AQA A-Level Computer Science: Communication and Networking — Revision Notes

Fundamentals of communication and networking (section 4.9 of AQA A-Level Computer Science 7517) covers how bits are transmitted, how networks are organised and secured, how the Internet routes packets, and how the TCP/IP stack, addressing and web protocols work. Paper 2 asks for precise definitions (baud rate, latency, gateway), short calculations such as subnet masking, and explanations of processes such as CSMA/CA or how a digital signature is checked.

Condensed for the final weeks. For the full explanation, use the Fundamentals of communication and networking study guide. Practise with the communication and networking practice questions.

Syllabus coverage

AQA 7517 specification (Version 1.6, 31 July 2023), section 4.9, assessed on Paper 2:

• 4.9.1 Communication – serial/parallel, synchronous/asynchronous, start and stop bits; baud rate, bit rate, bandwidth, latency, protocol
• 4.9.2 Networking – physical star and logical bus topologies; peer-to-peer and client-server; Wi-Fi, its security, CSMA/CA and RTS/CTS, SSID
• 4.9.3 The Internet – packet switching, routers, gateways, URLs, domain names, DNS, Internet registries; firewalls, encryption, digital certificates and signatures, malware
• 4.9.4 TCP/IP – the four layers, sockets, MAC addresses, ports; FTP, HTTP, HTTPS, POP3, SMTP, SSH; IP addresses, subnets, IPv4/IPv6, public/private addresses, DHCP, NAT, port forwarding; client-server, WebSocket, CRUD and REST, JSON vs XML; thin vs thick clients

Communication: the definitions

Term	Definition
Serial	Bits sent one after another along a single wire
Parallel	Several bits sent simultaneously along several wires
Synchronous	Sender and receiver share a timing (clock) signal so data is sent at regular intervals
Asynchronous	No shared clock; each character is framed by a start bit (wakes the receiver and synchronises its clock) and a stop bit (marks the end, gives time to process)
Baud rate	The rate at which the signal changes (signal changes per second)
Bit rate	The number of bits transmitted per second
Bandwidth	The range of frequencies a medium can carry; bit rate is directly proportional to bandwidth
Latency	The time delay between sending and the start of receipt
Protocol	A set of rules for communication between devices

Serial over parallel: no skew (bits on parallel wires arriving at different times), no crosstalk between wires, cheaper cable, works over longer distances – so serial can be run at higher speeds. Bit rate can exceed baud rate if more than one bit is encoded per signal change: 1,000 baud with 4 bits per change = 4,000 bit/s.

Networks and Wi-Fi

• Physical star: each device has its own cable to a central switch. Logical bus: a network wired as a star can behave as a bus if it uses a bus protocol – every device receives every frame and only the addressed device keeps it.
• Peer-to-peer: every computer has equal status and shares its own resources (small home networks, file sharing). Client-server: clients request services from one or more servers (file server, email server) – central backup, security and management.
• Wi-Fi: a wireless LAN based on international standards. Needs a wireless network adapter in each device and a wireless access point. SSID = the network’s name, identifying it to devices.
• Securing Wi-Fi: strong encryption with WPA/WPA2, disable SSID broadcast, MAC address allow list.

CSMA/CA (with and without RTS/CTS)

1. Listen: is the channel idle?
2. If busy, wait a random back-off period, then listen again.
3. If idle, transmit the frame.
4. Receiver sends an acknowledgement (ACK); no ACK -> assume collision, back off, retry.
With RTS/CTS: before sending data the device sends Request to Send to the access point;
the AP replies Clear to Send; other devices hear the CTS and stay silent. This solves
the hidden node problem, where two devices cannot hear each other but both reach the AP.

CSMA/CD (Ethernet) is not required.

The Internet

• Packet switching: data is split into packets that may take different routes and are reassembled in order at the destination. A packet has a header (source and destination IP address, packet number, time to live), the payload, and a trailer (checksum).
• Router: forwards packets between networks towards their destination using routing tables. Gateway: connects networks that use different protocols, translating between them.
• URL: the full address of a resource, e.g. https://www.example.com/docs/page.html (protocol, FQDN, path). FQDN: the complete domain name including host, e.g. www.example.com. Domain names are organised hierarchically (root -> top-level .com/.uk -> second-level -> subdomain).
• DNS: domain name servers translate a domain name into an IP address. Internet registries allocate IP addresses and domain names so each is globally unique.

