Chapter 6: Errors, Error Detection, and Error Control

INTRODUCTION

Noise can creep in and disrupts data transmission even with the highest quality fiber-optic cable. When this occurs, error detection techniques become valuable tool. There are different form of noise that commonly occur during data transmission. So having a better understanding of different types of noise and what causes them will enable better application of noise-reduction techniques to communicate with the system. There are three error-control options when an error is detected: (1) toss the frame/packet (ignore the error), (2)return an error message to the transmitter, or (3)correct the error without help from the transmitter

NOISES AND ERRORS

1. White Noise

• White noise: also called thermal noise or Gaussian noise, is a relatively continuos noise and is much like the static we hear when a radio is being turned between stations.
• It is always present to some degree in transmission and depend on the temperature of the medium.
• It can be significantly reduce but never completely

2. Impulse noise:

• Impulse noise: or noise spike is a noncontinuous noise and one of the most difficult errors to detect because it can occur randomly
•  The noise is typically an analog burst of energy. If  impulse spike interferes with an analog signal, removing it without affecting the original signal  can be difficult
• If impulse noise interferes with a digital signal, often the original data can be recognized and recovered, but not recoverable if the noise is completely obliterates the digital signal

3. Crosstalk

• Crosstalk: is unwanted coupling between two different signal paths
• Telephone signal crosstalk is example. When crosstalk happens, you can hear another person talks back
• It can be reduced with proper precautions and hardware

4. Echo

• Echo: is the reflective feedback of a transmitted signal as the signal moves through a medium. 
• Occurs mot often at junctions where wires are connected or at the open end of a coaxial cable.
• Echo suppressor can be attached to a line to reduce echo

5. Jitter

• Jitter: is the result of small timing irregularities that become magnified during the transmission of digital signals as the signals are passed from one device to another
• Jitter can cause video devices to flicker, audio transmissions to click and break up, and transmitted computer data to arrive with errors
• If serious enough, jitter can cause the system to slow down transmission rates
• Causes can include electromagnetic interference, crosstalk, passing the signal through many repeaters and the use of lower quality equipment
• Possible solution involve installing proper shielding

6. Attenuation

• Attenuation: is the continuos loss of a signal;s strength as it travels through a medium
• Can be eliminated with the use of amplifiers for analog systems or repeaters for digital systems

ERROR PREVENTION

To prevent the occurrence of may types of transmission errors, several techniques can be applied:

• Install wiring with proper shielding to reduce electromagnetic interference and crosstalk
• Be aware that many different types of wireless applications share the same wireless frequencies
• Replace older equipment with more modern, digital equipment
• Use the proper number of digital repeaters and analog, amplifiers to increase signal strength
• Observe the stated capacities of a medium and to reduce the error, avoid pushing transmission speeds beyond their recommended limits

ERROR DETECTION

• Despite best attempts to prevent, errors still occur
• Error detection can be performed in several places within a communications model. The most common place is data link layer

1. Parity Check

• Simple parity: is the easiest error-detection method to incorporate into a transmission system.
• Even Parity: the 0 and 1 are added to the string produces an even number of binary 1s
• Odd Parity: the 0 and 1 added to the string produces an odd number of binary 1s

2. Longitudinal parity

• Longitudinal parity: tries to solve the main weakness of simple parity, that all even numbers of errors are not detected.
• Adds a parity bit to each character then adds a row of parity bits after a block of character. 
• The row of parity bits is actually a parity bit for each “column” of character. 
• The row of parity bits plus the column parity bits add a great amount of redundancy to a block of characters

• Both simple and longitudinal parities do not catch all errors
• Simple parity only catches odd numbers of bit errors
• Longitudinal parity is better at catching errors but requires too many check bits added to a block of data

3. Arithmetic checksum

• Used in TCP and IP on the Internet
• Characters to be transmitted are converted to numeric form and summed
• Sum is placed in some form at the end of the transmission
• Receiver performs same conversion and summing and compares new sum with sent sum

4. Cyclic Redundancy Checksum

• CRC or cyclic checksum: adds 8 to 32 check bits to potentially large data packets and yields an error-detection capability approaching 100%
• Transmitter takes the message polynomial and using polynomial arithmetic, divides it by a given generating polynomial
• Generating polynomial: is an industry-approved bit string used to create the cyclic checksum remainder
• Quotient is discarded but the remainder is “attached” to the end of the message
• The message (with the remainder) is transmitted to the receiver
• The receiver divides the message and remainder by the same generating polynomial
• If a remainder not equal to zero results, there was an error during transmission
• If a remainder of zero results, there was no error during transmission

ERROR CONTROL

1. Toss the frame/packet

• Doesn't seem like an option, but has became a mode of operation for some newer wide area network transmission techniques
• If a data frame arrives at a frame relay switch and an error is detected, the frame is simply discarded
• Frame relay assumes a higher protocol (such as TCP/IP) will detect the tossed frame and ask for retransmission

2. Return a message

• Stop and Wait Error Control: the simplest of the error control protocols
  - A transmitter sends a frame then stops and waits for an acknowledgement
  a) If a positive acknowledgment (ACK) is received, the next frame is sent
  b) If a negative acknowledgement (ACK) is received, the same frame is transmitted again

• Siding Window Error Control: a flow control scheme that allows a station to transmit a number of data packets at one time before receiving some form of acknowledgement.
  - These techniques assume that multiple frames are in transmission at one time
  - When a receiver does acknowledge receipt, the returned ACK contains the number of the frame expected next
  - Using TCP/IP, there are some basic rules concerning ACKs:
  

  a) Rule 1: If a receiver just received data and wants to send its own data, piggyback an ACK along with that data

  b) Rule 2: If a receiver has no data to return and has just ACKed the last packet, receiver waits 500 ms for another packet

  *If while waiting, another packet arrives, send the ACK immediately 

  c) Rule 3: If a receiver has no data to return and has just ACKed the last packet, receiver waits 500 ms

  * No packet, send ACK

3. Correct the error

• For a receiver to correct the error with n further help from the transmitter requires a large amount of redundant information to accompany the original data
  - this redundant information allows the receiver to determine the error and make corrections
• This type of error control is often called forward error and involves codes called Hamming Codes
• Hamming Code is a specially designed code in which special check bits have been added to data bits such that, if an error occurs during transmission, the receiver might be able to correct the error using the included check and data bits

ERROR DETECTION IN ACTION

• FEC is used in transmission of radio signals, such as those used in transmission of digital television (Reed-Solomon and Trellis encoding) and 4D-PAM5 (Viterbi and Trellis encoding)
• Some FEC is based on Hamming Codes

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