Networks and their types · ⁨网络及其类型⁩

Two computers, six hundred kilometres, one message

• On 29 October 1969 a student at UCLA typed the first two letters of LOGIN to a computer at Stanford, 600 kilometres away. The link crashed after "LO". An hour later it worked.
• That was ARPANET, four machines by the end of the year. Every network since, from a classroom of laptops to the internet itself, answers the same question: how do separate computers share work and data?
• This lesson is the vocabulary of that question: what a network 网络 is for, how big it is, who is in charge on it, and how much work the machines at its edge do.

The purpose and benefits of a network

• A network is a set of computing devices connected so they can communicate and share resources.
• Share resources: one printer, one file server, one internet connection for everyone, cheaper than equipping each computer.
• Share data: many users open the same files. Central management: install software, manage user accounts and back up once, on a server.
• Communication by email, messaging and video calls, and remote access to files and systems from anywhere.

LAN and WAN

• A local area network 局域网 (LAN) covers a small area, a home, office or school site. The organisation owns the hardware, so data rates are high and latency 延迟 is low.
• A wide area network 广域网 (WAN) covers a large area, a city, a country or the world. It runs over third-party infrastructure, leased lines, fibre or the Public Switched Telephone Network 公共交换电话网 (PSTN), so it is slower and its latency higher.
• A WAN connects LANs together, and the internet is the largest WAN there is.

A WAN joins sites that are far apart over infrastructure someone else owns

Worked example: "give two characteristics"

Client-server

• In the client-server 客户端-服务器 model, powerful machines act as servers 服务器 providing a service: files, web pages, email, printing. The other machines are clients 客户端 that request those services.
• Everything is central: one login works from any machine, software and security are managed once, files are backed up in one place, and the server can be a powerful machine.
• The drawbacks are the cost of the server and of the technician to run it, and that the server is a single point of failure unless it is duplicated.

Clients ask, the server answers

Peer-to-peer

• In a peer-to-peer 对等网络 (P2P) network every machine is an equal peer: each can be both client and server, and resources are spread across them with no central server.
• No server to buy or manage, and no single failure brings the network down.
• But there is no central control: files live on individual machines that may be switched off, security is set on each peer separately, and there is no one backup.

Every peer serves and every peer requests

Worked example: justify the model

Thin and thick clients

• A thin client 瘦客户端 does little processing itself: the server runs the software and stores the data, and the client only sends input and shows the output. A cheap terminal is enough and everything is updated and backed up on the server, but nothing works if the network or the server fails.
• A thick client 胖客户端 has its own processing power and storage and runs full applications locally, like an ordinary desktop PC. It keeps working without the network, but each machine must be maintained and secured on its own.
• Thin clients load the server and the network heavily; thick clients load them lightly.

Cloud computing

• Cloud computing 云计算 is on-demand computing services, storage, processing and software, provided over the internet by a third party.
• A public cloud 公有云 is owned by a provider and shared by many customers; a private cloud 私有云 is dedicated to one organisation, on its own hardware or hosted for it alone.
• Benefits: scalability 可扩展性, paying only for what is used, access from anywhere, and redundant data centres. Drawbacks: it needs an internet connection, the data is held by someone else, and changing provider can be hard.

Wired against wireless

• Wired networks use Ethernet 以太网 over twisted-pair 双绞线 copper or fibre-optic 光纤 cable: higher speed, lower latency, fewer errors, and more secure, because a cable has to be physically tapped.
• Wireless networks use radio waves, Wi-Fi, Bluetooth or the cellular network: no cabling and devices can move, but the shared frequency means interference and lower speed, and the signal can be intercepted, so it needs encryption.
• Copper is cheap but weakens with distance and picks up interference; fibre carries light, is immune to interference and covers long distances; microwaves need line of sight; satellites reach anywhere but with a long delay.

Marks that slip away

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