Error Detection Flashcards
Error correction definition ?
Is a systems ability to detect the presence of errors cause by the noise or other impairments during a transmission
What do all error detections codes transmit ?
More bits than contained in the original data
Most codes are systematic, what does the transmitter send if this is the case ?
A fixed number of original data but followed by a fixed number of check bits
What does the receive terminal do with the check bits ?
Applies them to the data received and compares them to see if the values match
What are repetition schemes ?
Data is broken up into blocks and each block is sent a set number of times
What problems can arise in repetition schemes ?
If there is an error in the same place in all repeated blocks
What are parity schemes ?
They are error detecting schemes that add extra. I art digits at the end of each word called a parity check bit
What two versions of parity schemes are there ?
And what does each one do ?
Even and odd parity schemes
Even makes sure there is an even amount of 1’s
Odd makes sure there is an odd number of 1’s
What is not detectable within party checks ?
If there is an even amount of errors
Error correction definition ?
The ability of the receive terminal to reconstruct the original transmitted data stream error free
What are the two basic forms of error correction systems ?
Automatic repeat request or ARQ
And
Forward error correction or FEC
What is the basic premise of automatic repeat request systems ?
The transceiver and receiver uses acknowledgements and time outs to tell each other if data is being sent error free
What are the three ARQ’s automatic repeat requests ?
Stop and wait ARQ
Go back N ARQ
Selective repeat ARQ
What is stop and wait ARQ
The simplest method of automatic repeat request,
The transmit terminal send one frame at a time and waits to receive an acknowledgement from the receiver before sending the next one
What happens if the stop and wait ARQ doesn’t receive an acknowledgment?
It will send the frame again until is does receive an acknowledgment or reaches a predetermined limit of attempts to send the data
What is go back N ARQ
The transmit terminal sends a number of frames predetermined, without waiting for a acknowledgement
The receiver keeps track of the sequence of frames it receives, if the sequence doesn’t match it will ignore, and send an acknowledgement of which frames it’s received
The transmit terminal will go back and send from the last acknowledgement
What are the advantages and disadvantages of go back n ARQ ?
It is more efficient than stop and wait ARQ as the connection is still being utilised
However it could result in frames being sent multiple times
Explain selective repeat ARQ ?
The receive terminal will work like go back n ARQ however will continue to accept frames sent after a frame error is detected
It will let the transmit terminal know which frames it is missing which will send that one, before going back to where it left off
In forward error correction what does the transmit terminal encode the data with ?
An error correcting code or ECC
What doesn’t the receive terminal do in FEC ?
It does not send any messages back to the transmit terminal
Explain forward error correction?
FEC is accomplished by adding redundancy to the transmitted information
That is it looks at the most common sample and deduces what if thinks the right output is
How many bits do FEC codes normally look at ?
Anywhere between a few dozen to several hundred received bits
What are the advantages of forward error correction ?
A back channel is not required and retransmission of data can often be avoided
When is forward error correction often employed ?
In situations where retransmission of data is either relatively costly or impossible
What are two forms of error detection schemes?
Repetition schemes
Parity schemes
