Study Unit 6 - Chapter Six Errors, Error Detection, and Error Control

Question 1

White Noise

Answer 1

White Noise
* Also known as thermal or Gaussian noise
* Relatively constant and can be reduced
* If white noise gets too strong, it can completely disrupt the
signal

Question 2

Impulse Noise

Answer 2

Impulse Noise
* One of the most disruptive forms of noise
* Random spikes of power that can destroy one or more bits of
information
* Difficult to remove from an analog signal because it may be
hard to distinguish from the original signal
* Impulse noise can damage more bits if the bits are closer
together (transmitted at a faster rate)

Question 3

Crosstalk

Answer 3

Crosstalk
* Unwanted coupling between two different signal paths
– For example, hearing another conversation while talking on the
telephone
* Relatively constant and can be reduced with proper
measures

Question 3

Echo

Answer 3

Echo
* The reflective feedback of a transmitted signal as the signal
moves through a medium
* Most often occurs on coaxial cable
* If echo bad enough, it could interfere with original signal
* Relatively constant, and can be significantly reduced

Question 4

Jitter

Answer 4

Jitter
* The result of small timing irregularities during the
transmission of digital signals
* Occurs when a digital signal is repeated over and over
* If serious enough, jitter forces systems to slow down their
transmission
* Steps can be taken to reduce jitter

Question 5

Delay Distortion

Answer 5

Delay Distortion
* Occurs because the velocity of propagation of a signal
through a medium varies with the frequency of the signal
– Can be reduced

Question 6

Attenuation

Answer 6

Attenuation
* The continuous loss of a signal’s strength as it travels
through a medium

Question 7

Error Detection

Answer 7

Error Detection
* Despite the best prevention techniques, errors may still
happen
* To detect an error, something extra has to be added to the
data/signal
– This extra is an error detection code
* Three basic techniques for detecting errors: parity checking,
arithmetic checksum, and cyclic redundancy checksum

Question 8

Error Prevention

Answer 8

Error Prevention
* To prevent errors from happening, several techniques may
be applied:
– Proper shielding of cables to reduce interference
– Telephone line conditioning or equalization
– Replacing older media and equipment with new, possibly digital
components
– Proper use of digital repeaters and analog amplifiers
– Observe the stated capacities of the media

Question 9

Parity Checks

Answer 9

Parity Checks
* Simple parity
– If performing even parity, add a parity bit such that an even number of
1s are maintained
– If performing odd parity, add a parity bit such that an odd number of
1s are maintained
– For example, send 1001010 using even parity
– For example, send 1001011 using even parity

Question 10

Longitudinal parity

Answer 10

Longitudinal parity
– Adds a parity bit to each character then adds a row of parity bits after
a block of characters
– The row of parity bits is actually a parity bit for each “column” of
characters
– The row of parity bits plus the column parity bits add a great amount
of redundancy to a block of characters

Question 11

Parity Checks (continued)
* Simple parity (continued)

Answer 11

Parity Checks (continued)
* Simple parity (continued)
– What happens if the character 10010101 is sent and the first two 0s
accidentally become two 1s?
* Thus, the following character is received: 11110101
* Will there be a parity error?
– Problem: Simple parity only detects odd numbers of bits in error

Question 12

Parity Checks (continued)

Answer 12

Parity Checks (continued)
* Both simple parity and longitudinal parity 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
* We need a better error detection method
– What about arithmetic checksum?

Question 13

Arithmetic Checksum

Answer 13

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

Question 14

Arithmetic Checksum
Simplified example:

Answer 14

Arithmetic Checksum
Simplified example:
56
72
34
48
210
Then bring 2 down and add to right-most position
10
2
12

Question 15

Arithmetic Checksum

Answer 15

Arithmetic Checksum
* Receiver performs same conversion and summing and
compares new sum with sent sum
* TCP and IP processes a little more complex but idea is the
same
* But even arithmetic checksum can let errors slip through. Is
there something more powerful yet?

Question 15

Cyclic Redundancy Checksum

Answer 15

Cyclic Redundancy Checksum
* CRC error detection method treats the packet of data to be
transmitted as a large polynomial
* Transmitter takes the message polynomial and using
polynomial arithmetic, divides it by a given generating
polynomial
* Quotient is discarded but the remainder is “attached” to the
end of the message

Question 16

Cyclic Redundancy Checksum

Answer 16

Cyclic Redundancy Checksum
(continued)
* 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

Question 17

Cyclic Redundancy Checksum
(continued)

Answer 17

Cyclic Redundancy Checksum
(continued)
Some standard generating polynomials:
CRC-12: x12 + x11 + x3 + x2 + x + 1
CRC-16: x16 + x15 + x2 + 1
CRC-CCITT: x16 + x15 + x5 + 1
CRC-32: x32 + x26 + x23 + x22 + x16 + x12 + x11 + x10 + x8 + x7 + x5 + x4 + x2 + x + 1
ATM CRC: x8 + x2 + x + 1

