• Describe the measures that are taken to minimize electronic interface– Twisting of wires, FCC regulations etc.

Continued

Continuation of Chapter Objectives

• Discuss the effect of electromagnetic field on security– Eavesdropping etc.

• Explain the following error checking techniques– VRC, LRC and CRC

• Describe all the different points of error detection in a typical communication link

Chapter Modules

• Types of error• Understanding electromagnetic

interference• Minimizing electromagnetic interference• Effect of electromagnetic field on security• Error checking technique: VRC• Error checking technique: LRC• Error checking technique: CRC• Points of error detection in communication


Module

Causes and Types of Error

Causes Of Error• Interference

– Largely due to external electromagnetic field – Corrupts the information carried by the

electromagnetic signals

• Temperature– Affects the transmission quality and

capability of the medium– For instance, temperature variations

influence the conductivity of the communication medium

Error Types

• The three major types of errors are as follows:– Attenuation– Distortion– Noise

• The term Noise is frequently encountered in communication

• It reflects the cumulative effect of interference on the signal

Attenuation

• Refers to the weakening of the signal with distance

• Signals must be of sufficient strength at the receiving point to overcome noise

• Attenuation is greater at higher frequencies

• Line drivers and boosters are often used to minimize the effect of attenuation

Distortion

• Distortion is a general term used to describe the distortion of a signal

• A special type of distortion is know as delayed distortion

• It affects signals of different frequencies and results in them arriving at the destination at different times

• The above causes timing related problems in data transmission

Distortion Due to Noise

Origins Destination

Noise represents the cumulative effect of a wide variety of factors that have an influence on the signal transmitted.

Signal affected by noise.

Noise Factors• Interference and crosstalk occur due to

electromagnetic interference • Thermal noise, for instance, is proportional

to the temperature and bandwidth• Impulse noise that is induced as a result of

a surge in signal strength– Encountered immediately after the power is

switched on a communication line• Bulb fusing phenomenon is a practical example

Expression of the Quality of a Line

• Signal to noise ratio

Digital Advantage

• Although the signal changes due to various factors discussed, the original digital pattern can be extracted

End of ModuleEnd of Module


Module

Electro-Magnetic Interference (EMI)

Understanding Interference

Wall ElectricalOutlet

Electric Bulb

1

2

1.Wired cylinder connected to a bulb2. Wired cylinder connected to an electrical outlet

Interference Observation

• Bulb lights up as the smaller cylinder is lowered into the big cylinder

• The important point to note is that the cylinders do not touch one another

• This means that the electricity in one cylinder generates electricity in the other cylinder– Signifies the very definition of

interference

Role of Electromagnetic Field in Interference

• Electricity in the larger cylinder creates an electromagnetic field

• Electromagnetic field in turn creates electricity in the smaller cylinder

• The bulb thus lights up as a result indicating induced electricity by an adjacent carrier of electricity

Effect of Interference on Communication Signals

Electro- MagneticField

Cable 1

Cable 2

Creates Electricity

Creates Electricity



Module

Minimizing Electromagnetic Interference

Major Source of Interference in Practice

• Major sources of interference– Cables adjacent to one another– Misbehaving electrical equipment

• Minimizing interference at the Medium– Twisting and shielding of cables

• Minimizing interference from an electronic/electrical device– FCC compliance and certification is often

required of devices

Twisting of Wires to Minimize Interference

Twisting Counterbalance

•Unshielded Twisted Pair (UTP)

•Shielded Twisted Pair (STP)Cat 5 : Higher quality

Cat 2 and 3: Lower quality

Twisting of Wires

• The higher the number of twists per foot, the better the quality of the twisted pair wires– Category 5e wires are of better

quality compared to Category 5 wires

The Shielding of Cables

Shield the cable from outside interference (Conductive Material).

Minimize outside interference

Minimize emission of from the cable

Minimizing the Interference from

Equipment

• Ensure that the electrical and electronic equipment are FCC certified

• FCC certification implies that the equipment is in compliance with FCC regulations concerning the emission of the electromagnetic field

FCC Regulations Requirement and

Certification• Requires that the device be

housed in a proper casing • Example of Certification

– Class B certification for microcomputers for instance

Examples of FCC Regulated Equipment

• Computers and communication devices

• Electrical equipment such as fans etc.

In Summary

• Better the resistance to electromagnetic field, better the quality of the medium in terms of carrying the information at higher speeds over a longer distance

Fiber Optic Advantage• Does not generate electromagnetic field• No electromagnetic interference• Signal loss minimized• Data travels faster and further• Transmission of data is secure• Fiber strands can be strung closely

together– Large number of transmission lines in a fiber

optic cable of reasonable diameter



Module

Securing Transmission

Data Eavesdropping

Ground

Listening Device

CableElectro-MagneticWave

(Eavesdropping device)

(Below ground cable)

Data Eavesdropping

• It is based on monitoring the surrounding electromagnetic field to tap the data being transmitted over the cable

• A listening device need not directly touch the cable

• For example, the data can be monitored above ground and by devices that are implanted in a building as well

Securing Electronically Transmitted Data

• Encryption• Information to be transmitted is

encrypted at the origin and decrypted at the destination

• Encryption has been pushed to the forefront following the extensive use of the Internet

Some Encryption Details

• Encrypted data is unintelligible– May resemble a binary file

• Encrypted data will only make sense when it is decrypted with a key

• Keys are used both for the encryption and decryption of information– Public key– Private key

Role of Public and Private Keys in Encryption

Private Key Public Key

Example of a key is A10012A.

