Mission 1 · Spec 4.9.1
Communication methods and rates
Serial and parallel, synchronous and asynchronous transmission, start and stop bits, and baud rate, bit rate, bandwidth, latency and protocols.
- Starter 5 min
- Learn 15 min
- Lab 10 min
- Quiz 10 min
- Exam 15 min
Eight lanes or one?
An 8-lane road moves more cars than a 1-lane road. So why did computers switch from parallel cables to serial ones like USB?
Reveal
Over distance, bits in parallel wires arrive at slightly different times (skew) and interfere with each other (crosstalk). A single serial line can be run at a much higher clock rate reliably, and is cheaper.
Key ideas
Serial vs parallel
Serial: one bit at a time on one line. Parallel: several bits at once on several lines; suffers skew and crosstalk, so used only over short distances.
Synchronous vs asynchronous
Synchronous: sender and receiver share a timing signal. Asynchronous: each byte is framed with a start bit (to synchronise the receiver's clock) and a stop bit (giving time to process the byte).
Rates
Baud rate: signal changes per second. Bit rate: bits per second = baud rate × bits per signal. Bandwidth: range of frequencies; bit rate is directly proportional to bandwidth. Latency: delay between an action and its effect.
Protocol
A set of rules governing communication between devices.
Match the term
Exam-style questions
1. A signal has a baud rate of 1,000 and each signal change represents 4 bits. What is the bit rate?
[1 mark]Mark scheme
- 4,000 bits per second (1)
2. Explain why serial transmission is used over long distances instead of parallel transmission.
[2 marks]Mark scheme
- In parallel transmission bits can arrive at different times (skew) / wires interfere (crosstalk), especially over long distances (1)
- Serial uses fewer wires, so it is cheaper and can run at higher frequencies reliably (1)
TUTOR NOTES
- Common slip: confusing baud rate and bit rate. They are equal only when each signal carries one bit.
Mission 2 · Spec 4.9.2
Networking and wireless
Physical and logical topologies, client-server and peer-to-peer, Wi-Fi, CSMA/CA with RTS/CTS, and SSIDs.
- Starter 5 min
- Learn 15 min
- Lab 20 min
- Quiz 10 min
- Exam 15 min
Everyone talking at once
In a busy room everyone wants to speak. How do polite people avoid talking over each other?
Reveal
Listen first, wait for silence, then speak, and back off if someone else starts. Wi-Fi does the same with CSMA/CA (carrier sense multiple access with collision avoidance).
Key ideas
Topologies
Physical star: each device connects to a central switch. Physical bus: devices share one backbone cable. A network can be physically a star but logically a bus (e.g. if the central device broadcasts to everyone).
Client-server vs peer-to-peer
Client-server: central servers provide services and security. Peer-to-peer: peers have equal status and share resources directly.
Wireless networking
Wi-Fi: a wireless LAN based on international standards. Needs a wireless network adapter and a wireless access point. Secured with strong encryption (WPA2), SSID broadcast disabled, and MAC address whitelists.
CSMA/CA
Listen until the channel is idle, wait a random back-off, then transmit. With RTS/CTS, a station sends Request to Send and waits for Clear to Send from the access point before transmitting, avoiding the hidden node problem.
Star and bus
CSMA/CA with RTS/CTS
Exam-style questions
1. Explain how a network can be physically a star but logically a bus.
[2 marks]Mark scheme
- Physically, each device is wired to a central node (1)
- but the central node (e.g. a hub) broadcasts all data to every device, so it behaves like a shared bus (1)
2. Describe three methods of making a Wi-Fi network more secure.
[3 marks]Mark scheme
- Strong encryption, e.g. WPA2, so intercepted data can't be read (1)
- Disable SSID broadcast so the network isn't advertised (1)
- MAC address whitelist so only approved devices can connect (1)
TUTOR NOTES
- Exam habit: CSMA/CA questions are usually 4–6 marks; learn the sequence in order.
Mission 3 · Spec 4.9.3
The Internet and IP addressing
Packet switching, routers and gateways, URLs, FQDNs and DNS, IPv4 and IPv6, subnet masks, public and private addresses, DHCP, NAT and port forwarding.
- Starter 5 min
- Learn 20 min
- Lab 20 min
- Quiz 10 min
- Exam 15 min
One address, many devices
Your home has a laptop, two phones and a TV, but the rest of the internet sees just one IP address. How?
