International Baccalaureate · IB Diploma · Computer Science · SL Тема 2 6:28 Английское озвучивание · Английский + китайские субтитры (встроенные)
Главы
Транскрипт
You type a web address on your phone at school.
你在学校用手机输入一个网址。
In under a second your request crosses the school Wi-Fi, a router, your internet provider and several countries, and reaches a server.
不到一秒，请求就穿过学校的无线网络、路由器、你的网络服务商和好几个国家，到达服务器。
The page comes back in hundreds of small pieces that are put together on your screen.
网页被分成几百个小块传回来，在屏幕上重新拼好。
Every step follows agreed rules.
每一步都遵守约定的规则。
A LAN covers one site, such as a school.
局域网覆盖一个地点，比如学校。
A WAN covers a large area and joins LANs; the internet is the biggest.
广域网覆盖大片区域，把多个局域网连起来，互联网就是最大的广域网。
A PAN links one person's devices, like a phone and earbuds over Bluetooth.
个人区域网连接一个人的设备，比如手机和蓝牙耳机。
And a VPN is a private, encrypted link across a public network, so you can work from home on the school network.
虚拟专用网是在公共网络上的加密专线，让你在家也能用学校的网络。
Name each device by its job.
说设备时要说出它的作用。
A switch joins devices in a LAN and sends each frame only to the device it is for.
交换机连接局域网内的设备，只把数据帧发给目标设备。
A router joins networks and forwards packets along a route.
路由器连接不同网络，沿着路径转发数据包。
An access point lets wireless devices join.
无线接入点让无线设备接入。
A modem converts data for the provider's line.
调制解调器为服务商的线路转换数据。
A firewall blocks traffic that breaks its rules.
防火墙拦截违反规则的流量。
In a client-server network, servers provide services and clients request them.
在客户机-服务器网络中，服务器提供服务，客户机提出请求。
Security, backups and updates are in one place, but the server is a single point of failure.
安全、备份和更新集中在一处，但服务器是单点故障。
In peer-to-peer, every device is equal. There is no central control, so it is hard to secure and back up.
在对等网络中，每台设备地位平等，没有集中控制，所以很难保证安全和备份。
A topology is the layout of the connections.
拓扑结构是连接的布局。
In a star, every device has its own cable to a central switch: one broken cable affects one device, but if the switch fails, the whole network fails.
星型结构中，每台设备都有一根线连到中央交换机：一根线断了只影响一台设备，但交换机坏了整个网络都瘫痪。
A mesh has many paths, so data can route round a failure; the internet's core is a partial mesh.
网状结构有很多路径，数据可以绕开故障；互联网的核心就是部分网状结构。
A protocol is an agreed set of rules, so devices from different makers can talk.
协议是一套约定的规则，让不同厂商的设备能够通信。
Networking is split into four layers.
网络分成四层。
The application layer is the service you see, like HTTP.
应用层是你看到的服务，比如 HTTP。
Transport splits data into segments and checks delivery.
传输层把数据分成段并检查送达。
Internet addresses and routes packets.
网际层给数据包编址并选路。
Link sends frames over the cable or Wi-Fi.
链路层通过网线或无线网络发送数据帧。
Going down the layers, each one wraps the data in its own header. The data gains a TCP header, then an IP header, then a frame header.
数据往下经过各层时，每一层都给它包上自己的首部：先加 TCP 首部，再加 IP 首部，最后加帧首部。
The receiver removes them in reverse.
接收方按相反顺序去掉。
Because only the link layer differs, a browser works the same over Wi-Fi or cable.
因为只有链路层不同，浏览器用无线网络或网线都一样工作。
TCP gives reliable delivery: numbered segments, acknowledgements and resends, so it suits a web page or a file, where every byte must arrive.
TCP 提供可靠传输：给数据段编号、确认、重发，适合网页或文件，因为每个字节都必须到达。
UDP is fast, with no checks. It suits a video call or a game, where a late packet is useless, so it is better to drop it than wait.
UDP 速度快，不做检查，适合视频通话或游戏，因为迟到的数据包已经没用，与其等待不如丢掉。
An IP address identifies a device's position on a network. It is set by the network, often by DHCP, and changes when the device moves.
IP 地址标识设备在网络中的位置，由网络设定，通常通过 DHCP，设备移动时会改变。
A MAC address identifies the network card itself. The maker sets it, and it never changes.
MAC 地址标识网卡本身，由制造商设定，永远不变。
IPv4 uses thirty-two bits, about four point three billion addresses. That is too few for every phone, laptop and sensor.
IPv4 用三十二位，大约四十三亿个地址，对所有的手机、电脑和传感器来说太少了。
IPv6 uses a hundred and twenty-eight bits, written in hex, which gives far more addresses than we will ever need.
