Positive acknowledgement

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Quick Questions

Shade in the A or B circle for each question.

1. What does a positive acknowledgement tell the sender?

A The data arrived with an error and must be resent

B The data arrived without error

2. Who sends a positive acknowledgement?

A The receiver

B The sender

3. How is a positive acknowledgement often written?

A NAK

B ACK

4. When is a positive acknowledgement sent?

A Only after the error check passes

B Before the error check is done

5. The receiver gets a packet and its checksum matches. What should it send back?

A A positive acknowledgement (ACK)

B A negative acknowledgement

Match the Pairs

Draw a line connecting each item on the left to its match on the right.

Left column		Right column
1ACK		AAn ACK means the data arrived without error, so the sender should send the next data. Resending the same data is what happens after an error, not after an ACK.
2receiver sends ACK to sender		BIF ACK received THEN send next packet
3A positive acknowledgement is sent only after the error check passes		CThe receiver tells the sender that the data passed the error check and the sender can send the next data
4When the sender receives an ACK, it can send the next data		DAn ACK must only be sent after the error check passes. Sending it before the check means the receiver cannot know the data arrived without error.
5Receiver sends ACK before checking the data for errors		EIF errorCheck = PASS THEN SEND ACK
6Sender gets ACK so it sends the same data again		FShort for positive acknowledgement: a message from the receiver telling the sender the data arrived without error

Definitions

positive acknowledgement

Fill in the Blanks

Complete each line of code by filling in the missing parts.

IF errorCheck = PASS THEN SEND ___ ENDIF

IF errorCheck = PASS ____ SEND ACK ENDIF

RECEIVE reply IF reply _ ACK THEN SEND nextData

RECEIVE reply IF reply = ACK THEN ____ nextData

IF errorFound = _____ THEN SEND ACK ENDIF

Spot the Error

Shade in the letter of the line that contains an error.

Question 1

A RECEIVE packet

B CHECK packet for ACK

C IF no error THEN

D SEND ACK to sender

E ENDIF

Question 2

A SEND packet 1

B WAIT for reply

C IF reply = NAK THEN

D SEND packet 2

E ENDIF

Question 3

A RECEIVE packet

B CHECK parity of packet

C IF parity is wrong THEN

D SEND ACK

Question 4

A RECEIVE byte

B CHECK parity bit

C IF parity is correct THEN

D SEND NAK to sender

E ENDIF

Question 5

A RECEIVE packet 3

B IF check fails THEN

C SEND ACK for packet 3

D ENDIF

Answer Key

Positive acknowledgement

Section A: Quick Questions

B — The data arrived without error

A — The receiver

B — ACK

A — Only after the error check passes

A — A positive acknowledgement (ACK)

Section B: Match the Pairs

ACK → Short for positive acknowledgement: a message from the receiver telling the sender the data arrived without error

receiver sends ACK to sender → The receiver tells the sender that the data passed the error check and the sender can send the next data

A positive acknowledgement is sent only after the error check passes → IF errorCheck = PASS THEN SEND ACK

When the sender receives an ACK, it can send the next data → IF ACK received THEN send next packet

Receiver sends ACK before checking the data for errors → An ACK must only be sent after the error check passes. Sending it before the check means the receiver cannot know the data arrived without error.

Sender gets ACK so it sends the same data again → An ACK means the data arrived without error, so the sender should send the next data. Resending the same data is what happens after an error, not after an ACK.

Section C: Definitions

positive acknowledgement — A message sent by the receiver to tell the sender that data arrived without error. Often written ACK. It is sent only after the error check passes, so the sender can send the next data.

Section D: Fill in the Blanks

IF errorCheck = PASS THEN
    SEND ACK
ENDIF

IF errorCheck = PASS THEN
    SEND ACK
ENDIF

RECEIVE reply
IF reply = ACK THEN
    SEND nextData

RECEIVE reply
IF reply = ACK THEN
    SEND nextData

IF errorFound = FALSE THEN
    SEND ACK
ENDIF

Section E: Spot the Error

B — The receiver checks the packet for errors, not for an ACK. The ACK is what the receiver sends after the check passes. It should be CHECK packet for errors.

C — The sender may send the next packet only when a positive acknowledgement (ACK) arrives. A NAK means an error was found. It should be IF reply = ACK THEN.

C — An ACK is sent only when the error check passes. The condition must test that the parity is correct, not wrong.

D — When the check passes, the receiver sends a positive acknowledgement (ACK). NAK is the negative acknowledgement used when an error is found. It should be SEND ACK to sender.

B — The ACK means the data arrived without error, so it is sent only when the check passes. It should be IF check passes THEN.