Automatic Repeat Query (ARQ) (Cambridge International IGCSE Computer Science)

Spec point — 2.2.4 · "4 Describe how an automatic repeat query (ARQ) can be used to establish that data is received without error"

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

Exam Hint 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