Circuit Switching · Packet Switching · Domain Name System · DNS Hierarchy
CSZoneCambridge International AS & A Level Computer Science 9618
Circuit Switching
Dedicated End-to-End Connection
In circuit switching, a dedicated physical path is established between sender and receiver for the entire duration of the communication — like a telephone call.
ADVANTAGES
Guaranteed bandwidth — consistent quality
No delays once connected
DISADVANTAGES
Inefficient — bandwidth wasted when idle
Connection must be set up before use
If path fails, entire call drops
Packet Switching
Data Split into Packets, Routed Independently
In packet switching, data is divided into packets. Each packet may take a different route through the network and is reassembled at the destination.
ADVANTAGES
Efficient — multiple users share bandwidth
Fault tolerant — packets can be rerouted
No dedicated connection needed
DISADVANTAGES
Variable delay (latency/jitter)
Packets may arrive out of order
Used by: the Internet — all data (web, email, video) is sent as packets
Packet Structure
What's Inside a Packet?
PACKET COMPONENTS
Source IP address — where the packet came from
Destination IP address — where the packet is going
Packet number — for reassembly in correct order
Total number of packets — so receiver knows when all arrive
Payload (data) — the actual content being transmitted
Checksum — for error detection
DNS — Domain Name System
Translating Names to IP Addresses
DNS converts human-readable domain names (e.g. www.cszone.co.uk) into IP addresses (e.g. 185.42.19.1) — like a phone book for the internet.
1
User types www.cszone.co.uk in browser
2
Browser checks local DNS cache — if found, use cached IP
3
If not cached, query sent to recursive resolver (ISP's DNS server)
4
Resolver queries root name server → TLD server (.uk) → authoritative server
5
IP address returned, cached, and browser connects
Exam Practice
Cambridge-style questions
Question 1
Describe two advantages of packet switching over circuit switching.
4 marks
1 mark
Efficient use of bandwidth — multiple packets from different users share the same links simultaneously
1 mark
No dedicated path needed — links not reserved, so no wasted capacity when no data is being sent
1 mark
Fault tolerant — if a router fails, packets can be rerouted via alternative paths
1 mark
Circuit switching: bandwidth wasted when no data is being sent (idle connection)
Exam Practice
Question 2
Explain the role of DNS in accessing a website. Include what happens if the IP address is not in the local cache.
4 marks
1 mark
DNS translates a domain name (e.g. www.example.com) into an IP address
1 mark
Browser first checks local DNS cache — if found, uses cached IP immediately
1 mark
If not cached, queries recursive resolver (ISP's DNS server)
1 mark
Resolver queries root server → TLD server → authoritative name server until IP address is found and returned
Common Mistakes
Don't lose easy marks
1
Saying packets always take the same route — in packet switching, each packet is routed independently and may take different paths through the network. That's why they may arrive out of order.
2
Saying DNS "stores" websites — DNS only maps domain names to IP addresses. It does NOT store web content. The web server stores the actual page content.
3
Confusing circuit switching with "being connected to a circuit" — circuit switching refers to a dedicated reserved path (like PSTN telephone), not simply using a physical cable.
Topic Summary — 1.2.6
What You Need to Know
SWITCHING METHODS
Circuit: dedicated path, reserved, wasteful Packet: data split, different routes, efficient Packets: src IP, dst IP, number, payload, checksum Internet uses packet switching
DNS
Maps domain names → IP addresses Check cache → resolver → root → TLD → authoritative Result cached for future requests Without DNS: must type raw IP addresses
COMPARISON
Circuit: consistent quality, wastes bandwidth Packet: efficient, fault tolerant, variable delay Packet switching enables the modern internet Jitter: variable delay between packets