Packet trailer

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Quick Questions

Shade in the A or B circle for each question.

1. Which part of a packet comes last?

A The trailer

B The header

2. What does the packet trailer do?

A Stores the destination address

B Marks the end of the packet

3. True or false: a packet trailer can hold error checking information.

A False

B True

4. Which item belongs in the packet trailer?

A An end of packet marker

B The packet number

5. Which shows the packet parts in the correct order?

A Header, payload, trailer

B Header, trailer, payload

Match the Pairs

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

Left column		Right column
1packet trailer		AThe last part of a packet, which marks where the packet ends
2trailer: end_marker, check_value		Bif check_value(payload) != trailer.check: resend()
3The trailer is the last part of a packet, after the header and the payload		Cpacket = header + payload + trailer
4The trailer can hold error checking information that the receiver compares with the data it received		DA trailer holding an end of packet marker and error checking information
5packet = trailer + payload + header		EThe trailer is placed at the start of the packet. It should be the last part, after the payload, so the packet is header + payload + trailer.
6trailer = destination address, packet number		FThe destination address and packet number belong in the header, not the trailer. The trailer holds the end marker and error checking information.

Definitions

packet trailer

Fill in the Blanks

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

packet = [header, payload, _______]

endPart = packet._______

IF packet._______ = "END" THEN OUTPUT "Packet has ended" ENDIF

trailer = [endMarker, __________]

IF _______.errorCheck = "OK" THEN OUTPUT "Data arrived correctly" ENDIF

Spot the Error

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

Question 1

A last_part ← payload

B trailer.marks ← end_of_packet

C OUTPUT last_part

Question 2

A order ← [head, data, trailer]

B last ← order[1]

C OUTPUT last

Question 3

A READ trailer

B IF trailer found THEN

C end_reached ← FALSE

Question 4

A READ part

B IF part = payload THEN

C OUTPUT "End of packet"

D ENDIF

Question 5

A trailer ← check_value

B received ← calc(payload)

C IF received ≠ trailer THEN

D OUTPUT "Data OK"

Answer Key

Packet trailer

Section A: Quick Questions

A — The trailer

B — Marks the end of the packet

B — True

A — An end of packet marker

A — Header, payload, trailer

Section B: Match the Pairs

packet trailer → The last part of a packet, which marks where the packet ends

trailer: end_marker, check_value → A trailer holding an end of packet marker and error checking information

The trailer is the last part of a packet, after the header and the payload → packet = header + payload + trailer

The trailer can hold error checking information that the receiver compares with the data it received → if check_value(payload) != trailer.check: resend()

packet = trailer + payload + header → The trailer is placed at the start of the packet. It should be the last part, after the payload, so the packet is header + payload + trailer.

trailer = destination address, packet number → The destination address and packet number belong in the header, not the trailer. The trailer holds the end marker and error checking information.

Section C: Definitions

packet trailer — The last part of a packet. It marks where the packet ends and can hold error checking information, so the receiving device can check the data arrived correctly.

Section D: Fill in the Blanks

packet = [header, payload, trailer]

endPart = packet.trailer

IF packet.trailer = "END" THEN
  OUTPUT "Packet has ended"
ENDIF

trailer = [endMarker, errorCheck]

IF trailer.errorCheck = "OK" THEN
  OUTPUT "Data arrived correctly"
ENDIF

Section E: Spot the Error

A — The trailer is the last part of a packet. The payload is the data in the middle, so line 1 should be last_part ← trailer.

B — The trailer is the third and final part of the packet, so the last part is order[3]. order[1] would give the header.

C — Once the trailer is found, the end of the packet has been reached, so end_reached should be set to TRUE.

B — The trailer marks the end of the packet, not the payload. The test should be IF part = trailer THEN.

C — The data is only OK if the check value calculated on arrival matches the one in the trailer. The test should use = not ≠.