OxfordAQA A-Level Computer Science: Networking and cyber security (9645) – Revision Notes

Every idea here is taught in full, with worked examples, in the study guide. These notes condense section 3.14 (Networking and cyber security) of the OxfordAQA International AS and A-level Computer Science (9645) specification, Version 1.1, for International AS exams May/June 2025 onwards and International A-level exams May/June 2026 onwards. Treat every line as International A-level only content; it sits in Unit 4, the written paper on sections 3.12 to 3.16.

Test yourself with the practice questions, tick off topics on the printable checklist, browse the course hub, or find gaps with a free diagnostic.

3.14.1 Communications

Term	Definition to learn
Serial	Bits sent one at a time down a single line
Parallel	Several bits sent at once down several lines
Synchronous	Continuous data stream; sender and receiver clocks kept in step by a timing signal
Asynchronous	Characters sent individually, framed by start and stop bits
Baud rate	Signal changes per second
Bit rate	Bits transmitted per second
Bandwidth	Range of frequencies a medium can carry
Latency	Time delay between sending and receiving
Protocol	Agreed set of rules for communication

Why serial beats parallel over distance: no skew between bits, no crosstalk between neighbouring wires, cheaper cable with fewer conductors.

Start bit: brings the receiver’s clock into phase with the sender’s. Stop bit: allows the next start bit to be recognised; in older devices it gave the receiver time to process the data.

Method: bit rate from baud rate

1. Count the signal levels, n.
2. Bits per signal change = log₂(n).
3. Bit rate = baud rate × bits per change.

Reminder: 1,200 baud with 8 levels → log₂(8) = 3 bits per change → 3,600 bit/s.

Bit rate and bandwidth: directly proportional. Double the bandwidth and the achievable bit rate doubles.

Asynchronous overhead: with 1 start bit, 8 data bits and 2 stop bits, each character costs 11 bits, so only 8/11 ≈ 72.7% of the bits are data.

3.14.2 Networking

Peer-to-peer vs client-server

	Peer-to-peer	Client-server
Status of computers	All equal	Servers provide services; clients request them
Typical use	Small home network, sharing files between a few users	School or business with central files, email, logins
Strength	Cheap, no dedicated server	Central backups, security and user management
Weakness	No central control; files scattered	Server cost; server failure affects everyone

Thin vs thick client

	Thin client	Thick client
Processing and storage	Mostly on the server	On the local machine
Hardware needs	Simple, cheap terminals; powerful server	Capable local hardware
Network needs	Fast, reliable network essential	Lighter network load
Management	Central updates and security	Each machine maintained separately

Copper vs fibre: copper is cheaper and easy to fit but slower, lower capacity and open to electrical interference; fibre is costlier but faster, higher capacity, longer reach and immune to electrical interference. Pick fibre for long or high-traffic links, copper for short, low-cost runs.

Method: CSMA/CD (wired)

1. Listen to the medium; wait while it is busy.
2. When idle, transmit and keep listening.
3. If a collision is detected, send a jam signal.
4. Wait a random back-off time, then return to step 1.

Method: CSMA/CA (wireless), no RTS/CTS

1. Listen; if busy, wait a random back-off time and listen again.
2. When idle, transmit the frame.
3. Wait for an acknowledgement from the receiver.
4. No acknowledgement → assume a collision, back off, retry.

With RTS/CTS: sender transmits a short Request to Send to the access point → access point broadcasts Clear to Send → all other stations stay quiet → data frame → acknowledgement. Fixes the hidden-node problem.

Wireless kit and security: wireless network adapter in each device, wireless access point for the network. The SSID names the network. Secure with WPA2 encryption, SSID broadcast disabled, MAC address allow list.

Wired vs wireless: wireless wins on mobility, set-up cost and adding devices; wired wins on speed, reliability and security.

3.14.3 The Internet

• Structure: a network of networks joined by high-capacity backbone links and routers; ISPs connect users.
• Packet switching: data split into packets that travel independently, may take different routes, and are reordered by sequence number.
• Routers: forward each packet towards its destination using routing tables; routing happens one hop at a time.
• Packet contents: source address, destination address, sequence number, time to live, payload, error detection/correction data.
• Time to live: reduced by 1 at each router; packet discarded at 0.

Must-know distinctions

Term	Meaning
IP address	Numeric address of a device on the network
Domain name	Readable name for an organisation’s network area
FQDN	Domain name plus host name, identifying exactly one host
URL	Complete address of a resource: protocol, FQDN, path

Domain hierarchy: root → top-level domain (generic such as .com, or country code) → organisation’s domain → subdomains/hosts. Read right to left.

Method: DNS lookup

1. Check the local cache.
2. Ask the DNS resolver.
3. Resolver asks a root server → referred to the top-level domain server.
4. Top-level domain server → refers to the domain’s authoritative name server.
5. Authoritative server returns the IP address; resolver caches it and replies.

