Learning Objectives
- Describe every stage in packet switching from message division to reconstruction.
- Explain the role of a router in selecting a route.
- Explain why packets from the same message can take different routes.
- Explain why packets may arrive out of order.
- Describe how the destination reorders packets after the last packet has arrived.
Key Terms
- Packet switching
- A transmission method in which a message is divided into packets that are routed independently.
- Router
- A network device that examines addressing information and directs a packet along a route.
- Route
- The path taken by a packet through interconnected networks.
- Reordering
- Placing received packets into their original sequence using packet numbers.
- Congestion
- Heavy network traffic that can make a route slower or less available.
The Packet-Switching Process
Packet switching begins when the sending device divides the data into packets. Each packet receives a header containing the destination address, originator address and packet number. The packets are then passed into the network.
Routers receive packets and decide where to forward them next. A router uses addressing information and the routes currently available. It does not need to keep every packet from one message together. Therefore, packet 1 may be sent through one route while packet 2 is sent through another.
At the destination, packets are collected. They can arrive in a different order because their paths and delays may differ. After the final required packet has arrived, the packet numbers are used to reorder the payloads. The pieces are then combined to recreate the original data.
- Data is broken down into packets.
- Each packet can take a different route.
- Routers control the route taken by packets.
- Packets may arrive out of order.
- After the last packet arrives, the packets are reordered.
Why Routes Can Differ
A network usually contains several interconnected routes between a sender and a receiver. Conditions can differ from moment to moment. One link may be busy, unavailable or slower than another. A router can forward a packet using an available route rather than waiting for a single fixed path.
The packets are independent units because each carries the destination address. A later packet can therefore be sent along a route that is different from an earlier packet. The route is not stored as a complete end-to-end plan inside the packet; routers make forwarding decisions as the packet travels.
Different routes can have different numbers of links, different traffic levels and different delays. This explains why the sending order is not guaranteed to be the receiving order.
Arrival, Checking And Reconstruction
The receiving system records the packet numbers that arrive. If packets 4, 2, 1 and 3 are received, they are stored until they can be placed in the sequence 1, 2, 3, 4. The payload from each packet is then joined in that order.
If a packet is missing or found to contain an error, a retransmission process may be required. Error detection and automatic repeat query are covered later in this volume. In a packet-switching answer, the key focus is independent routing, possible out-of-order arrival and reordering at the destination.
The destination must know when it has the full set of packets. Once the last packet and all preceding numbered packets are present, reconstruction can be completed.
Advantages Of Independent Routing
Independent routing allows the network to continue operating when one path is busy or unavailable, provided another route exists. It also allows links to be shared among many transmissions.
The examination may present a network diagram and ask which route a packet could take. There may be more than one acceptable path. A good explanation states that a router directs each packet and that packets belonging to the same message do not have to follow the same route.
Packet Switching In Order
| Stage | What Happens |
|---|---|
| 1 | The sender divides the data into packets. |
| 2 | A header is added to every packet. |
| 3 | Routers direct packets through the network. |
| 4 | Different packets may follow different routes. |
| 5 | Packets may arrive out of order. |
| 6 | The receiver uses packet numbers to reorder them and reconstruct the message. |
Cause And Effect
| Cause | Effect |
|---|---|
| Different routes have different delays | Packets can arrive out of order. |
| A route becomes busy or unavailable | A router may send later packets by another route. |
| A packet number is missing | The receiver can identify that a packet has not arrived. |
Worked Examples
Tracing Two Possible Routes
Question: A packet can travel S-A-C-D or S-B-D. Explain why two packets may arrive in reverse order.
- Packet 1 may be sent along S-A-C-D.
- Packet 2 may be sent later along the shorter or less busy route S-B-D.
- Packet 2 can reach D before packet 1.
- The destination uses packet numbers to restore the original order.
Answer: Independent routes and different delays can reverse the arrival order.
Reordering Packets
Question: Packets arrive in the order 5, 2, 1, 4, 3. State the reconstruction order.
- Read each packet number.
- Sort the packet numbers into ascending sequence.
- Join the payloads using that sequence.
Answer: 1, 2, 3, 4, 5.
Router Role
Question: A link becomes unavailable after packet 6 is sent. What happens to later packets?
- The next router examines the destination address.
- It selects another available route if one exists.
- Later packets can still reach the destination by the alternative path.
Answer: The router can direct later packets along another route; the whole message does not need to restart from the sender.
Examination Guidance
- Write packet switching as a sequence of stages.
- Always mention routers when explaining route selection.
- State that each packet could take a different route, not that it must.
- Finish the process by explaining packet reordering and message reconstruction.
Common Mistakes
- Saying packets are guaranteed to arrive in order.
- Claiming the sender chooses one permanent route for the whole message.
- Forgetting to mention the router.
- Saying packets are reordered before transmission rather than after reception.
Knowledge Check
1. What device controls the route a packet takes?
2. Why may packet 4 arrive before packet 3?
3. What information allows reordering?
4. When is the original message reconstructed?
5. Can two packets from the same message take different routes?
