Data transmission and packets · ⁨数据传输与数据包⁩

Sending data around

• Data transmission 数据传输 moves data from one place to another.
• The data is broken into small equal pieces called packets 数据包.
• We also choose how it travels: serial 串行/parallel 并行 and the direction.

Packets and packet switching 数据包交换

• A packet has three parts: a header 报头 (sender + receiver IP addresses, packet number), the payload 有效载荷 (the data), and a trailer (end marker + error check).
• Benefits of packets: only a lost packet is resent (not the whole file); packets can take different routes; many users share the connection.
• In packet switching, routers 路由器 read each header and pick a route; packets may arrive out of order and are reordered by packet number.

A packet has a header, a payload (the data) and a trailer; the header carries the addresses and packet number.

Routers forward packets, and different packets can take different routes to the same receiver.

Serial vs parallel

• Serial sends one bit at a time down one wire — cheaper, and good over long distances (bits stay in order).
• Parallel sends several bits at once down several wires — faster but only over short distances (bits can arrive at slightly different times).

An Ethernet cable with an RJ45 plug, often used to carry data on a wired network

Direction, and USB

• Simplex 单工 — one direction only (computer → printer).
• Half-duplex 半双工 — both directions, one at a time (walkie-talkie).
• Full-duplex 全双工 — both directions at once (phone call).
• USB: one standard connector, auto-detects the device and loads the driver, carries power, and only fits one way.

A fibre-optic cable carries data as pulses of light along thin glass fibres

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