Automatic Repeat Query (ARQ) (Cambridge (CIE) O Level Computer Science)
Revision Note
Written by: Robert Hampton
Reviewed by: James Woodhouse
Automatic Repeat reQuests(ARQ)
What is an automatic repeat request(ARQ)?
An automatic repeat request is a protocol that notifies the sender that an error has occurred and that the data received is incorrect
It works as follows:
If an error is detected the receiver sends a negative acknowledgement transmission to indicate the data is corrupted
If no error is detected the receiver sends a positive acknowledgement transmission meaning the data is correct
If the receiver does not send any acknowledgement transmission then the sender waits for a certain time period known as a time-out before automatically resending the data
This process is repeated until all data has been received and acknowledged
Examiner Tips and Tricks
In this specification ARQ is referred to as Automatic Repeat Query but in past exam questions it has been referred to as Automatic Repeat reQuests
Both words are interchangeable and should not cause any confusion!
Worked Example
Explain how Automatic Repeat reQuests (ARQ) are used in data transmission and storage
[2]
Answer
Any two from:
Set of rules for controlling error checking/detection // it’s an error detection method // used to detect errors
Uses acknowledgement and timeout
Request is sent (with data) requiring acknowledgement
If no response/acknowledgment within certain time frame data package is resent
When data received contains an error a request is sent (automatically) to resend the data
The resend request is repeatedly sent until packet is received error free/limit is reached/acknowledgement received
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