Sender Receiver

A B

Degrees of Sophistication of Encryption

• Level of sophistication – Expressed in bits

• A 32 bit encryption algorithm is less sophisticated compared to the 64 bit algorithm

• At higher bit lengths, it may take hundred of years to decode the information using a supercomputer– 128 bit encryption is the most

sophisticated encryption

Encryption Applications

• PGP (Pretty Good Privacy)• PointSec

Enhancing the Security

• Use a secure transmission protocol– Point-to-Point Tunneling Protocol

(PPTP)

• PPTP is used for transmitting information over the Internet

• VPNs are created based on the PPTP Encryption/Tunneling protocol

• IPSec



Module

Error Checking Technique: VRC

(Vertical Redundancy Code)

Major Error Correction Techniques

• Vertical Redundancy Checking (VRC)– Also known simply as parity checking

• Longitudinal Redundancy Checking (LRC)– Similar in principle to VRC– Operates on a block of data

• Cyclic Redundancy Checking (CRC)– Sophisticated error checking procedure – Performed on a block of data– Used extensively

Vertical Redundancy Checking (VRC)

• Two implementations of VRC– Odd– Even

Odd Parity Checking

• Add a parity bit• Either a one or a zero is added

– Total number of ones adds an odd number

• Example – Before parity 0110100 – After parity 01101000

Parity bit

Even Parity Checking

• Even– The total number of ones should add

up to an even number

• Example– Before parity 0101010– After parity 01010101

Parity bit

Error Detection Capability of VRC

• Errors can still escape detection– When two bits change value in which

case VRC becomes ineffective

• Even parity example– 11010111 Before transmission– 10000111After transmission– Parity count remains the same– But, the data has changed as shown by

the colored digits

Probability of Error Not Being by VRC

• Probability is indeed very low• Assumption that the probability of

a single digit changing value is .0001 – Probability of two digits changing is

• P1 x P1 • .0001 X .0001 = .000000001



Module

Error Checking Technique: LRC(Longitudinal Redundancy

Checking)

Overview Of Longitudinal Redundancy Check (LRC)

• LRC is somewhat similar to parity checking except for the fact that the error checking is performed on a block of data

• The difference, in this case is that the number of ones across the block must add up to an:– even number for even parity– odd number for odd parity

Longitudinal Redundancy Code (LRC)

10111011

10101010110 110100100010001001011

Block of characters (data).

Parity Check (VRC)

LRC Check

8-bit data representing one character.

LRC Error Detection Capability

• Errors can still escape detection• For example, LRC cannot detect

the error when four digits change value– Two may change along the row– Two may change along the column

Illustration of an Undetected Error in LRC

1 0 00 1 00 1 00 1

1 0 01 0 01 0 00 1

Parity

When four digits change value, the parity countstill remains as an odd number both along the block as well as within a character.

Origin Destination

Probability Of Error Being Undetected In LRC

• Probability of an error not being detected is indeed very very low

• Assumption • Probability of one digit changing value

is .00001

• Probability of all four digits changing values • P1 X P1 X P1 X P1• .00000000000000000001 • A very low number



Module

Error Checking Technique CRC

(Cyclic Redundancy Code)

Overview Of CRC • CRC is performed on a block of data• It operates by adding a tail-end bit to the

block of data• Uses an advanced formula to derive the

parity bit– A polynomial is fitted to the data– CRC bit is added based on the characteristics of

the polynomial

• CRC supports a near error-free transmission of information

Cyclic Redundancy Code (CRC)

10111011

10101010110 110100100010001001011

CRC Check

Block of characters (data).

Parity Check(VRC)

8-bit data representing one character.

ITU Standardized CRC Techniques

• Implemented in most high speed analog modems

• CRC 16 – Signifies that the degree of polynomial used in

error checking is 16

• CRC 32 – Corresponds to a polynomial of degree 32 being

used for error checking

• CRC 32 has a better error detection capability compared to CRC 16

In Summary• CRC is the most sophisticated of

all error checking techniques• The higher the degree of

implantation– The better the error detection

capability– The more the resources required to

implement CRC



Module (Appendix A)

Points of Error Detection in Communication

Micro

Points Of Error Detection

Modem

* Communication Software* Application Software

* ROM basedSoftware in the modem.

Hardware Based Error Detection in Modem

• MNP Level 5 detection• Built within a number of modems• Incorporated in the new standard

Error Detection Performed by Communication

Software

• Protocols incorporated in a communication software performs this error detection

• Sample communication protocols that incorporate error detection – Xmodem– Zmodem – Kermit

Error Detection in Application Software

• Applications may have programmed error checking capabilities– A bank transaction

• In summary– Combined effect of these error

checking techniques is the error free communication that is now possible

Error Detection in Digital and Analog Environments

• Analog technology– Error detection is done in some detail – An example is X.25

• Digital technology– Some protocols may not perform

extensive error detection– Error detection may be left to the

client

Digital Technologies

• Frame Relay– Error detection and correction on the

network are not as intensive as in the case of X.25

– Digital transmission is less prone to errors

• UDP (May be used in place of TCP)– Connection less protocols that does not

guarantee error free transmission


END OF MODULE END OF MODULE

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© N. Ganesan, Ph.D., All rights reserved. Chapter Error Causes and Detection