Circuit Switching & Packet Switching
When data travels across a network, the path it takes depends on the switching method. Circuit switching reserves a dedicated path for the entire communication, while packet switching breaks data into packets that find their own way independently.
Learning Objectives
- 12.6.1.1 Explain the difference between packet switching and circuit switching
Conceptual Anchor
Phone Call vs Post Office
Circuit switching is like a phone call — a dedicated line is reserved between you and the other person for the entire conversation. No one else can use that line. Packet switching is like sending postcards — each postcard can take a different route through the postal system, and they may arrive out of order, but the recipient reassembles them.
Rules & Theory
Circuit Switching
1 How It Works
1. A dedicated path is established BEFORE data transfer
2. The entire path is reserved for the duration of communication
3. Data flows continuously along this fixed path
4. The connection is released when communication ends
Example: Traditional telephone call
Phone A ──── Switch 1 ──── Switch 2 ──── Phone B
(dedicated circuit held open for entire call)Packet Switching
2 How It Works
1. Data is broken into small PACKETS (each with a header)
2. Each packet is routed INDEPENDENTLY — may take different paths
3. Routers forward packets based on destination address
4. Packets may arrive out of order — reassembled at destination
Example: Sending an email
Packet 1 → Router A → Router C → Destination
Packet 2 → Router A → Router B → Router C → Destination
Packet 3 → Router A → Router D → Destination
(packets reassembled in correct order)Comparison Table
| Feature | Circuit Switching | Packet Switching |
|---|---|---|
| Connection | Dedicated path established first | No dedicated path — packets routed independently |
| Path | Fixed for entire session | Each packet may take a different route |
| Bandwidth | Reserved — wasted if not used | Shared — efficient use of bandwidth |
| Delay | Setup delay, then constant speed | No setup delay, but variable delivery time |
| Reliability | If a node fails, entire connection breaks | Packets can be re-routed around failures |
| Order | Data arrives in order | Packets may arrive out of order |
| Cost | Expensive — resources reserved | Cost-effective — resources shared |
| Best for | Real-time: voice calls, video calls | Data: web, email, file transfer |
| Example | Traditional telephone (PSTN) | The Internet |
Packet Structure
3 What's Inside a Packet?
┌─────────────────────────────────────────┐
│ PACKET │
├──────────────┬──────────────────────────┤
│ HEADER │ PAYLOAD (Data) │
├──────────────┤ │
│ Source IP │ │
│ Destination IP│ "Hello, how are you?" │
│ Packet number │ │
│ Total packets │ │
│ TTL / Checksum│ │
└──────────────┴──────────────────────────┘Common Pitfalls
"Packet Switching Is Always Better"
Not always! For real-time communication (phone calls, live video), circuit switching provides consistent quality with no jitter or delays. Packet switching is better for data because it's more efficient, but voice/video can suffer from packet loss and reordering.
Thinking All Packets Take the Same Route
In packet switching, each packet is routed independently. Two packets from the same message may travel completely different routes and arrive at different times.
Tasks
Define circuit switching and packet switching in your own words.
Explain why a traditional phone call uses circuit switching while the Internet uses packet switching.
Draw a diagram showing 3 packets being sent from device A to device B taking different routes through 4 routers.
Compare circuit switching and packet switching. Give 2 advantages and 2 disadvantages of each.
Self-Check Quiz
Q1: What happens to bandwidth in circuit switching when no data is being sent?
Q2: Can packets arrive out of order in packet switching?
Q3: Which switching method does the Internet use?