The shape of the lesson
By the end of the lesson, learners can
- Why packets?: A large file is split into many small packets of data before it is sent.
- Inside a packet: Look at a packet in the lab.
- Packet switching: Packets from one file travel independently.
- Header: destination address, originator's address, packet number.
- Payload: the actual data.
- Trailer: end of packet marker and an error check.
- Packet switching: split, number, different routes via routers, may arrive out of order, reorder when the last one arrives.
One picture every idea returns to
Data sent over a network is broken down into packets. Each packet has a header (destination address, originator's address and packet number), a payload (the actual data) and a trailer (marks the end of the packet and holds an error check). In packet switching each packet may take a different route, chosen by routers, so packets can arrive out of order; once the last packet arrives they are reordered using their packet numbers.
Forty-five minutes, phase by phase
| Time | Phase | What happens in the room | Grouping |
|---|---|---|---|
| 0 to 5 min | Hook: predict | Open with the question in the title, Split it into packets.. When you send a photo across the internet it does not travel as one big lump. Put the figure on the board: it is the one picture every idea in the lesson returns to. | Whole class, then pairs |
| 5 to 13 min | Teach and check: Why packets? | A large file is split into many small packets of data before it is sent. | Whole class, then pairs |
| 13 to 21 min | Teach and check: Inside a packet | Look at a packet in the lab. Project tab 1 of the lab (Packets and packet switching) and let a student drive it while the class predicts. | Pairs |
| 21 to 30 min | Teach and check: Packet switching | Packets from one file travel independently. Model the worked example Reordering on the board, one line at a time. Your turn: Why might packet 2 arrive after packet 4? | Jigsaw |
| 30 to 36 min | Check | The four Stage 1 questions from the student lesson, answered on mini whiteboards or by show of fingers. Re-teach the one most get wrong. | Individual, then whole class |
| 36 to 43 min | Plenary | The exam-style questions from Stage 2, one at a time: thirty seconds alone, then a random call. The feedback lines in the key give the examiner's wording. | Individual, random call |
| 43 to 45 min | Exit and homework | Set the worksheet below (6 questions, 16 marks). Learners self-assess against the objectives before they leave. | Individual |
Protect the parts that carry the learning
The sequence carries about fifty minutes of material, so the lab is driven by one student while the class predicts rather than explored freely. Times are a guide.
Protected: the check at 30 minutes. It is the only individual read on whether the key idea landed, and it is not cut.
Cut first if the build overruns: the last check in Packet switching, never the check or the plenary.
In a 60 minute block: run worksheet questions 1 to 3 in class as a Rally Coach set and model both the hardest worked example and its check live.
Jigsaw
Split the content into expert pieces. Expert groups master one piece each, then regroup so every home team has one expert per piece, and each expert teaches their piece to the team.
The full structure, with its variations and the other twelve, is on the cooperative-learning page.
What to head off, and how
| Trap learners fall into | Teaching move that pre-empts it |
|---|---|
| Packets do not make the data itself smaller. | The total amount of data is the same (in fact slightly more, because every packet adds a header and trailer). |
| The packet number goes in the header, not the trailer. | Learn the three header items exactly: destination address, originator's address, packet number. |
| It is the router that controls the route a packet takes. | A packet does not "choose" its own path. |
Packets and packet switching: worksheet
6 questions, 16 marks. Written in the style of the papers; no past paper question is reproduced.
Identify the three items of data stored in a packet header.
State what is stored in the payload and in the trailer of a packet.
Describe the process of packet switching when a file is sent from one computer to another across the internet.
Explain why the packets of one file may arrive at the destination in a different order from the order they were sent.
A video file is sent as 500 packets. Packet 237 is corrupted on the way. Explain one advantage of packet switching in this situation.
A student says: "The receiving computer uses the originator's address to reorder the packets." Explain why this is wrong.
Answer key and marking notes
- Destination address.
- Originator's (sender's) address.
