Switching Methods (Cambridge (CIE) A Level Computer Science): Revision Note
Exam code: 9618
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 typically contains:
Header – contains control information such as:
Source IP address – who sent the packet
Destination IP address – where it's going
Sequence number – used to reassemble packets in the correct order
Protocol – e.g. TCP or UDP
Packet length – the size of the packet
Checksum – used for error checking
Payload – the actual data being sent
Footer (trailer) – may include extra error-checking or control information
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 is another communication method where data is broken into packets and sent across a shared network
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 |
Examiner Tips and Tricks
Try to avoid talking about the speed of data transmission in an answer to a question on packet or circuit switching. This will not get you a mark in the exam and, in some questions, is explicitly stated as not worthy of a mark. It is better to talk about higher bit rates or bandwidth (the number of bits sent per second) or the efficiency of the transmission
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