Reveal
Your router uses network address translation (NAT): devices get private IP addresses inside the home, and the router swaps them for its single public address on the way out, keeping track of which reply belongs to which device.
Key ideas
Packet switching
Data is split into packets, each with a header (source and destination IP, sequence number, TTL) and payload, routed independently and reassembled. Routers forward packets between networks; gateways connect networks using different protocols.
URLs and DNS
A URL identifies a resource (protocol, domain name, path). A fully qualified domain name includes the host name, e.g. www.example.co.uk. Domain name servers resolve names to IP addresses; domain names are registered with Internet registrars.
IP addresses
IPv4: 32 bits, split into a network identifier and a host identifier. IPv6: 128 bits, introduced because IPv4 addresses were running out. A subnet mask identifies the network part: IP AND mask = network address.
Public, private, DHCP, NAT
Private addresses are used within a LAN and aren't routable on the internet; public addresses are. DHCP automatically assigns IP addresses from a pool. NAT maps private to public addresses. Port forwarding lets outside clients reach a server inside a private network.
Subnet calculator
Packet switching
Exam-style questions
1. A device has IP address 192.168.4.200 and subnet mask 255.255.255.192. Calculate its network address.
[2 marks]Mark scheme
- Last octet: 200 = 11001000 AND 11000000 (1)
- = 11000000 = 192, so the network address is 192.168.4.192 (1)
2. Explain the purpose of DHCP.
[2 marks]Mark scheme
- Automatically assigns an IP address (and settings such as subnet mask and gateway) to a device joining the network (1)
- from a pool of available addresses, avoiding manual configuration and conflicts (1)
TUTOR NOTES
- Check Q1 in the subnet calculator: 192.168.4.200 with prefix /26.
Mission 4 · Spec 4.9.3 – 4.9.4
Security, TCP/IP and the client-server model
Firewalls, encryption, digital certificates and signatures, malware; the TCP/IP stack, sockets and ports; application protocols; WebSockets, REST, JSON and XML; thin and thick clients.
- Starter 5 min
- Learn 20 min
- Lab 20 min
- Quiz 10 min
- Exam 15 min
Is this really my bank?
Encryption stops eavesdroppers. But how does your browser know the site it's encrypting to really is your bank?
Reveal
The bank presents a digital certificate, issued and signed by a trusted certificate authority, which links the bank's identity to its public key.
Key ideas
Firewalls
Packet filtering (by IP address and port), proxy servers, and stateful inspection (tracking connections).
Encryption, certificates, signatures
Symmetric and asymmetric encryption. A digital signature: a hash of the message encrypted with the sender's private key; the receiver decrypts it with the public key and compares hashes, proving authenticity and integrity. A digital certificate verifies the owner of a public key.
Malware
Worms self-replicate across networks. Viruses attach to other programs. Trojans pretend to be useful. Protection: code quality, monitoring, patches, anti-malware.
TCP/IP
Application, transport, network, link layers. A socket = IP address + port. Well-known ports (0–1023) for servers, e.g. HTTP 80, HTTPS 443, FTP 20/21, SMTP 25, POP3 110, SSH 22. Clients use temporary (ephemeral) ports.
Client-server model
WebSocket: a persistent full-duplex connection. REST maps CRUD to HTTP: Create = POST, Retrieve = GET, Update = PUT, Delete = DELETE. JSON is more compact and easier to read and parse than XML.
Thin vs thick client
Thin: most processing on the server; cheap devices, central control, needs a reliable network. Thick: processing on the client; works offline, more powerful devices needed.
Down the TCP/IP stack
Ports and methods
Public-key encryption
Exam-style questions
1. Explain how a digital signature is used to verify that a message came from the stated sender and has not been altered.
[5 marks]Mark scheme
- The sender produces a hash (digest) of the message (1)
- and encrypts it with their private key to form the signature (1)
- The receiver decrypts the signature using the sender's public key (1)
- The receiver hashes the received message (1)
- If the two hashes match, the message is authentic and unaltered (1)
2. Compare JSON and XML for exchanging data between a client and a server.
[3 marks]Mark scheme
- JSON is more compact, so less data is transmitted (1)
- JSON is easier for humans to read and quicker to parse (1)
- XML is more verbose but more flexible / supports schemas and attributes (1)
TUTOR NOTES
- Misconception: a digital signature encrypts the whole message. It signs a hash; the message itself may be sent in plain text.