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© 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
© N. Ganesan, Ph.D. , All rights reserved.
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
© N. Ganesan, Ph.D. , All rights reserved.
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
End of ModuleEnd of Module
© N. Ganesan, Ph.D. , All rights reserved.
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
End of ModuleEnd of Module
© N. Ganesan, Ph.D. , All rights reserved.
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
End of ModuleEnd of Module
© N. Ganesan, Ph.D. , All rights reserved.
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
End of ModuleEnd of Module
© N. Ganesan, Ph.D. , All rights reserved.
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
End of ModuleEnd of Module
© N. Ganesan, Ph.D. , All rights reserved.
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
End of ModuleEnd of Module
© N. Ganesan, Ph.D. , All rights reserved.
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 ModuleEnd of Module
END OF MODULE END OF MODULE
END OF CHAPTEREND OF CHAPTER
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