Network topologies and hardware · 网络拓扑与硬件
| English | 中文 | Pinyin · 拼音 |
|---|---|---|
| collision/kəˈlɪʒn/ | 冲突 | chōng tū |
| topology/təˈpɒlədʒi/ | 拓扑 | tuò pū |
| bus/bʌs/ | 总线 | zǒng xiàn |
| bandwidth/ˈbændwɪdθ/ | 带宽 | dài kuān |
| star/stɑː/ | 星形 | xīng xíng |
| switch/swɪtʃ/ | 交换机 | jiāo huàn jī |
| mesh/meʃ/ | 网状 | wǎng zhuàng |
| fault-tolerant/fɒlt ˈtɒlərənt/ | 容错 | róng cuò |
| packet/ˈpækɪt/ | 数据包 | shù jù bāo |
| network interface card/ˈnetwɜːk ˈɪntəfeɪs kɑːd/ | 网络接口卡 | wǎng luò jiē kǒu kǎ |
| MAC address/mæk əˈdres/ | MAC地址 | MAC dì zhǐ |
| wireless network interface card/ˈwaɪələs ˈnetwɜːk ˈɪntəfeɪs kɑːd/ | 无线网络接口卡 | wú xiàn wǎng luò jiē kǒu kǎ |
| hub/hʌb/ | 集线器 | jí xiàn qì |
| wireless access point/ˈwaɪələs ˈækses pɔɪnt/ | 无线接入点 | wú xiàn jiē rù diǎn |
| bridge/brɪdʒ/ | 网桥 | wǎng qiáo |
| repeater/rɪˈpiːtə/ | 中继器 | zhōng jì qì |
| router/ˈruːtə/ | 路由器 | lù yóu qì |
| IP address/ˌaɪ ˈpiː əˈdres/ | IP地址 | IP dì zhǐ |
| routing table/ˈraʊtɪŋ ˈteɪbl/ | 路由表 | lù yóu biǎo |
| packet switching/ˈpækɪt ˈswɪtʃɪŋ/ | 分组交换 | fēn zǔ jiāo huàn |
| network address translation/ˈnetwɜːk əˈdres trænˈsleɪʃn/ | 网络地址转换 | wǎng luò dì zhǐ zhuǎn huàn |
| DHCP/ˌdiː eɪtʃ siː ˈpiː/ | 动态主机配置协议 | dòng tài zhǔ jī pèi zhì xié yì |
| firewall/ˈfaɪəwɔːl/ | 防火墙 | fáng huǒ qiáng |
| full-duplex/fʊl ˈdjuːpleks/ | 全双工 | quán shuāng gōng |
| CSMA/CD/ˌsiː es em ˈeɪ ˌsiː ˈdiː/ | 载波侦听多路访问/冲突检测 | zài bō zhēn tīng duō lù fǎng wèn chōng tū jiǎn cè |
The cable that was a party line
- The first Ethernet, in 1973, was a single coaxial cable snaking through a Xerox research building. Every computer tapped into the same wire and shouted onto it.
- If two shouted at once, both messages were ruined, so the designers borrowed a rule from polite conversation: listen first, and if you collide, stop, wait a random moment, and try again.
- That rule is still on the syllabus, even though modern networks arrange their cables so that collisions cannot happen at all.
- This lesson is the shape of a network, the boxes that make it work, and the Ethernet rules for sharing a wire.
一根像"共线电话"的线缆
- 1973 年的第一个以太网,是一根蜿蜒穿过施乐研究楼的同轴电缆。每台计算机都接在同一根线上,朝它"喊话"。
- 如果两台同时喊,两条消息都会毁掉,所以设计者借用了礼貌交谈的规则:先听,如果撞上了,就停下,随机等一会儿,再试。
- 这条规则至今还在大纲上,尽管现代网络已经把线缆安排得根本不会发生冲突。
- 这一课讲网络的形状、让它工作的那些盒子,以及以太网共享一根线的规则。
The four topologies
- A network's topology 拓扑 is how its nodes and links are arranged.
- Bus 总线: every device on one shared cable with a terminator at each end. Cheap, but if the cable fails the whole LAN fails, and performance drops as more devices share the bandwidth 带宽.
- Star 星形: every device on its own cable to a central switch. A failed cable affects one device; a failed switch brings everything down. The most common LAN today.
