MODULE 7

4/18PREPARED BY: ENGR. JO-ANN

C. VIÑAS 1

MODULE 7

ERROR DETECTION


C. VIÑAS 2

TRANSMISSION ERRORSTransmission errors are caused by:– thermal noise {Shannon}– impulse noise (e..g, arcing relays)– signal distortion during transmission

(attenuation)– crosstalk– voice amplitude signal compression

(companding)– quantization noise (PCM)– jitter (variations in signal timings)– receiver and transmitter out of synch


C. VIÑAS 3

OBJECTIVES:

1. Introduce Data Communications2. Define Data Communication

Codes3. Discuss the Types of Data

Transmission andTypes of Errors

4. Explain Error Detection Schemes


C. VIÑAS 4

DATA COMMUNICATIONS

- It is the process of transferring digital information (usually binary form) between two or more points


C. VIÑAS 5

FORMS OF DATA

a. Alphabetical informationb. Numeric informationc. Symbolic information


C. VIÑAS 6

DATA CODE

- is a set of rules that translates alphanumeric characters into binary numbers

- also called character codes, character sets, character languages or symbol codes


C. VIÑAS 7

DATA COMMUNICATION CODES

1. Baudot Code - 1st fixed-length character - developed for machine rather than

people - 5-bit character code primarily used for

low- speed teletype system equipment


C. VIÑAS 8


2. ASCII Code (American Standard for Information Interchange) - 7-bit fixed length character set - is the standard character set for source coding the alphanumeric character set that humans understand but computers do not.


C. VIÑAS 9


3. EBCDIC Code (Extended Binary-Coded Decimal Interchange Code)

- 8-bit fixed length character set

developed in 1962 by IBM- used almost exclusively with IBM

mainframe computers and peripheral equipment


C. VIÑAS 10


4. Bar Codes - is a series of vertical black bars separated

by vertical white bars (called spaces)


C. VIÑAS 11

SERIAL and PARALLEL DATA TRANSMISSION

1. Parallel by bit or Serial by character

2. Serial by bit


C. VIÑAS 12

SERIAL DATA TRANSMISSION


C. VIÑAS 13

PARALLEL DATA TRANSMISSION


C. VIÑAS 14

TYPES OR ERRORS

1. Single bit error- 1 bit of error has occurred

2. Burst error - 2 or more bits in the data unit have

changed from 1 to 0 or from 0 to 1


C. VIÑAS 15

SINGLE BIT ERROR


C. VIÑAS 16

BURST ERROR


C. VIÑAS 17

“Error detection uses the concept of redundancy,

which means adding extra bits for detecting errors at

the destination.”


C. VIÑAS 18

ERROR DETECTION

- Is the process of monitoring data transmission and determining when errors have occurred.

Purpose:“ Not to prevent error from occurring but to prevent UNDETECTED ERROR.”


C. VIÑAS 19

GENERAL ERROR DETECTION SYSTEM


C. VIÑAS 20

ERROR DETECTION METHODS

1. REDUNDANCY

- Transmitting each character twice

2. EXACT COUNT ENCODING

- The number of 1’s in each character is the same


C. VIÑAS 21

REDUNDANCY


C. VIÑAS 22

PARITY BIT

- Bit added to each character to make all bits add up to an even number (even parity) or odd number (odd parity)


C. VIÑAS 23

EVEN PARITY CONCEPT


C. VIÑAS 24

“Simple parity check can detect all single-bit errors. It can detect burst errors only if the total number of errors

in each data unit is odd.”


