Module 3.2 - Error, Error Detection, and Error Control

Midterm Topic

TRUE

• Noise is always present; If a communications line experiences too much noise, the signal will be lost or corrupted

White Noise

• Also known as thermal or Gaussian noise

• Relatively constant and can be reduced

• If this gets too strong, it can completely disrupt the signal

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

• It can damage more bits if the bits are closer together (transmitted at a faster rate)

Crosstalk

• Unwanted coupling between 2 different signal paths

• For example, hearing another conversation while talking on the telephone

• Relatively constant and can be reduced with proper measures

Echo

• The reflective feedback of a transmitted signal as the signal moves through a medium

• Most often occurs on coaxial cable

• If this is bad enough, it could interfere with the original signal

• Relatively constant, and can be significantly reduced

Jitter

• The result of small timing irregularities during the transmission of digital signals

• Occurs when a digital is repeated over and over

• If serious enough, it forces systems to slow down their transmission

• Steps can be taken to reduce it

• the unwanted, short-term deviation of a digital signal's active edges from their ideal positions in time. It causes timing uncertainty in electronics and networks, which can lead to data errors, bit flips, or audio/video distortion

Delay Distortion

• Occurs because the velocity of propagation of a signal through a medium varies with the frequency of the signal

• Can be reduced

Attenuation

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

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

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 is called an error detection code

• 3 basic techniques for detecting errors: parity checking, arithmetic checksum, and cyclic redundancy checksum

Simple Parity

• An error detection technique:

• If performing an even parity, add a parity bit such that an even number of 1s are maintained

• Only catches odd numbers of bit errors

Longitudinal Parity

• An error detection technique:

• 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

• Is better at catching errors but requires too many check bits added to a block of data

TRUE

• Both simple and longitudinal parity do not catch all errors

Arithmetic Checksum

• An error detection technique:

• Used in TCP and IP on the Internet

• Characters to be transmitted are converted to numeric form and summed

• The 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

• TCP and IP process is a little more complex but the idea is the same

• It can still let errors slip through

Cyclic Redundancy Checksum (CRC)

• An error detection technique:

• It treats the packet of data to be transmitted as a large polynomial

• The transmitter takes the message polynomial and using polynomial arithmetic, divides it by a given generating polynomial

• The 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

• 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

Do Nothing (Toss the Frame/Packet)

(Error Control)

• Seems like a strange way to control errors but some lower-layer protocols such as frame relay perform this type of error control

• For example, if frame relay detects an error, it simply tosses the frame. (Frame relay assumes a higher protocol (such as TCP/IP) will detect the tossed frame and ask for retransmission)

• No message is returned

Return a Message

(Error Control)

• Once an error is detected, an error message is returned to the transmitter

• 2 Basic forms: Stop-and-wait error control, sliding window error control

Stop and Wait Error Control

(Error Control - Return a Message)

• Is the simplest of the error control protocols

• A transmitter sends a frame then stops and waits for an acknowledgement

• If a positive ACK is received, the next frame is sent, if a negative ACK is received, the same frame is transmitted again

Sliding Window Error Control

(Error Control - Return a Message)

• Allows the transmitter to send a number of data packets at one time before receiving any acknowledgements; depends on window size

• When a receiver does acknowledge the receipt, the returned ACK contains the number of the frame expected next

Forward Error Correction

(Error Control)

• For a receiver to correct the error w/ no further help from the transmitter requires a large amount of redundant information to accompany the original data: this redundant info allows the receiver to determine the error and make corrections

• This involves codes called Hamming codes: add additional check bits to a character

• This is used in the transmission of radio signals such as those used in the transmission of digital television