Networks

What Networks covers

Networks is examined on component 01 of OCR Computer Science, Computer systems. 92 recall questions across 8 subtopics.

The 8 subtopics

Each one has its own page.

• Characteristics of networks, LANs and WANs, protocols and standardsRecall questions on what makes a network, bandwidth and latency, protocols and standards, common internet protocols, error handling, open standards, LANs and WANs, the internet as a WAN, and wired and wireless LANs.Open →
• Protocol layering and the TCP/IP stackRecall questions on protocol layering, the application, transport, internet and link layers of TCP/IP, what each layer adds, port numbers, IP and hardware addresses, TCP and UDP, encapsulation, and why layering eases maintenance.Open →
• DNS and the structure of the internetRecall questions on the internet and the world wide web, the Domain Name System and its hierarchy, resolving a domain name, DNS caching and cache poisoning, authoritative name servers, and the role of ISPs and the backbone.Open →
• Packet and circuit switchingRecall questions on packets and their headers, packet switching and circuit switching compared, out-of-order and lost packets, variable latency, why packet switching suits data traffic, when circuits are better, and hop limits.Open →
• Network security and threatsRecall questions on confidentiality, integrity and availability, viruses, worms and trojans, phishing, denial of service, brute force, packet sniffing, man in the middle attacks, unpatched software, insiders, and layered defences.Open →
• Firewalls, proxies and encryptionRecall questions on firewalls, packet filtering and stateful inspection, proxy servers, what encryption achieves, symmetric and asymmetric keys, certificates in HTTPS, what each measure does not protect against, and default-deny rules.Open →
• Network hardwareRecall questions on network interface cards, hubs, switches and routers, wireless access points, modems, twisted pair, coaxial and fibre optic cable, MAC and IP addresses, and choosing hardware for a small office.Open →
• Client-server and peer to peerRecall questions on client-server and peer to peer networks, how each works, when each is the right choice, why peer to peer downloads speed up with more users, and evaluating a move from peer to peer to client-server.Open →

What you need to know

Answer each one before you reveal it.

Take Networks further

The same content, four other ways.

Commonly confused

Pairs that lose marks when they are blurred together.

• A virus vs A worm

  A virus: malware that attaches to a host file and spreads only when a user runs that file. A worm: malware that is self-contained and spreads by exploiting a vulnerability without user action.

• A LAN vs A WAN

  A LAN: a network covering a single site, whose infrastructure is owned by the organisation using it. A WAN: a network spanning geographically separate sites, using infrastructure it does not own.

• Packet switching vs Circuit switching

  Packet switching: sending data as separate units that are routed independently and reassembled on arrival. Circuit switching: reserving a dedicated path for the whole of a communication before it begins.

Questions from this topic

Each one with the model answer underneath.

1. 1

   What actually makes a set of computers a network?

   Model answer

   Two or more computing devices connected together, by cable or wirelessly, so they can exchange data and share resources such as files, printers, storage, an internet connection or processing power. The connection alone is not enough: the devices must also follow a common set of rules (protocols) for how data is formatted, addressed, transmitted and acknowledged, otherwise the signals arriving mean nothing to the receiver.

2. 2

   Describe the benefits and the drawbacks of networking an organisation's computers rather than leaving them standalone.

   Model answer

   Benefits: hardware and software can be shared (one printer, one shared storage array, site licences); files are held centrally so everyone works on the same version and one backup covers everyone; user accounts, permissions and software updates are managed centrally; communication is possible (email, messaging, voice over IP); one internet connection serves all machines. Drawbacks: cabling, switches, servers and installation cost money; the network needs skilled administration; central components are single points of failure so one failed switch or server can stop many users; malware that reaches one machine can spread to others; a single security breach can expose the whole organisation's data.

3. 3

   What is the difference between the bandwidth and the latency of a network link?

   Model answer

   Bandwidth is the maximum rate at which data can be carried, measured in bits per second - how much can flow per second. Latency is the delay between data being sent and arriving, measured in milliseconds - how long the first bit takes to get there. They are independent: a satellite link can have very high bandwidth and very high latency, while a short copper link can have low bandwidth and very low latency. Throughput is the rate actually achieved in practice, which is always less than or equal to the bandwidth because of protocol overheads, contention and retransmissions.

4. 4

   Define a protocol, and say what a protocol has to specify.

   Model answer

   A protocol is an agreed set of rules governing communication between devices. It must specify the format of a message (which fields appear, in what order, how long each is); how devices are addressed; the order in which messages are exchanged and what response each message requires; how the start and end of a message are recognised; timing and transmission speed; how errors are detected and what happens when one is found; and how the receiver acknowledges what it has received.

5. 5

   What is the difference between a protocol and a standard?

   Model answer

   A protocol is a specific set of rules for a specific kind of communication, such as HTTP for requesting web pages. A standard is a published, agreed specification that manufacturers and software developers implement so that independently made products work together; a standard may define a protocol, but it may equally define a physical connector, a voltage, a wireless frequency, a character encoding or a file format. Protocols are normally published as standards, but standards cover more ground than protocols do.

6. 6

   A student says standards are just bureaucracy. Say concretely what goes wrong when two devices communicate with no agreed standard.

   Model answer

   They may not physically connect at all, because connectors, voltages or radio frequencies differ. If they do connect, the receiver cannot tell where one message ends and the next begins, so the data stream is meaningless. Bits may be interpreted in the opposite order, so numbers come out wrong. Text may be decoded with a different character encoding, so it appears as nonsense. With no rule about who may transmit and when, both devices talk at once and the signals collide. With no common addressing scheme, data cannot be directed to the right machine or the right application. With no agreed error detection, corrupted data is accepted silently as if it were correct. With no agreed acknowledgement, the sender never learns whether anything arrived. The practical result is that only equipment from a single manufacturer works together, so customers are locked to one vendor and separate networks cannot be joined.

What is inside Levo

Six ways to work the same material.

Networks revision questions

What does Networks cover in OCR A-Level Computer Science?

8 subtopics: characteristics of networks, lans and wans, protocols and standards, protocol layering and the tcp/ip stack, dns and the structure of the internet, packet and circuit switching, network security and threats, firewalls, proxies and encryption, network hardware and client-server and peer to peer. Each one has its own page with questions and model answers.

How is Networks organised?

By the OCR H446 specification, subtopic by subtopic, so it matches the order you are taught it in.