Methods Of Error Detection

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Notes

Why Errors Occur

Errors occur due to **interference** in wired or wireless transmission (e.g., wire degradation, electrical fields, weather, physical barriers).
Results of interference: **data loss**, **data gain**, or **data change** (bits flipped).
Computers expect data in specific **formats**; unexpected data causes errors or corruption.

Parity Check

A **parity bit** is added to each byte; sender and receiver agree on **odd** or **even** parity.
For **even parity**, total number of 1s (including parity bit) must be even; for **odd parity**, total must be odd.
If parity doesn't match, an error is detected, but the location is unknown.
Limitation: parity check fails if an **even number of bits** are flipped or bits are transposed.

Parity Block & Parity Byte

A **parity block** uses horizontal (per byte) and vertical (per column) parity calculations.
The **parity byte** contains vertical parity bits for each column.
Cross-referencing horizontal and vertical parity **pinpoints the exact error location**.
The error can be corrected automatically or a retransmission requested.

Checksum

A **checksum** is a value calculated from data using an algorithm and sent with the data.
Receiver recalculates the checksum and compares it to the transmitted value.
If they **do not match**, an error is assumed; checksum does not indicate error location.

Echo Check

The receiver **echoes** (sends back) the received data to the sender.
Sender compares echoed data to original; if different, an error occurred and data is retransmitted.
Limitation: an error could occur during the echo transmission itself.

Check Digits

A **check digit** is the last digit in a numeric code (e.g., ISBN, barcode) used to detect entry errors.
Detects: incorrect digits, omitted/extra digits, phonetic errors.
Calculated using a standard algorithm; if the result is a whole number, the code is valid.

Automatic Repeat Query (ARQ)

ARQ is a protocol using **acknowledgements** and **timeout** to ensure error-free transmission.
Receiver sends **positive acknowledgement** (ACK) if data is correct, or **negative acknowledgement** (NAK) if corrupted.
If no acknowledgement is received within a **timeout** period, the sender automatically retransmits the data.
Process repeats until data is received correctly or a limit is reached.

Even parity check: parity bit ensures total number of 1s is even.

Automatic Repeat Query (ARQ) process.

Practice questions

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1.Which of the following is a result of interference during data transmission?
Easy
- AData loss
- BData encryption
- CData compression
- DData indexing
2.What is the purpose of a parity bit in data transmission?
Easy
- ATo detect errors in the transmitted data
- BTo correct errors in the transmitted data
- CTo encrypt the transmitted data
- DTo compress the transmitted data
3.A byte is transmitted with even parity. The byte received is 10110010. Does this byte contain an error?
Medium
- AYes, because there are four 1s
- BNo, because there are four 1s
- CYes, because there are five 1s
- DNo, because there are five 1s
4.Why might a parity check fail to detect an error?
Medium
- AIf an even number of bits are flipped
- BIf an odd number of bits are flipped
- CIf the parity bit itself is corrupted
- DIf the data is encrypted
5.In a parity block check using odd parity, the parity byte is calculated as 01111111. If the received parity byte is 01111110, what does this indicate?
Hard
- AAn error has occurred in bit column 8
- BAn error has occurred in bit column 7
- CAn error has occurred in byte row 8
- DNo error has occurred
6.What is a checksum used for?
Easy
- ATo determine if data has been corrupted
- BTo encrypt data
- CTo compress data
- DTo convert data to a different format
7.In an echo check, the receiver sends the received data back to the sender. Why is this method not fully reliable?
Medium
- AAn error could occur when the receiver transmits the data back
- BThe sender cannot detect errors
- CThe receiver cannot detect errors
- DIt requires too much bandwidth
8.What is a check digit?
Easy
- AThe last digit in a code used to detect errors in numeric data entry
- BThe first digit in a code used to identify the data type
- CA digit used to encrypt the data
- DA digit used to compress the data