Internet registries: allocate IP addresses and record domain names so each is unique. IANA delegates to five Regional Internet Registries.

3.14.4 TCP/IP

Layer	Key job	Adds
Application	Program-level protocol (HTTP, SMTP, FTP)	Application data
Transport	Splits into segments, reliability, resends	Port numbers, sequence numbers
Internet	Routing between networks	Source and destination IP addresses
Link	Delivery across the physical network	MAC addresses

Why layers: each layer is independent, so one can change without the others; complex problem divided up; standard interfaces let different makers’ products work together.

Socket = IP address + port number. MAC address = unique hardware address of a network interface, used inside the local network; the frame’s MAC addresses are replaced at each hop.

Protocol	What it does
FTP	File transfer, anonymous or authenticated; unencrypted, being replaced by SFTP
HTTP / HTTPS	Fetch web resources / the same, encrypted
SMTP	Sends email from client to server and server to server
POP3	Downloads email to one device, normally deleting it from the server
IMAP	Leaves email on the server, synchronised across devices
SSH	Encrypted remote login to run commands and manage servers

Web: the server sends pages in text form; the browser requests the page and its resources, then renders them.

Method: network identifier from a subnet mask

1. Write each octet where the mask is neither 255 nor 0 in binary.
2. AND the address with the mask bit by bit.
3. Octets under 255 are copied; octets under 0 become 0.

Reminder: 192.168.37.77 with 255.255.255.224: 77 = 01001101, 224 = 11100000, AND = 01000000 = 64, so the network is 192.168.37.64 with 5 host bits.

IPv4 vs IPv6: 32-bit vs 128-bit addresses; IPv6 was introduced because the IPv4 supply ran out.

Routable vs non-routable: public addresses are unique and forwarded across the Internet; private ranges (10.x.x.x, 172.16–31.x.x, 192.168.x.x) are reused within local networks and not forwarded.

DHCP: Discover (broadcast) → Offer → Request → Acknowledge. Address leased for a period. Beats manual set-up: no duplicates or typos, address reuse, instant connection for visitors, central changes.

3.14.5 Cyber security

Firewall method	How it works
Packet filtering	Compares addresses, ports and protocol with rules
Proxy server	Requests on users’ behalf; hides internal addresses
Stateful inspection	Admits only packets that belong to a tracked connection

	Symmetric	Asymmetric
Keys	One shared key	Public and private key pair
Speed	Fast	Slower
Problem	Getting the key to the receiver safely	Slow for bulk data

Key exchange: encrypt a fresh symmetric key with the recipient’s public key; send it; use symmetric encryption from then on.

Digital signature: hash → encrypt hash with sender’s private key → receiver decrypts with sender’s public key, re-hashes, compares. Digital certificate: public key + identity sent to a Certificate Authority → identity checked → certificate signed by the CA.

Malware: virus (needs a host file to run), worm (self-spreading over networks), trojan (disguised as useful software). Vulnerabilities: unpatched software, weak passwords, excess privileges, human error.

Quick self-test

1. Define latency.
2. Give two reasons serial is preferred to parallel over long cables.
3. A link uses 4 signal levels at 1,500 baud. Find the bit rate.
4. What does a stop bit allow?
5. Name the problem RTS/CTS solves.
6. Which part of a packet stops it circulating forever?
7. Find the network identifier of 172.20.9.130 with mask 255.255.255.128.
8. State one difference between POP3 and IMAP.
9. Which key does a sender use to create a digital signature?
10. How does a worm differ from a virus?

Answers

1. The delay between data being sent and being received.
2. No skew between bits; no crosstalk; cheaper cable (any two).
3. log₂(4) = 2 bits per change; 1,500 × 2 = 3,000 bit/s.
4. The next start bit to be recognised (and, in older devices, time to process the data).
5. The hidden-node problem.
6. The time to live.
7. 130 = 10000010; AND 10000000 = 128; network 172.20.9.128.
8. POP3 downloads and usually deletes from the server; IMAP keeps mail on the server.
9. The sender’s own private key.
10. A worm spreads by itself across networks; a virus needs a host file to be run.

Where marks are usually lost

• Defining bandwidth as “speed” instead of a range of frequencies.
• Saying bit rate always equals baud rate.
• Mixing up the start bit (clock phase) and stop bit (recognise next start bit).
• Describing CSMA/CA as detecting collisions while sending.
• Missing the random back-off step from either CSMA method.
• Using “domain name” when the question asks for an FQDN.
• Forgetting that the resolver caches the DNS answer.
• Claiming private addresses are “secure” rather than non-routable.
• Saying a signature uses the receiver’s keys.
• Calling every piece of malware a virus.

Official syllabus

OxfordAQA International AS and A-level Computer Science (9645) specification, Version 1.1, for International AS exams May/June 2025 onwards and International A-level exams May/June 2026 onwards, published by OxfordAQA. Section 3.14 Networking and cyber security.