IPv6 用一百二十八位，用十六进制书写，地址多得远远用不完。
When you type a web address, your device asks a DNS server for its IP address.
你输入网址时，设备向 DNS 服务器询问它的 IP 地址。
If that server does not know, it asks other DNS servers in turn.
如果那台服务器不知道，就依次问其他 DNS 服务器。
The IP address comes back and your browser connects.
IP 地址返回后浏览器建立连接。
The answer is cached, so the next visit is faster.
答案会被缓存，下次访问更快。
The internet uses packet switching.
互联网使用分组交换。
The data is split into packets, each with a header holding the addresses and a sequence number.
数据被分成数据包，每个包的首部有地址和序号。
Each router forwards a packet along the best route it knows.
每台路由器沿它所知的最佳路径转发数据包。
Packets may arrive out of order, so TCP puts them back by sequence number and asks again for any that are missing.
数据包可能乱序到达，TCP 按序号重新排好，并重新请求丢失的包。
Keep three terms apart.
这三个术语要分清。
Bandwidth is the most data a link can carry per second.
带宽是一条链路每秒最多能传的数据量。
Latency is the delay before data starts to arrive, in milliseconds.
延迟是数据开始到达前的等待时间，单位毫秒。
Throughput is what is actually delivered per second, always at or below the bandwidth, because of other users, errors and resends.
吞吐量是每秒实际送达的数据量，因为有其他用户、错误和重发，它总是不超过带宽。
How long does a twelve-megabyte file take on an eight-megabit link?
一个十二兆字节的文件，在八兆比特每秒的链路上要传多久？
First convert bytes to bits: twelve times eight is ninety-six million bits.
先把字节换成位：十二乘八等于九千六百万位。
Divide by eight million bits per second: twelve seconds.
除以每秒八百万位，得十二秒。
That is the ideal; latency and overhead make the real time longer.
这是理想时间；延迟和额外开销会让实际时间更长。
Malware, such as ransomware, damages or locks data.
恶意软件，比如勒索软件，会破坏或锁住数据。
Phishing uses a fake message to trick a user into giving away a password.
网络钓鱼用假消息骗用户交出密码。
Interception reads data as it travels, for example on open Wi-Fi.
截获是在数据传输时读取它，比如在开放的无线网络上。
And a denial-of-service attack floods a server with requests until real users cannot reach it.
拒绝服务攻击用大量请求淹没服务器，直到真正的用户无法访问。
Match each defence to a threat.
每种防御都对应一种威胁。
A firewall checks each packet against rules.
防火墙按规则检查每个数据包。
Multi-factor authentication means a stolen password alone is not enough.
多因素认证让仅仅偷到密码不够用。
Updates close known weaknesses, and backups allow recovery from ransomware.
更新修补已知漏洞，备份可以从勒索软件中恢复。
Training reduces phishing, and encryption makes intercepted data unreadable.
培训减少钓鱼，加密让截获的数据无法读懂。
Symmetric encryption uses one shared secret key. It is fast, but the key must be passed on safely.
对称加密使用一把共享的密钥，速度快，但密钥必须安全地传递。
Asymmetric encryption uses a pair: anyone can lock a message with the public key, but only the private key unlocks it.
非对称加密使用一对密钥：任何人都能用公钥加密，但只有私钥能解密。
Ana encrypts with Ben's public key, and only Ben can decrypt it.
安娜用本的公钥加密，只有本能解密。
HTTPS uses both.
HTTPS 两种都用。
First a digital certificate proves the public key really belongs to the website.
先用数字证书证明公钥确实属于这个网站。
Then asymmetric encryption is used to agree a secret key safely.
然后用非对称加密安全地商定一把密钥。
After that, fast symmetric encryption protects the rest of the session.
之后用快速的对称加密保护整个会话。
Try these. Choose TCP or UDP for an online game and explain. Describe packet switching. Work out the time for thirty megabytes on a twenty-megabit link. And name the threat in a fake password e-mail.
试试这些：为在线游戏选择 TCP 或 UDP 并解释；描述分组交换；算出三十兆字节在二十兆比特每秒链路上的传输时间；说出假冒“确认密码”邮件属于哪种威胁。
In the exam, name each device by its job.
考试时，说设备要说出它的作用。
Link TCP or UDP to the scenario.
选 TCP 还是 UDP 要联系情境。
Describe packet switching in order.
按顺序描述分组交换。
Keep bandwidth, throughput and latency apart, and multiply bytes by eight before dividing by a speed in bits per second.
分清带宽、吞吐量和延迟，并且先把字节乘八，再除以每秒位数。