More questions

9.Which of the following is an example of a check digit algorithm?
Medium
- AISBN
- BParity check
- CChecksum
- DEcho check
10.In Automatic Repeat reQuest (ARQ), what does the receiver send if an error is detected?
Easy
- AA negative acknowledgement
- BA positive acknowledgement
- CA timeout signal
- DA checksum
11.What happens in ARQ if the sender does not receive any acknowledgement within a certain time period?
Medium
- AThe sender waits for a timeout period and then automatically resends the data
- BThe sender assumes the data was received correctly
- CThe sender sends a negative acknowledgement
- DThe sender terminates the transmission
12.A parity block uses odd parity. Byte 3 has a parity bit of 1 and the main bits are 0101111. The total number of 1s in the byte (including parity) is 6. Does this indicate an error?
Hard
- AYes, because 6 is even
- BNo, because 6 is even
- CYes, because 6 is odd
- DNo, because 6 is odd
13.A parity block uses odd parity. Byte 3 has a parity bit of 1 and the main bits are 0101111. The total number of 1s in the byte (including parity) is 6. Does this byte contain an error?
Medium
- AYes, because 6 is even and odd parity is required
- BNo, because 6 is even and odd parity is required
- CYes, because 6 is odd and odd parity is required
- DNo, because 6 is odd and odd parity is required
14.A parity block uses even parity. Byte 2 has a parity bit of 0 and the main bits are 1010101. How many 1s are in the main bits?
Medium
- A4
- B3
- C5
- D2
15.A parity block using odd parity is transmitted. The received parity block is shown. Byte 4 row has a parity bit of 1 and main bits 1011001. The parity byte column has bits 01111110. Which bit is most likely in error?
Hard
- AByte 4, bit 3
- BByte 4, bit 5
- CByte 3, bit 4
- DByte 2, bit 6
16.What is a checksum?
Easy
- AA value used to determine if data has been corrupted or altered
- BA method that retransmits data if an error is detected
- CA bit added to each byte to ensure even or odd number of 1s
- DA protocol that uses acknowledgements and timeouts
17.Which of the following is an advantage of using a parity block (with parity byte) over a simple parity check?
Medium
- AIt can detect and correct single-bit errors
- BIt can detect multiple-bit errors but not correct them
- CIt does not require additional bits to be transmitted
- DIt is faster to compute
18.In Automatic Repeat reQuest (ARQ), what happens if the sender does not receive an acknowledgement within a certain time period?
Medium
- AThe sender automatically resends the data
- BThe sender waits indefinitely for an acknowledgement
- CThe sender sends a negative acknowledgement to the receiver
- DThe sender aborts the transmission
19.A file is transmitted using ARQ. The receiver detects an error in a packet and sends a negative acknowledgement. The sender resends the packet. What is the next possible action by the receiver?
Hard
- ASend a positive acknowledgement if the resent packet is error-free
- BSend another negative acknowledgement if the resent packet still has an error
- CBoth A and B are possible
- DIgnore the packet and wait for the next one
20.Which of the following is an example of a check digit usage?
Easy
- AISBN book numbers
- BParity bits
- CChecksums
- DARQ
21.An echo check involves the receiver sending the received data back to the sender. Why is this method not fully reliable?
Medium
- AAn error could occur during the echo transmission as well
- BIt is too slow for practical use
- CIt only detects errors in the original transmission, not the echo
- DIt requires too much bandwidth
22.A check digit algorithm is applied to the number 9780141036144. The last digit is the check digit. The algorithm multiplies each digit by 1 or 3 alternately, sums the products, and the check digit makes the total divisible by 10. If the sum of products for the first 12 digits is 85, what should the check digit be?
Hard
- A5
- B0
- C3
- D8
23.What is the purpose of a check digit?
Easy
- ATo detect errors in numeric data entry
- BTo correct errors in transmitted data
- CTo encrypt data for secure transmission
- DTo compress data for faster transmission
24.A 7-bit binary value 1001101 is transmitted with an even parity bit. What is the transmitted 8-bit byte?
Medium
- A11001101
- B01001101
- C10011011
- D10011010
25.A parity block uses odd parity. The received parity block is shown below. Byte 3 has a parity bit of 1 and main bits 1011001. The parity byte column for bit 5 is 1. Which cell contains the error?
Medium
- AByte 3, bit 5
- BByte 3, bit 4
- CByte 5, bit 5
- DByte 3, parity bit
26.A 7-bit binary value 1101010 is transmitted with an even parity bit. During transmission, two bits are flipped: bit 3 (from 0 to 1) and bit 6 (from 1 to 0). Will the parity check detect this error?
Hard
- AYes, because the total number of 1s changes
- BNo, because the number of 1s remains even
- CYes, because two bits flipped always cause a parity error
