📡 Paper 1 · 1.2 Communication and Internet Technologies
1.2.6 Circuit and Packet Switching, and DNS
Cambridge 9618 · International A Level Computer Science · ~14 min read
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Circuit Switching
In circuit switching, a dedicated physical path (circuit) is established between sender and receiver before communication begins, and this path is maintained for the entire duration of the communication.
The path is reserved exclusively — no other traffic can use those links
Guaranteed constant bandwidth and low latency once connected
Connection setup time required before data transfer
If no data is being sent, the circuit is still held — wasteful
Example: traditional telephone (PSTN) network
Packet Switching
In packet switching, data is split into small units called packets. Each packet is sent independently across the network and may take different routes to the destination, where packets are reassembled.
Each packet contains: header (source IP, destination IP, packet number, total packets), payload (data), and trailer (error checking)
Packets may arrive out of order — destination reassembles using packet numbers
No dedicated circuit — network resources are shared efficiently
If one route is congested, packets can take alternative routes
Used by the internet (IP, TCP)
Feature
Circuit Switching
Packet Switching
Path
Dedicated circuit reserved
No dedicated path — packets routed independently
Resource use
Inefficient — circuit held even if idle
Efficient — bandwidth shared between many users
Delay
Setup delay, then constant low latency
Variable latency (packets may queue at routers)
Reliability
Guaranteed bandwidth
No guaranteed bandwidth — best effort
Packet ordering
Not applicable
Packets may arrive out of order — must be reassembled
Example
Traditional phone call (PSTN)
Internet (TCP/IP)
DNS — Domain Name System
DNS is a hierarchical, distributed database system that resolves human-readable domain names (e.g. www.bbc.co.uk) into IP addresses (e.g. 151.101.128.81). Without DNS, users would need to remember IP addresses for every website.
DNS Resolution Process
User types www.example.com in browser
Browser checks its local cache — if found, uses cached IP
If not cached, query goes to the local DNS resolver (usually provided by ISP)
Resolver checks its cache; if not found, queries the root name server
Root server directs to the appropriate TLD (top-level domain) server (e.g. .com)
TLD server directs to the authoritative name server for that domain
Authoritative server returns the IP address
Browser connects to the IP address; response is cached for future use
DNS Structure
Root servers: top of the hierarchy — know the TLD name servers
TLD servers: for each top-level domain (.com, .org, .uk, .ac)
Authoritative name servers: know the IP addresses for their specific domain
DNS records: A record (domain → IPv4), AAAA record (domain → IPv6), MX record (mail server), CNAME (alias)
Exam tip: Cambridge often asks to describe DNS resolution step by step, or compare circuit vs packet switching. For circuit switching: dedicated path, constant bandwidth, wasteful if idle, used for phone calls. For packet switching: packets routed independently, efficient, out-of-order arrival, used by the internet. For DNS: know the hierarchy (root → TLD → authoritative) and the caching mechanism.
⚠️ Common Mistakes
Saying packet switching guarantees delivery — it does not; TCP (at the Transport layer) adds reliability on top of IP
Forgetting that DNS results are cached — this is crucial for efficiency
Saying circuit switching is used on the internet — the internet uses packet switching
Forgetting that packets can take different routes and arrive out of order — they must be reassembled using packet numbers
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Worksheet — 1.2.6 Circuit/Packet Switching & DNS
8 questions · instantly marked · Cambridge 9618 standard
Q1Describe how circuit switching works and give one advantage and one disadvantage.[4]
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A dedicated physical path is established before communication and held for the entire session [1]; Advantage: guaranteed constant bandwidth / no variable latency / no out-of-order packets [1]; Disadvantage: bandwidth wasted when no data is being transmitted — circuit still held / inefficient use of network resources [1]; [4th mark: suitable example e.g. traditional telephone network] [1].
Q2Explain what a packet contains and why packet numbers are needed.[4]
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A packet contains a header (source/destination IP, packet number, total packets) [1]; a payload (data being transmitted) [1]; and a trailer (error-checking data such as a checksum) [1]; Packet numbers are needed because packets may take different routes and arrive out of order — the destination uses packet numbers to reassemble them in the correct order [1].
Q3A company wants to set up a video conferencing system. Compare circuit switching and packet switching and explain which is more suitable.[4]
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Circuit switching provides guaranteed bandwidth and constant latency — good for real-time communication; packet switching has variable latency but is more efficient [2]; For video conferencing, either could be justified [1]; Packet switching is the modern standard and allows efficient sharing of network resources while codecs handle minor quality variation; circuit switching would give guaranteed quality but is wasteful and expensive [1].
Q4Explain the purpose of DNS. Why is it necessary?[3]
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DNS (Domain Name System) resolves domain names (e.g. www.bbc.co.uk) into IP addresses [1]; it is necessary because computers communicate using IP addresses but humans find it easier to remember domain names than numerical IP addresses [1]; DNS acts as a distributed phone book for the internet [1].
Q5Describe the steps that occur when a browser resolves www.example.com to an IP address using DNS.[5]
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Browser checks local cache — if IP found, uses it [1]; if not, query sent to local DNS resolver (ISP) [1]; resolver queries root name server, which directs to TLD server (.com) [1]; TLD server directs to the authoritative name server for example.com [1]; authoritative server returns IP address; result is cached [1].
Q6Why is DNS caching important for network performance?[3]
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DNS caching stores the results of previous DNS queries locally [1]; when the same domain is requested again, the IP address can be returned immediately without querying the DNS hierarchy again [1]; this reduces DNS query traffic on the network and speeds up page loading for frequently visited websites [1].
Q7State one advantage of packet switching over circuit switching for internet data transfer.[2]
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Packet switching is more efficient — bandwidth is only used when packets are actually being transmitted; in circuit switching the dedicated path is held even when no data is being sent [1]; Or: fault tolerance — if one route is congested or fails, packets can be rerouted via alternative paths [1].
Q8Describe the role of the authoritative name server in the DNS hierarchy.[2]
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The authoritative name server is the final authority for a specific domain [1]; it stores the actual DNS records (A records etc.) and returns the IP address for that domain to the requesting DNS resolver [1].
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Mini Test — 1.2.6 Switching & DNS
10 questions · 10 marks · 10 minutes
⏱ 10:00
10 marks
Section A — Multiple Choice [5 marks]
Q1Which switching method is used by the internet?
Q2In packet switching, packets may arrive out of order. What allows them to be reassembled correctly?
Q3What is the main disadvantage of circuit switching compared to packet switching?
Q4What does a DNS server do?
Q5What is the purpose of DNS caching?
Section B — Short Answer [5 marks]
Q6State what is stored in a packet's header.
Mark schemeSource IP address, destination IP address, packet number (sequence number), total number of packets [1 mark for any two of these].
Q7State one advantage of packet switching over circuit switching.
Mark schemeMore efficient use of bandwidth — links only used when packets are being transmitted / fault tolerant — packets can be rerouted if one path fails [1 mark for either].
Q8Name the three levels in the DNS hierarchy.
Mark schemeRoot name server, TLD (Top Level Domain) name server, Authoritative name server [1 mark for all three].
Q9What is the first thing a browser checks when resolving a domain name?
Mark schemeIts local cache — if the IP address has been previously looked up and cached, it is used immediately without querying DNS [1 mark].
Q10Traditional telephone networks use circuit switching. State one advantage this gives for voice calls.
Mark schemeGuaranteed constant bandwidth and low, predictable latency — no variation in call quality once the circuit is established [1 mark].