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Cambridge A-Level Computer Science 9618 – Topic 14.2 Circuit Switching, Packet Switching

14.2 Circuit Switching and Packet Switching

Learning Objective

Show a clear understanding of circuit switching, its operation, advantages, disadvantages and how it differs from packet switching.

What is Circuit Switching?

Circuit switching is a method of communication where a dedicated communication path is established between two endpoints for the duration of a session. The path consists of a series of physical or logical channels that remain reserved exclusively for the call until it is terminated.

How Circuit Switching Works

Connection Establishment – A signalling protocol (e.g., SS7) is used to reserve a continuous path through the network.

Data Transfer – Once the circuit is established, data flows continuously at a constant rate.

Connection Release – When the communication ends, the reserved resources are released for other users.

Key Characteristics

Fixed bandwidth is allocated for the whole session.

Latency is low and predictable because the path is dedicated.

Resource utilisation can be inefficient if the user does not use the full bandwidth.

Mathematical Representation

The total capacity \$C\$ of a link is divided among \$n\$ simultaneous circuits, each receiving a bandwidth \$B\$:

\$ B = \frac{C}{n} \$

If a circuit is idle for a fraction \$p\$ of the time, the effective utilisation \$U\$ of that circuit is:

\$ U = 1 - p \$

Advantages of Circuit Switching

Guaranteed bandwidth – no contention once the circuit is set up.

Predictable performance – constant delay and jitter.

Simplicity of protocol – data is transmitted as a steady stream.

Disadvantages of Circuit Switching

Inefficient use of resources – idle periods waste reserved bandwidth.

Scalability limits – the number of simultaneous circuits is bounded by the total link capacity.

Setup time – establishing a circuit adds initial latency before data can be sent.

Comparison with Packet Switching

Aspect	Circuit Switching	Packet Switching
Resource Allocation	Dedicated path for the whole session	Resources shared; packets routed independently
Bandwidth Utilisation	Often low due to idle periods	Higher overall utilisation; bandwidth used only when packets are present
Latency	Predictable, low after setup	Variable; depends on network congestion
Scalability	Limited by number of circuits that can be simultaneously reserved	Highly scalable; many users share the same links
Typical Use Cases	Traditional telephone networks, real‑time voice/video with strict QoS	Internet data traffic, email, web browsing, modern \cdot oIP (over packet networks)

Illustrative Example

Consider a 1 Gbps link that can support up to 100 simultaneous voice calls, each requiring 10 Mbps. If only 30 calls are active, the remaining 70 circuits are idle, wasting \$70 \times 10\text{ Mbps} = 700\text{ Mbps}\$ of potential capacity.

Suggested diagram: A schematic showing a dedicated path (circuit) from Sender A to Receiver B across several switching nodes, contrasted with a packet-switched network where packets take different routes.

Summary

Circuit switching provides a guaranteed, low‑latency communication channel by reserving a fixed portion of network resources for the entire duration of a session. While this ensures predictable performance, it can lead to inefficient utilisation of bandwidth, especially when traffic is bursty or intermittent. Understanding these trade‑offs is essential when comparing circuit‑switched systems with packet‑switched alternatives, which aim to maximise network utilisation at the cost of variable latency.

Show understanding of circuit switching