- DNo, because parity cannot detect errors in data
27.In Automatic Repeat reQuest (ARQ), what does the receiver send if it receives a data packet with an error?
Medium
- APositive acknowledgement
- BNegative acknowledgement
- CTimeout signal
- DNo acknowledgement
28.Which type of error can a parity check fail to detect?
Easy
- AAn odd number of bits flipped
- BAn even number of bits flipped
- CA single bit flipped
- DData loss
29.A file is transmitted using ARQ. The sender transmits a packet and waits. What happens if the sender does not receive any acknowledgement within a certain time period?
Medium
- AThe sender assumes the packet was received correctly
- BThe sender automatically resends the packet
- CThe sender stops transmitting
- DThe sender sends a negative acknowledgement
30.A checksum is calculated for a block of data and transmitted with it. The receiver recalculates the checksum and finds it matches the transmitted checksum. Which of the following is true?
Hard
- AThe data is definitely error-free
- BThe data may still contain errors
- CThe checksum algorithm can pinpoint the error location
- DThe receiver will request retransmission
31.A parity block uses even parity. Byte 1 has parity bit 0 and main bits 1010101. Byte 2 has parity bit 1 and main bits 0101010. The parity byte for bit 3 is 0. Which of the following is most likely true?
Hard
- ANo error occurred
- BAn error occurred in Byte 1, bit 3
- CAn error occurred in Byte 2, bit 3
- DAn error occurred in the parity byte
32.Which of the following is a possible result of interference during data transmission?
Easy
- AData gain
- BData compression
- CData encryption
- DData indexing
33.A parity block uses odd parity. The received parity block is shown: | | Parity bit | Bit 2 | Bit 3 | Bit 4 | Bit 5 | Bit 6 | Bit 7 | Bit 8 | |-------|------------|-------|-------|-------|-------|-------|-------|-------| | Byte 1| 0 | 1 | 1 | 0 | 1 | 0 | 1 | 1 | | Byte 2| 0 | 0 | 0 | 0 | 1 | 0 | 0 | 0 | | Byte 3| 1 | 0 | 1 | 0 | 1 | 1 | 1 | 1 | | Byte 4| 1 | 0 | 1 | 1 | 1 | 0 | 0 | 1 | | Byte 5| 1 | 1 | 0 | 1 | 0 | 1 | 0 | 1 | | Byte 6| 1 | 1 | 0 | 0 | 1 | 1 | 1 | 0 | | Byte 7| 0 | 0 | 1 | 1 | 1 | 1 | 1 | 0 | | Byte 8| 0 | 1 | 0 | 1 | 1 | 0 | 0 | 0 | |Parity | 0 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | Which cell contains the error?
Medium
- AByte 3, Bit 5
- BByte 5, Bit 6
- CByte 7, Bit 4
- DByte 2, Bit 6
34.A parity block uses even parity. The received parity block is shown: | | Parity bit | Bit 2 | Bit 3 | Bit 4 | Bit 5 | Bit 6 | Bit 7 | Bit 8 | |-------|------------|-------|-------|-------|-------|-------|-------|-------| | Byte 1| 0 | 1 | 1 | 0 | 1 | 0 | 1 | 1 | | Byte 2| 0 | 0 | 0 | 0 | 1 | 0 | 0 | 0 | | Byte 3| 1 | 0 | 1 | 0 | 1 | 1 | 1 | 1 | | Byte 4| 1 | 0 | 1 | 1 | 1 | 0 | 0 | 1 | | Byte 5| 1 | 1 | 0 | 1 | 0 | 1 | 0 | 1 | | Byte 6| 1 | 1 | 0 | 0 | 1 | 1 | 1 | 0 | | Byte 7| 0 | 0 | 1 | 1 | 1 | 1 | 1 | 0 | | Byte 8| 0 | 1 | 0 | 1 | 1 | 0 | 0 | 0 | |Parity | 0 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | If the error is in Byte 3, which bit is incorrect?
Hard
- ABit 5
- BBit 3
- CBit 7
- DBit 2
35.Which of the following is a limitation of a simple parity check?
Easy
- AIt cannot detect an error if an even number of bits are flipped.
- BIt cannot detect any errors in the data.
- CIt requires a large amount of additional data to be transmitted.
- DIt can locate the exact position of the error.
36.A 7-bit binary value 1001101 is transmitted with an even parity bit. During transmission, two bits are flipped: bit 3 (from 0 to 1) and bit 6 (from 1 to 0). Will the parity check detect this error?
Medium
- ANo, because an even number of bits have changed, so the parity remains even.
- BYes, because any change in bits will be detected by parity.
- CNo, because parity can only detect errors in the parity bit itself.
- DYes, because the total number of 1s changes from 4 to 4, so parity is still even.
37.A file is transmitted using ARQ. The sender transmits a packet and waits. If the receiver detects an error, what does it send back?
Hard
- AA negative acknowledgement (NAK)
- BA positive acknowledgement (ACK)
- CThe corrupted packet itself
- DA timeout signal
38.What is the purpose of a check digit in a barcode?
Easy
- ATo detect errors in the numeric data entry
- BTo increase the speed of scanning
- CTo encode the price of the item
- DTo identify the manufacturer only
39.An ISBN-10 number is 965-448-765-9. The check digit is 9. Which of the following would be detected as an error by the check digit algorithm?
Medium
- A965-448-756-9 (transposition of last two digits)
- B965-448-765-9 (correct number)
- C965-448-765-8 (check digit changed to 8)
- D965-448-765-0 (check digit changed to 0)
40.Which of the following is an example of interference that can cause errors in data transmission?
Easy
- AWire degradation
- BUsing a router
- CIncreasing bandwidth
- DUsing a firewall