- Packet number.
- Payload: the actual data being transmitted.
- Trailer: an end of packet marker and/or an error check such as a checksum.
- The file is broken down into packets.
- Each packet is given a header including a packet number and the destination address.
- Each packet could take a different route.
- Routers control the route each packet takes.
- Packets may arrive out of order; once the last packet arrives they are reordered using the packet numbers.
- Each packet can take a different route.
- Some routes are longer or busier (more traffic), so a packet sent later can arrive earlier.
- Only packet 237 needs to be resent.
- The other 499 packets do not need to be sent again, saving time and bandwidth.
- Every packet from the file has the same originator's address, so it cannot show the order.
- The packet number in the header is used to reorder the packets.
6 multiple-choice questions on this lesson
From the topic question bank. Assign them in the app for a marked score in your gradebook, or run the topic as a class game.
Which part of a packet contains the actual data being sent, such as part of an image file?
- AThe payload
- BThe header
- CThe trailer
- DThe packet number
Which set of items is stored in the header of a packet?
- ADestination address, originator's address and packet number
- BDestination address, payload and checksum
- CPacket number, end of packet marker and payload
- DOriginator's address, error check and route to follow
A video file is sent across the internet using packet switching. Packets 1 to 6 arrive in the order 2, 1, 5, 3, 6, 4. What happens next?
- AThe packets are reordered using their packet numbers
- BThe receiver asks for the whole file to be sent again
- CThe packets are joined in the order they arrived
- DThe router sends the packets back to be renumbered
In packet switching, what decides the route each packet takes across the network?
- ARouters along the way
- BThe trailer of the packet
- CThe receiving computer
- DThe packet number
Why is it an advantage that packets from the same file can take different routes?
- APackets can avoid a busy or broken route
- BPackets always arrive in the order in which they were sent
- CEach packet no longer needs a destination address
- DThe packets cannot be intercepted by a hacker
A large file is sent as 500 packets. Packet 237 is corrupted on the way. What is the main benefit of having split the file into packets?
- AOnly packet 237 needs to be sent again
- BThe corrupted packet is repaired by the router
- CThe other 499 packets are not checked for errors
- DThe file can be read without packet 237
Quiz key
The payload is the actual data being transmitted. The header holds addressing and ordering information (destination address, originator's address, packet number), not the file data. The trailer marks the end of the packet and holds an error check. The packet number is just one item inside the header.
The header holds the destination address, the originator's (sender's) address and the packet number. The payload is the data itself, not part of the header. The checksum and end of packet marker belong in the trailer. The route is not stored in the packet at all: routers decide it as the packet travels.
In packet switching, packets can take different routes and so arrive out of order. Once all have arrived, the receiver puts them back in order using the packet number in each header. Out-of-order arrival is not an error, so there is no need to resend the whole file. Joining them in arrival order would scramble the video. Routers forward packets; they do not renumber them.
Routers read the destination address and decide where to forward each packet, so packets from the same file may take different routes. The trailer only marks the end and holds an error check. The receiving computer only reassembles the packets after they arrive. The packet number is used for reordering, not routing.
Because each packet can be routed separately, routers can send packets around busy or failed links, so the data still gets through and the network is used efficiently. Different routes actually make out-of-order arrival more likely, not less. Every packet still needs a destination address so routers know where to send it. Taking different routes does not stop interception; encryption is needed for that.
Only the faulty packet has to be retransmitted, not the whole file, which saves time and bandwidth. Routers forward packets; they do not repair corrupted data. Every packet is still checked for errors (for example using the error check in its trailer). The file is incomplete without packet 237, so it must be resent before the file can be reassembled.
What to project
The student lesson embeds these tabs. Open them full screen on the projector; every slider has a drag handle on the stage.
Slides for this lesson
A projectable deck for every pack is on the way. Until then, present the student lesson full screen: its sections, figure and lab tabs follow the same order as the plan above.