Internet security

Defence	How it works
Packet filtering	Allows or blocks packets by source/destination IP address, port and protocol
Proxy server	Intermediary that makes requests on the client’s behalf, hiding internal addresses; can filter and cache
Stateful inspection	Tracks open connections and only allows packets that belong to an established, legitimate connection

• Symmetric encryption: the same key encrypts and decrypts – fast, but the key must be shared securely. Asymmetric: a public key encrypts and only the matching private key decrypts. In practice, asymmetric encryption is used for key exchange of a symmetric session key.
• Digital signature: the sender hashes the message and encrypts the hash with their private key. The receiver decrypts it with the sender’s public key and compares it with their own hash of the message – a match proves who sent it and that it was not altered.
• Digital certificate: issued by a Certificate Authority; binds a public key to an identity and is signed by the CA, so others can trust the key.
• Malware: a virus attaches to a file and needs a user action to spread; a worm replicates itself across a network without a host file; a trojan pretends to be useful software. They exploit vulnerabilities such as unpatched software, buffer overflows and user trust. Defences: improved code quality (testing, code review), monitoring (traffic and behaviour), protection (antivirus, firewalls, patching).

TCP/IP

Layer	Role
Application	The protocol the program uses (HTTP, FTP, SMTP…) and the data it sends
Transport	TCP splits data into segments, numbers them, adds port numbers, handles acknowledgements and retransmission
Network (Internet)	Adds source and destination IP addresses; routing between networks
Link	Adds MAC addresses and transmits on the physical network

• Socket = IP address + port number; identifies one end of a connection to a specific process.
• MAC address: unique hardware address of a network interface, used to deliver frames on the local network.
• Well-known ports (0-1023) identify standard server services: FTP 21, SSH 22, SMTP 25, HTTP 80, POP3 110, HTTPS 443. Client ports are temporary, assigned by the client’s OS for one connection.
• Protocols: FTP file transfer (anonymous or with a login); HTTP/HTTPS web pages (HTTPS encrypted); SMTP sends email; POP3 retrieves email from a server; SSH secure remote login and commands – an SSH client can also open a TCP connection to a port and send application commands such as HTTP GET.

IP addressing – worked example

IP address 192.168.12.77 with subnet mask 255.255.255.192:

Last byte of address  77  = 01001101
Last byte of mask    192  = 11000000
AND                        = 01000000 = 64
Network identifier: 192.168.12.64   Host identifier: 001101 = 13

• IPv4 is 32 bits; IPv6 (128 bits) was introduced because IPv4 addresses were running out.
• Public (routable) addresses are unique on the Internet; private (non-routable) addresses, e.g. 192.168.x.x, are used inside local networks.
• DHCP automatically assigns an IP address (and subnet mask, gateway, DNS server) to a device from a pool.
• NAT lets many devices with private addresses share one public address; the router rewrites addresses and keeps a translation table. Port forwarding directs incoming traffic on a given public port to a specific private device, e.g. a game server.

Client-server, WebSocket and REST

• Client-server: the client sends a request message, the server replies with a response.
• WebSocket: a full-duplex, persistent connection between browser and server over TCP, so either side can send at any time – used for chat, live games and live data feeds.
• REST uses HTTP request methods so browser JavaScript can call an API on the server. CRUD mapping: GET -> SELECT, POST -> INSERT, PUT -> UPDATE, DELETE -> DELETE. Data is sent as JSON or XML.
• JSON vs XML: JSON is easier for humans to read, more compact, easier to create, and easier and quicker for computers to parse.
• Thin client: most processing and storage on the server – cheap terminals, central management, but depends on the network and server. Thick client: processing and storage on the client – works offline and reduces server load, but each machine is more expensive and harder to manage.

Common mistakes / exam traps

• Defining baud rate as “bits per second” – that is bit rate; baud is signal changes per second.
• Saying a gateway and a router are the same: a gateway joins networks using different protocols.
• Signing with the receiver’s key: signatures use the sender’s private key.
• Forgetting the AND step in subnet questions, or ANDing in denary.
• Writing that NAT gives each device a public address – it lets many share one.

Frequently asked questions

Why is serial transmission used for fast, long connections? Serial uses one wire per direction, so there is no skew or crosstalk between wires. That allows higher clock speeds and cheaper cabling over long distances than parallel transmission.

What is the difference between a well-known port and a client port? Well-known ports (0 to 1023) identify standard services on servers, such as 80 for HTTP. A client port is a temporary number chosen by the client’s operating system for a single connection.

Why was IPv6 introduced? The 32-bit IPv4 address space allows about 4.3 billion addresses, which was not enough for every connected device. IPv6 uses 128-bit addresses.

Related resources

Communication and networking study guide | Communication and networking practice questions | Fundamentals of databases revision notes

Official syllabus

AQA AS and A-level Computer Science (7516/7517) specification, Version 1.6 (31 July 2023), section 4.9 – aqa.org.uk/7517.