Question 18

Do Nothing (Toss the Frame/Packet)

Answer 18

Do Nothing (Toss the Frame/Packet)
* Seems like a strange way to control errors but some lowerlayer protocols such as frame relay perform this type of error
control
* For example, if frame relay detects an error, it simply tosses
the frame
– No message is returned
* Frame relay assumes a higher protocol (such as TCP/IP) will
detect the tossed frame and ask for retransmission

Question 19

Error Control

Answer 19

Error Control
* Once an error is detected, what is the receiver going to do?
– Do nothing (simply toss the frame or packet)
– Return an error message to the transmitter
– Fix the error with no further help from the transmitter

Question 20

Return A Message

Answer 20

Return A Message
* Once an error is detected, an error message is returned to
the transmitter
* Two basic forms:
– Stop-and-wait error control
– Sliding window error control

Question 20

Stop-and-Wait Error Control

Answer 20

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

Question 21

Sliding Window Error Control

Answer 21

Sliding Window Error Control
* These techniques assume that multiple frames are in
transmission at one time
* A sliding window protocol allows the transmitter to send a
number of data packets at one time before receiving any
acknowledgments
– Depends on window size
* When a receiver does acknowledge receipt, the returned
ACK contains the number of the frame expected next

Question 22

Sliding Window Error Control (continued)

Answer 22

Sliding Window Error Control (continued)
* Older sliding window protocols numbered each frame or
packet that was transmitted
* More modern sliding window protocols number each byte
within a frame
* An example in which the packets are numbered, followed by
an example in which the bytes are numbered:

Question 23

Sliding Window Error Control (continued)

Answer 23

Sliding Window Error Control (continued)
* Notice that an ACK is not always sent after each frame is
received
– It is more efficient to wait for a few received frames before returning
an ACK
* How long should you wait until you return an ACK?

Question 24

Sliding Window Error Control (continued)

Answer 24

Sliding Window Error Control (continued)
* Using TCP/IP, there are some basic rules concerning ACKs:
– Rule 1: If a receiver just received data and wants to send its own
data, piggyback an ACK along with that data
– 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
– 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

Question 25

What happens when a packet is lost?

Answer 25

What happens when a packet is lost?
– As shown in the next slide, if a frame is lost, the following frame will
be “out of sequence”
* The receiver will hold the out of sequence bytes in a buffer and request
the sender to retransmit the missing frame

Question 26

What happens when an ACK is lost?

Answer 26

What happens when an ACK is lost?
– As shown in the next slide, if an ACK is lost, the sender will wait for
the ACK to arrive and eventually time out
* When the time-out occurs, the sender will resend the last frame

Question 27

Correct the Error

Answer 27

Correct the Error
* For a receiver to correct the error with no 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
correction and involves codes called Hamming codes

Question 28

Correct the Error (continued)

Answer 28

Correct the Error (continued)
* Hamming codes add additional check bits to a character
– These check bits perform parity checks on various bits
* Example: One could create a Hamming code in which 4
check bits are added to an 8-bit character
– We can number the check bits c8, c4, c2 and c1
– We will number the data bits b12, b11, b10, b9, b7, b6, b5, and b3
– Place the bits in the following order: b12, b11, b10, b9, c8, b7, b6, b5,
c4, b3, c2, c1

Question 29

Correct the Error (continued)

Answer 29

Correct the Error (continued)
* Example (continued):
– c8 will perform a parity check on bits b12, b11, b10, and b9
– c4 will perform a parity check on bits b12, b7, b6 and b5
– c2 will perform a parity check on bits b11, b10, b7, b6 and b3
– c1 will perform a parity check on bits b11, b9, b7, b5, and b3
* The next slide shows the check bits and their values

Question 30

Correct the Error (continued)

Answer 30

Correct the Error (continued)
* The sender will take the 8-bit character and generate the 4
check bits as described
– The 4 check bits are then added to the 8 data bits in the sequence as
shown and then transmitted
* The receiver will perform the 4 parity checks using the 4
check bits
– If no bits flipped during transmission, then there should be no parity
errors
* What happens if one of the bits flipped during transmission?

Question 31

For example, what if bit b9 flips?

Answer 31

For example, what if bit b9 flips?
– The c8 check bit checks bits b12, b11, b10, b9 and c8 (01000)
* This would cause a parity error
– The c4 check bit checks bits b12, b7, b6, b5 and c4 (00101)
* This would not cause a parity error (even number of 1s)
– The c2 check bit checks bits b11, b10, b7, b6, b3 and c2 (100111)
* This would not cause a parity error

Question 32

For example, what if bit b9 flips? (continued)

Answer 32

For example, what if bit b9 flips? (continued)
– The c1 check bit checks b11, b9, b7, b5, b3 and c1 (100011)
* This would cause a parity error
– Writing the parity errors in sequence gives us 1001, which is binary
for the value 9
* Thus, the bit error occurred in the 9th position

Question 33

Error Detection In Action

Answer 33

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