Networks and topologies — OCR GCSE Computer Science

A network links devices so they can share data and resources. A LAN covers a small geographical area such as one school, office or home, and the hardware is usually owned and managed by the organisation. A WAN covers a large geographical area, often connecting LANs across towns, countries or continents; the communication infrastructure is usually owned by third parties such as telecom companies. The internet is the largest WAN. A school with computers in several rooms linked by its own switches and cabling is a LAN; the same school connecting to a branch in another city through leased lines forms a WAN. Performance depends on the type chosen, so identifying LAN or WAN helps explain bandwidth, ownership and cost.

Network performance is how quickly and reliably data moves between devices. Bandwidth is the maximum data transfer rate of a connection, so a higher bandwidth allows more data per second. The number of users or devices sharing the network affects performance because bandwidth is divided between them; more simultaneous users means each gets less. Wired connections such as Ethernet are generally faster and more stable than wireless connections such as Wi-Fi, which can suffer from interference, distance from the access point and physical obstacles. The type and quality of transmission media, network topology, hardware quality and signal interference all matter. A home network with one user streaming video may perform well, but the same connection with several users gaming and downloading at once can slow down noticeably.

In a client-server network, one or more powerful computers act as servers and the remaining computers are clients. A client requests services or resources; the server stores data, manages user accounts, applies security and responds to requests. For example, a school file server holds pupil work and checks login details before allowing access. In a peer-to-peer network, every computer has an equal role: each can act as both client and server, sharing files or a printer directly with others. No central server controls the network, so each peer must be switched on for its shared resources to be available. The roles differ in where control, storage and security sit, which affects cost, performance, management and suitability for a given task.

A stand-alone computer works alone, so connecting computers into a Local Area Network requires hardware that moves data between them. A NIC gives each computer a network interface and a MAC address. Transmission media carry the signals: copper cable such as Cat5e or Cat6 uses electrical signals, fibre optic cable uses light, and wireless uses radio waves. A switch connects devices within one LAN and forwards frames only to the intended port. A wireless access point lets wireless devices join the wired LAN by converting between radio and wired signals. A router connects the LAN to other networks, such as the internet, and directs packets between them. Together these devices allow reliable, addressable communication.

The Internet is a worldwide collection of computer networks, not a single network: independent LANs and WANs interconnect through routers and exchange data using common protocols such as TCP/IP. When you type a web address, a DNS server translates the human-readable domain name into the numeric IP address that identifies the destination server. Hosting means storing a website or service on a server that is reachable over the Internet; the host may be owned locally or rented from a provider. The Cloud is the delivery of computing resources, such as storage, processing and software, over the Internet on demand, so users access them remotely rather than owning the hardware. Web servers respond to requests from web clients, typically browsers, which send requests and render the returned pages.

A network topology is the arrangement of devices and connections in a network. In a star topology, every device connects to a central node, usually a switch or hub; all traffic passes through that central device, which makes faults easier to isolate but creates a single point of failure. In a mesh topology, devices connect to multiple other devices, so data can take alternative routes; a full mesh connects every device to every other device, while a partial mesh connects some devices by more than one path. Mesh networks are robust and can spread traffic, but they need more cabling or wireless links and are more complex and costly to install and manage. Star networks are simpler to add devices to and to troubleshoot, but performance and availability depend on the central node.