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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

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  1. Chapter Error Causes and Detection

  2. 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

  3. 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

  4. 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

  5. Module Causes and Types of Error

  6. 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

  7. 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

  8. 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

  9. 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

  10. Distortion Due to Noise Signal affected by noise. Origins Destination Noise represents the cumulative effect of a wide variety of factors that have an influence on the signal transmitted.

  11. 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

  12. Expression of the Quality of a Line • Signal to noise ratio

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

  14. End of Module

  15. Module Electro-Magnetic Interference (EMI)

  16. Understanding Interference Electric Bulb 1 Wall Electrical Outlet 1.Wired cylinder connected to a bulb 2. Wired cylinder connected to an electrical outlet 2

  17. 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

  18. 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

  19. Effect of Interference on Communication Signals Creates Electricity Electro- Magnetic Field Cable 1 Cable 2 Creates Electricity

  20. End of Module

  21. Module Minimizing Electromagnetic Interference

  22. 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

  23. Twisting of Wires to Minimize Interference Twisting Counterbalance • Unshielded Twisted Pair (UTP) Cat 5 : Higher quality Cat 2 and 3: Lower quality • Shielded Twisted Pair (STP)

  24. 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

  25. The Shielding of Cables Minimize emission of from the cable Minimize outside interference Shield the cable from outside interference (Conductive Material).

  26. 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

  27. 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

  28. Examples of FCC Regulated Equipment • Computers and communication devices • Electrical equipment such as fans etc.

  29. 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

  30. 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

  31. End of Module

  32. Module Securing Transmission

  33. Data Eavesdropping Listening Device (Eavesdropping device) Ground Cable Electro- Magnetic Wave (Below ground cable)

  34. 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

  35. 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

  36. 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

  37. Role of Public and Private Keys in Encryption Private Key Public Key Sender Receiver A B Example of a key is A10012A.

  38. 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

  39. Encryption Applications • PGP (Pretty Good Privacy) • PointSec

  40. 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

  41. End of Module

  42. Module Error Checking Technique: VRC (Vertical Redundancy Code)

  43. 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

  44. Vertical Redundancy Checking (VRC) • Two implementations of VRC • Odd • Even

  45. 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

  46. 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

  47. 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 • 10000111 After transmission • Parity count remains the same • But, the data has changed as shown by the colored digits

  48. 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

  49. End of Module

  50. Module Error Checking Technique: LRC (Longitudinal Redundancy Checking)

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