C. VIÑAS 25


3. PARITY CHECKING

- adds 1 additional bit to each byte in the message

A) Odd ParityB) Even Parity


C. VIÑAS 26

PARITY GENERATORS

I. Serial Transmission Parity Generator

II. Parallel Transmission Parity Generator


C. VIÑAS 27

SERIAL TRANSMISSION PARITY GENERATOR


C. VIÑAS 28

PARALLEL TRANSMISSION PARITY GENERATOR


C. VIÑAS 29

PARITY CHECKING

Advantage:1. Simple

Disadvantages:1. If even number of errors has occurred it

can not be detected2. 50% efficiency


C. VIÑAS 30


4. VRC/LRC

A) LRC - message parity

B) VRC - character parity


C. VIÑAS 31

VERTICAL REDUNDANCY CHECKING

VRC entails the appending of a parity bit at the end of each transmitted character of value to create an odd or even total mathematical bit value.


C. VIÑAS 32

LONGITUDINAL REDUNDANCY CHECKING OR BLOCK CHECKING CHARACTER

LRC adds another level of reliability, as data is viewed in a block or data set, as though the receiving device were viewing data set in a matrix format.

LRC adds a significant measure of reliability. Also known as checksum, the LRC is sent as an extra character at the end of each data block.


C. VIÑAS 33

VRC/LRC

Advantage:1. Simple2. 98% reliability


C. VIÑAS 34

EXAMPLE

Determine the VRC and LRC for the message “SANTINO”. Use ASCII Character, and also use odd parity for VRC and even parity for LRC. Determine What ASCII character is to be transmitted for checking the message.


C. VIÑAS 35

“In two-dimensional parity check, a block of bits is divided into rows and a redundant row of bits is

added to the whole block.”


C. VIÑAS 36


5. CRC

- Is generally used with 8-bit codes such as EBCDIC.

- CRC 16: most common used CRC code in US and is identical to CCITT V.41


C. VIÑAS 37

CRC GENERATING CIRCUIT


C. VIÑAS 38

CRC GENERATOR


C. VIÑAS 39


5. CRC

- CRC Character is the remainder of a division process.

G(x) - data messageP(x) - generator polynomial function


C. VIÑAS 40


CRC 16

P(x) = x16 + x12 + x5 + x0

Advantage:1. 99.95% efficiency


C. VIÑAS 41

CRC ALGORITHM

1. Multiply G(x) by xn-k (puts zeros in (n-k) low order positions)

2. Divide xn-k G(x) by P(x)

3. Add remainder B(x) to xn-k G(x) (puts check bits in the n-k low order positions):


C. VIÑAS 42

CRC CHECKING

1. Add the CRC to the end of G(x)

2. Divide the product obtained in Step 1 by P(x)

“The remainder of the process should be equal to 0, otherwise error has occurred.”


C. VIÑAS 43

EXAMPLE

Determine the BCS for the ff data and CRC generating polynomials.

G(x) = x7 + x5 + x4 + x2 + x1 + x0

P(x) = x5 + x4 + x1 + x0


C. VIÑAS 44

CRC-12 : x12 + X11 + X3 + X2 + X + 1

CRC-16: x16 + x 15 + x2 + 1

CRC-CCITT: x16 + x12 + x5 + 1

MOST COMMONLY USED CYCLICCODES GENERATOR POLYNOMIAL


C. VIÑAS 45

DATA UNIT AND CHECKSUM


C. VIÑAS 46

6. CHECKSUM


C. VIÑAS 47

CHECKSUM ALGORITHM AT THE SENDER

The sender follows these steps:

The unit is divided into k sections, each of n bits.

All sections are added using one’s complement to get the sum.

The sum is complemented and becomes the checksum.

The checksum is sent with the data.


C. VIÑAS 48

CHECKSUM ALGORITHM AT THE RECEIVER

The receiver follows these steps:

The unit is divided into k sections, each of n bits.

All sections are added using one’s complement to get the sum.

The sum is complemented.

If the result is zero, the data are accepted: otherwise, rejected.


C. VIÑAS 49

EXAMPLE

Suppose the following block of 16 bits is to be sent using a checksum of 8 bits.

10101001 00111001

Determine the checksum.

Prove that the receiver can detect the error.


C. VIÑAS 50

EXAMPLE

Now suppose there is a burst error of length 5 that affects 4 bits.

10101111 11111001 00011101

Prove that the receiver can detect the error.

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