- Mesh 网状: every device linked directly to others, so there are many paths and traffic reroutes round a fault. Very fault-tolerant 容错, but a great deal of cabling. A hybrid mixes them: a star in each office, mesh links between offices.
Bus, star and mesh: one cable, one centre, or many paths
四种拓扑
- 网络的拓扑(topology)是它的节点和链路的排列方式。
- 总线(bus):所有设备都在一根共享线缆上,两端各有一个终端器。便宜,但线缆一坏整个 LAN 都坏,而且设备越多共享带宽(bandwidth),性能就越差。
- 星形(star):每台设备用自己的线缆连到中央交换机。一根线缆坏了只影响一台设备;交换机坏了则全部瘫痪。今天最常见的 LAN。
- 网状(mesh):每台设备直接与其他设备相连,所以有许多路径,流量可以绕过故障。容错(fault-tolerant)性很强,但需要大量布线。混合(hybrid)把它们混起来:每个办公室一个星形,办公室之间用网状链路。
总线、星形和网状:一根线、一个中心,或许多路径
Compare the network topologies · 比较各网络拓扑
Tap through the four topologies. Each trades off cost, speed and how well it survives a failure — notice what breaks the whole network in each one. · 点击浏览四种拓扑。每种都在成本、速度和它在故障中存活得多好之间权衡——注意在每种中什么会破坏整个网络。
Match each topology to its defining feature. · 把每种拓扑与它的决定性特征配对。
Bus shares one cable, star uses a central switch, mesh has many paths, hybrid mixes them. · 总线共享一根电缆,星形用一个中央交换机,网状有许多路径,混合把它们混合。
In a star topology, what happens if one device's cable fails? · 在一个星形拓扑中,如果一个设备的电缆失效会发生什么?
Each device has its own link to the central switch, so one cable failing affects only that device. (If the switch fails, all go down.) · 每个设备有它自己到中央交换机的链接,所以一根电缆失效只影响那个设备。(如果交换机失效,全都瘫痪。)
How a packet travels in each topology
- Bus: the sender puts the packet on the shared cable; every device sees it, and only the one whose address matches accepts it. One transmission at a time, so collisions happen.
- Star: the sender passes the packet to the switch, which reads the destination address and forwards it only down the cable to that device. Other pairs can talk at the same time.
- Mesh: the packet is forwarded from node to node along one of several available paths; if a link is down, another path is used.
数据包在每种拓扑中怎样传输
- 总线:发送方把数据包放到共享线缆上;每台设备都看得到,只有地址匹配的那台接收它。一次只能有一台发送,所以会发生冲突。
- 星形:发送方把数据包交给交换机,交换机读出目标地址,只沿着通向那台设备的线缆转发。其他设备对可以同时通话。
- 网状:数据包沿多条可用路径之一从一个节点转发到另一个节点;某条链路断了,就用另一条路径。
In a star topology, how does a packet get from PC A to PC B? · 在星形拓扑中,数据包怎样从 PC A 到 PC B?
The switch reads the destination MAC address and forwards to one port, which is why other pairs can talk at the same time. · 交换机读出目标 MAC 地址并转发到一个端口,这就是其他设备对能同时通话的原因。
Worked example: justify a topology
- A school computer room with thirty PCs. Star: a failed cable affects one PC; a new PC is one more cable to the switch; the switch means no collisions.
- A hospital's critical systems. Mesh: a fault in one link is routed around, so no single failure stops the traffic.
- A temporary link between three devices where cost matters most. Bus: one cable and no switch.
- "Draw the star topology" means a switch in the middle and one line from it to each device: not a ring, not a chain.
例题:论证拓扑的选择
- *一间有三十台电脑的学校机房。*星形:一根线缆坏了只影响一台电脑;新加一台电脑只是多接一根线到交换机;有交换机就没有冲突。
- *医院的关键系统。*网状:一条链路的故障会被绕过,没有任何单一故障能让流量停下。
- *三台设备之间最看重成本的临时连接。*总线:一根线缆,不用交换机。
- "画出星形拓扑"意思是交换机在中间,从它到每台设备各一条线:不是环,不是链。
Match each situation to the topology that suits it. · 把每个情境与适合它的拓扑配对。
Star isolates a cable fault, mesh reroutes round any fault, bus is one cable and nothing else. · 星形隔离线缆故障,网状绕过任何故障,总线只有一根线缆别无其他。
LAN hardware
- A network interface card 网络接口卡 (NIC) lets a device send and receive on the network and carries its unique 48-bit MAC address MAC地址. A wireless device uses a wireless network interface card 无线网络接口卡 (WNIC).