Slides

Why Errors Occur

Errors can occur in wired or wireless transmission due to interference.
Interference includes wire degradation, electrical fields, weather, or physical barriers.
Results: data loss, data gain, or data change (bits flipped).
Computers expect data in agreed formats; errors cause corruption.

Which of the following is NOT a possible result of interference during data transmission?

Data loss
Data gain
Data encryption
Data change

Parity Check

A parity bit is added to each byte to make the total number of 1s either odd (odd parity) or even (even parity).
Sender and receiver agree on the parity type beforehand.
If the received byte's 1s don't match the agreed parity, an error is detected.
Parity checks detect errors but do not locate them.

Using even parity, what should the parity bit be for the byte 1011010?

0
1
Cannot determine
It depends on the sender

Parity Block and Parity Byte

A parity block adds both horizontal (per byte) and vertical (per bit column) parity bits.
The vertical parity bits form a parity byte sent with the data.
Cross-referencing horizontal and vertical parity can pinpoint the exact bit error.
This allows error correction or retransmission request.

Which of these are true about parity checks?

A parity check can detect if an even number of bits have been flipped.
A parity block can locate the position of an error.
Parity checks are quick and easy to implement.

Checksum

A checksum is a value calculated from data using an algorithm, sent with the data.
The receiver recalculates the checksum and compares it to the sent value.
If they match, data is assumed correct; if not, an error occurred.
Checksums detect errors but do not specify where.

In a checksum, the sender calculates a ____ value and transmits it with the data. The receiver ____ the checksum and compares it.

Echo Check

Echo check: the receiver sends the received data back to the sender.
The sender compares the echoed data with the original to detect errors.
This method is not fully reliable because errors can occur in either transmission.
If an error is found, the sender retransmits the data.

What is a disadvantage of echo check?

It cannot detect errors
It is slow and doubles transmission time
It requires a checksum
It only works with wireless

Check Digits

A check digit is the last digit in a code (e.g., ISBN, barcode) used to detect errors in numeric entry.
It is calculated using a standard algorithm so the result is a whole number.
Common errors detected: incorrect digits, omitted/extra digits, phonetic errors.
Examples: ISBN book numbers, barcodes.

Which of the following uses a check digit?

IP address
ISBN number
Email address
File name

Automatic Repeat Query (ARQ)

ARQ is a protocol that uses acknowledgements and timeouts to ensure reliable data transmission.
If data is received correctly, the receiver sends a positive acknowledgement (ACK).
If data is corrupted, the receiver sends a negative acknowledgement (NAK).
If no acknowledgement is received within a timeout, the sender automatically retransmits the data.

In ARQ, if the receiver detects an error, it sends a ____ acknowledgement. If no acknowledgement is received, the sender waits for a ____ before resending.

Which of these are true about ARQ?

ARQ stands for Automatic Repeat Query.
ARQ can correct errors without retransmission.
ARQ uses timeouts to trigger retransmission.

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