CSZoneCambridge International AS & A Level Computer Science 9618
Circuit Switching
Dedicated End-to-End Path
HOW IT WORKS
A dedicated physical circuit is established between sender and receiver before any data is sent
The path remains reserved for the entire duration of the call
Circuit released only when the call ends
Traditional telephone network (PSTN) uses circuit switching
ADVANTAGES
Guaranteed bandwidth — no contention once connected
No delays due to routing decisions during call
Consistent quality — good for voice/real-time calls
DISADVANTAGES
Circuit reserved even when no data flowing (wasted bandwidth)
Setup time before transmission begins
Inefficient for bursty data (web browsing)
Packet Switching
Data Split into Packets, Routed Independently
Data is broken into packets. Each packet travels independently across the network, potentially taking different routes. Reassembled at the destination.
Packet structure:
HEADER src IP · dest IP seq no · TTL
PAYLOAD Actual data
TRAILER Error check (CRC)
ADVANTAGES
Network bandwidth shared — very efficient
No setup time — first packet sent immediately
Resilient — packets reroute around failed nodes
Works well for bursty data (internet browsing)
DISADVANTAGES
Packets may arrive out of order — reordering overhead
Variable delay (jitter) — bad for real-time voice/video
Router processing adds latency for each hop
Routers & Routing
Directing Packets Across Networks
ROUTER FUNCTION
Examines destination IP address in each packet header
Looks up best next hop in routing table
Forwards packet to next router towards destination
Operates at Internet (IP) layer of TCP/IP stack
Connects different networks together
ROUTING TABLE
Stores: destination network → next hop router address. Built by routing protocols (e.g. OSPF, BGP). Updated dynamically when network topology changes.
TTL (TIME TO LIVE)
Each packet has a TTL counter. Decremented by 1 at each router. If TTL reaches 0, packet is discarded — prevents infinite loops.
Exam Practice
Cambridge-style questions
Question 1
Describe two differences between circuit switching and packet switching when transmitting data across a network. [4]
1+1
In circuit switching a dedicated physical path is established and reserved for the entire session; in packet switching no dedicated path exists — each packet is routed independently and can take different routes.
1+1
In circuit switching bandwidth is guaranteed (even if not fully used during idle periods), wasting resources; in packet switching bandwidth is shared between users efficiently, but variable delays (jitter) may occur.
Common Mistakes
Don't lose easy marks
1
Saying circuit switching is "for computers" and packet switching is "for phones" — it's the opposite historically. Traditional landline telephony = circuit switching. The Internet = packet switching. Modern VoIP uses packet switching over the internet.
2
Forgetting to mention the packet header contents — Cambridge mark schemes expect: source IP address, destination IP address, sequence number (to reorder packets at destination), TTL. Missing sequence number is a common omission.
3
Confusing router (layer 3, IP addresses) with switch (layer 2, MAC addresses). Routers connect different networks. Switches connect devices on the same network. Don't use them interchangeably.
Topic Summary — 3.2.2
What You Need to Know
CIRCUIT SWITCHING
Dedicated physical path established before transmission. Guaranteed bandwidth. Wasted capacity during idle. Traditional telephony (PSTN).
PACKET STRUCTURE
Header (src IP, dest IP, seq no, TTL) + Payload (data) + Trailer (CRC checksum). TTL prevents infinite loops.
PACKET SWITCHING
Data split into packets routed independently. Efficient bandwidth use. Resilient (reroute around failures). Variable delay (jitter). Packets may arrive out of order — sequence numbers used to reassemble. The Internet uses packet switching.
CSZone
Next Video
3.2.3
Wireless Networking
Wi-Fi · CSMA/CA · Security · Bluetooth · 5G
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