- A switch 交换机 forwards each frame only to the port for its destination MAC address; the obsolete hub 集线器 copied every frame to every port. A wireless access point 无线接入点 (WAP) lets wireless clients join the wired LAN.
- A server provides a service to the others; a bridge 网桥 joins two LAN segments into one network; a repeater 中继器 receives a weakened signal and retransmits it at full strength to extend a cable's reach.
The switch at the centre of a star
A switch reads the destination MAC address and forwards to one port
LAN 硬件
- 网络接口卡(network interface card,NIC)让设备在网络上收发,并携带它唯一的 48 位 MAC 地址(MAC address)。无线设备使用无线网络接口卡(wireless network interface card,WNIC)。
- 交换机(switch)只把每个帧转发到其目标 MAC 地址对应的端口;已淘汰的集线器(hub)把每个帧复制到每个端口。无线接入点(wireless access point,WAP)让无线客户端接入有线 LAN。
- 服务器为其他设备提供服务;网桥(bridge)把两段 LAN 连成一个网络;中继器(repeater)接收衰减的信号并以全强度重新发出,以延长线缆的覆盖范围。
星形中心的交换机
交换机读出目标 MAC 地址并转发到一个端口
A NIC's unique, fixed 48-bit hardware address is called its ______ address. · 一个 NIC 唯一的、固定的 48 位硬件地址叫做它的 ______ 地址。
The MAC address is burned into the network interface card. (DHCP hands out IP addresses, which can change.) · MAC 地址被烧进网络接口卡。(DHCP 分发 IP 地址,它可以改变。)
How does a switch differ from a hub? · 一个交换机与一个集线器有什么不同?
A switch sends each frame only out the destination's port (efficient, more secure). A hub copied to all ports. Routing by IP is a router's job. · 一个交换机只把每个帧从目标的端口发出(高效、更安全)。一个集线器复制到所有端口。按 IP 路由是路由器的工作。
A switch is smarter than a hub because it sends each frame only to the port for its destination, not to every port. · 一个交换机比一个集线器更聪明,因为它只把每个帧发到它目标的端口,而不是每个端口。
This makes a switched network faster and more secure than the old hub-based one. · 这使一个交换式网络比旧的基于集线器的更快、更安全。
Which devices extend or join a LAN's physical reach? Select all · 所有 that apply. · 哪些设备延长或连接 LAN 的物理覆盖?选出所有适用的。
A repeater lengthens a cable run, a bridge joins two segments, a WAP adds wireless clients. A NIC is inside one device. · 中继器延长线缆距离,网桥连接两个网段,WAP 加入无线客户端。NIC 在一台设备内部。
Worked example: describe the functions of a WNIC
- The four-mark answer, one point per line.
- It converts the device's data into radio signals and converts incoming radio signals back into data.
- It carries the device's unique MAC address, so frames can be addressed to it.
- It connects the device to a wireless access point and follows the wireless protocol, using the agreed channel and frequency.
例题:描述 WNIC 的功能
- 四分的答案,每行一个要点。
- 它把设备的数据转换成无线电信号,并把收到的无线电信号转换回数据。
- 它携带设备唯一的 MAC 地址,这样帧才能寻址到它。
- 它把设备连到一个无线接入点,并遵循无线协议,使用约定的信道和频率。
A WNIC converts the device's data into ____ signals and back again. · WNIC 把设备的数据转换成____信号,再转换回来。
Radio conversion, the MAC address, the connection to an access point and following the wireless protocol are the four functions the exam lists. · 无线电转换、MAC 地址、连接接入点以及遵循无线协议,是考试列出的四项功能。
The router
- A router 路由器 connects different networks and forwards data between them, usually at the boundary of the LAN and the internet.
- It reads each packet 数据包's destination IP address IP地址 and forwards it out of the right port using its routing table 路由表. That is packet switching 分组交换: the packets of one message may take different routes and are reassembled in order at the destination.
