Communication and internet technologies

A protocol 协议 is a set of rules for how devices communicate. Both ends must follow the same rules, or one side's signals are meaningless to the other. Protocols define the format of the data (where addresses and payload sit), the order of messages (who speaks first, when to acknowledge), the meaning of each message, the timing (timeouts, retransmits), and what to do on error. Without an agreed protocol, communication fails — like two people speaking different languages with no translator.

A protocol is the shared rules: format, order, timing and errors

Networking is complex, so it is split into layers 层, each with one focused job, talking only to the layer above and below. Benefits: modularity 模块化 (replace one layer — say Ethernet with Wi-Fi — without touching the others), standardisation (vendors interoperate), and abstraction 抽象 (you ignore details handled elsewhere). The internet uses the TCP/IP protocol suite 协议栈 (4 layers).

The four layers of the TCP/IP protocol suite

Application layer

The application layer 应用层 gives services to user programs and defines the protocols they speak (HTTP for web, SMTP for email). This is where a programmer most often works.

Transport layer

The transport layer 传输层 delivers data end-to-end between processes, identified by port numbers 端口号. Two protocols:

TCP connects and delivers in order; UDP sends and forgets

• TCP 传输控制协议 — connection-oriented 面向连接: sets up a connection, ensures all data arrives in order, retransmits lost packets 数据包, controls flow. Reliable but with overhead. Used by HTTP, HTTPS, SMTP, FTP.
• UDP 用户数据报协议 — connectionless 无连接: sends and forgets, with no acknowledgements or ordering. Low overhead, no guarantees. Used for streaming, DNS and gaming, where speed beats reliability.

Internet layer

The internet layer 网络层 carries packets between hosts using IP. Each packet has a source and destination IP address IP地址, and routers 路由器 forward it onward. It does not guarantee delivery — that is TCP's job.

A home router does this job for your house: it reads each packet's destination address and sends it on towards the internet, and back to the right device.

A home Wi-Fi router: it forwards packets between your devices and the internet

Before the router reaches the wider internet, a modem 调制解调器 connects the home to the internet provider over the provider's cable or phone line. Its lights show the link is up and online.

A cable modem connects a home network to the internet provider

Link layer

The link layer 链路层 sends bits over one physical link (Ethernet, Wi-Fi). It adds a frame header with MAC addresses MAC地址 and handles medium access (e.g. CSMA/CD 载波侦听多路访问 on Ethernet).

The parts of a typical Ethernet frame

On a wired local network, a switch 交换机 joins many devices together. Each device plugs into a port with an Ethernet cable (an RJ45 plug), and the switch uses the MAC addresses in each frame to send it only to the correct port.

A network switch connects many wired devices on a local network

The physical link can be a copper wire, a radio signal (Wi-Fi), or a fibre-optic cable 光纤. In a fibre-optic cable, the bits travel as flashes of light through very thin strands of glass, which is fast and carries data a long way.

A fibre-optic cable: data travels as light through thin glass strands

A radio link can reach much further. A satellite dish 卫星天线 sends and receives radio signals to and from a satellite, carrying data to places that wired links cannot easily reach.

A satellite dish sends and receives data by radio over a long distance

• HTTP 超文本传输协议 — browsers fetch web pages from servers (over TCP, port 80). HTTPS is HTTP over TLS — encrypted, port 443.
• FTP 文件传输协议 — transfer files between client and server.
• SMTP 简单邮件传输协议 — send email between client and server, and between servers. Receiving uses POP3 or IMAP.
• POP3 — downloads email and usually deletes it from the server. IMAP — leaves email on the server and syncs across devices, so the same inbox appears everywhere.
• BitTorrent — a peer-to-peer 对等网络 protocol; a file is split into pieces downloaded from many peers in parallel, so no single server carries all the load.

BitTorrent: a tracker helps peers find each other, then they share file pieces directly

Circuit switching

A dedicated path is set up between the two ends before any data is sent (circuit switching 电路交换), reserved for the whole conversation, then released. It gives reserved bandwidth 带宽 and in-order delivery, but is inefficient during silences and slow to set up. Classic example: the traditional telephone network.

Circuit switching: one dedicated path is reserved end to end

Packet switching

The data is split into packets, each sent independently (packet switching 分组交换). Each packet carries the destination address; routers make per-packet decisions, so packets may take different routes and arrive out of order, and the destination reassembles them. It is efficient (one link is multiplexed 多路复用 across many conversations), robust (reroute around a failure), but has variable latency 延迟 and possible loss (TCP handles reliability). Used by the internet.

Packet switching: packets travel independently and may take different routes

Modern networks use packet switching for its efficiency and resilience.

Describing packet switching in a few sentences

A good exam answer: "The message is broken into small packets. Each packet carries the destination and source addresses and a sequence number. Each packet travels through the network independently, with routers choosing the next hop per packet. Packets may take different paths and arrive out of order. The destination uses the sequence numbers to reassemble the message, and missing packets can be requested again."

Worked example. A phone call and a large file download share a network. Which switching method suits each, and why? A phone call needs a steady stream with low delay, and it would suffer badly if pieces arrived late or out of order - so circuit switching suits it: a dedicated path is set up for the whole call and its capacity is reserved for the duration. A file download does not care about timing or arrival order, because the receiver reassembles it, and it benefits from using whatever capacity happens to be spare - so packet switching suits it: the file is split into packets that travel independently, each carrying source and destination addresses and a sequence number, with routers choosing a next hop per packet. Name the property of the traffic that decides it: reserved capacity and low delay for the call, efficiency and resilience for the download.

• Explain why protocols and layers are used: each layer has one job and can change independently.
• Place common protocols in the TCP/IP stack (HTTP/FTP/SMTP application; TCP/UDP transport; IP internet).
• Compare circuit switching vs packet switching (a dedicated path vs independent packets) with a use for each.