Switching Methods (Cambridge International A-Level Computer Science)
Exam board Cambridge International · A-LevelUpdated 2026-10-01
Packetising
What is packetising?
- Packetising is the process of dividing a large message into smaller, manageable units called packets
- Each packet can be transmitted individually across a network
Structure of a packet
-
Each packet contains:
- A packet header, which contains key information such as:
- Source IP address: who sent the packet
- Destination IP address: where it is going
- Sequence number (packet number): used to reassemble packets in the correct order
- Checksum: used for error checking
- The data: the part of the message being sent in this packet
Packet transmission & reassembly
- Once packetised and encapsulated with headers, packets are transmitted independently
- They may travel along different routes through the network
- At the destination, packets are reassembled into the original message using sequence numbers
Circuit & packet switching
What is packet switching?
- Packet switching is a communication method where **data is broken into packets **and sent across a shared network
- Each packet may** take a different path to the destination** and is reassembled on arrival
Benefits:
- Efficient use of network bandwidth
- More reliable– only lost or corrupted packets need to be resent
- Lower cost – network resources are shared
Drawbacks:
- Not ideal for real-time applications like VoIP or video calls
- Packets can arrive out of order and require reassembly
- Network congestion may lead to packet delay or loss
What is circuit switching?
- Circuit switching sets up a dedicated communication path between two devices for the duration of a session (like a traditional phone call)
Benefits:
- Ideal for real-time communication – steady and continuous data flow
- No delays once the circuit is established
- Data arrives in order
Drawbacks:
- Less efficient – resources remain reserved even when not in use
- Higher cost due to dedicated lines
- Less scalable and flexible for large networks
Packet switching vs circuit switching
| Feature | Packet switching | Circuit switching |
|---|---|---|
| Definition | Data is split into packets and sent independently over shared routes | A dedicated path is set up for the duration of the communication |
| Transmission | Data is sent in individual packets | Data is sent in a continuous stream |
| Efficiency | High – uses bandwidth as needed, paths shared among many users | Low – resources are tied up for each communication, even when idle |
| Reliability | Robust – packets can be rerouted if a failure occurs | Less robust – if the path fails, the call is dropped and must be re-established |
| Scalability | Highly scalable – handles large volumes of data and users | Less scalable – more users require more physical paths |
| Use Cases | Best for emails, web pages, file downloads | Best for real-time communication (voice calls, video conferencing) |
| Data Order | May arrive out of order – requires reassembly | Always arrives in order |
| Cost | Lower – due to shared infrastructure | Higher – due to reserved bandwidth and infrastructure |
Exam Hint Speed can earn a mark when it is stated precisely and tied to the right method. Mark schemes have accepted "fast data transfer rate" as a benefit of circuit switching and "high data transmission rate is possible" as a benefit of packet switching. Back it up with the reason: circuit switching has the whole bandwidth of the channel available, while packet switching lets packets share bandwidth and take the best route depending on congestion.
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