- A home router also does network address translation 网络地址转换 (NAT), letting many private addresses share one public one; runs DHCP 动态主机配置协议 to hand out private addresses; and acts as a firewall 防火墙 against unwanted incoming traffic.
A switch works with MAC addresses inside the LAN; a router works with IP addresses between networks
路由器
- 路由器(router)连接不同的网络并在它们之间转发数据,通常位于 LAN 和互联网的边界。
- 它读出每个数据包(packet)的目标 IP 地址(IP address),并用它的路由表(routing table)从正确的端口转发出去。这就是分组交换(packet switching):同一条消息的各个数据包可能走不同的路线,在目的地按顺序重新组装。
- 家用路由器还做网络地址转换(network address translation,NAT),让许多私有地址共享一个公有地址;运行 DHCP(动态主机配置协议)分配私有地址;并充当防火墙(firewall)阻挡不需要的入站流量。
交换机在 LAN 内用 MAC 地址工作;路由器在网络之间用 IP 地址工作
The main job of a router is to: · 一个路由器的主要工作是:
A router joins networks (e.g. a LAN to the internet) and forwards packets using their destination IP and a routing table. · 一个路由器连接网络(例如一个 LAN 到互联网)并用它们的目标 IP 和一个路由表转发数据包。
Ethernet
- Ethernet is the standard for wired LANs: devices joined by twisted-pair or fibre cable, data sent in frames carrying the source and destination MAC addresses.
- On a shared medium two devices can transmit at once, and the two signals corrupt each other: a collision 冲突.
- Modern switched Ethernet uses full-duplex 全双工 point-to-point links between each device and the switch, so collisions no longer happen, but the protocol for handling them is still examined.
以太网
- 以太网是有线 LAN 的标准:设备用双绞线或光纤连接,数据以携带源和目标 MAC 地址的帧发送。
- 在共享介质上,两台设备可能同时发送,两个信号互相破坏:一次冲突(collision)。
- 现代交换式以太网在每台设备与交换机之间使用全双工(full-duplex)点对点链路,所以不再发生冲突,但处理冲突的协议仍然是考点。
CSMA/CD
- CSMA/CD 载波侦听多路访问/冲突检测 is Carrier Sense Multiple Access with Collision Detection.
- Carrier sense: listen before sending, and wait if the cable is busy. Multiple access: many devices share the medium.
- Collision detection: keep listening while sending. On a collision, stop, send a brief jam signal so every device knows, wait a random backoff time, then try again. The random wait is what stops the same two devices colliding again.
Listen, send, detect, jam, back off, retry
CSMA/CD
- CSMA/CD(载波侦听多路访问/冲突检测)是 Carrier Sense Multiple Access with Collision Detection。
- 载波侦听:发送前先听,线缆忙就等。多路访问:许多设备共享介质。
- 冲突检测:发送时持续监听。发生冲突时停下,发出一个短暂的阻塞信号让每台设备都知道,等待一个随机退避时间,然后再试。随机等待正是阻止同样两台设备再次相撞的东西。
侦听、发送、检测、阻塞、退避、重试
Put the CSMA/CD steps in order for a device that wants to transmit on a shared cable. · 为一个想在共享电缆上传输的设备把 CSMA/CD 的步骤按顺序排列。
Carrier sense → transmit → detect collision → jam → random backoff. The random wait stops both devices clashing again immediately. · 载波侦听 → 传输 → 检测冲突 → 阻塞 → 随机退避。随机等待阻止两个设备立即再次冲突。
Worked example: the three CSMA/CD tasks
- Describe how a device on a shared Ethernet transmits and handles a collision. [3]
- The device listens to the cable, senses the carrier, and transmits only when it is free.
- It keeps checking for a collision while it transmits.
- On a collision it stops, sends a jam signal, waits a random time and retransmits. Three tasks, three marks, in that order.
例题:CSMA/CD 的三项任务
- 描述共享以太网上的设备怎样发送并处理冲突。[3]
- 设备监听线缆,侦听载波,只在空闲时发送。
- 它在发送时持续检查是否发生冲突。
- 发生冲突时它停下,发出阻塞信号,等待随机时间后重发。三项任务,三分,按这个顺序。
After a collision, CSMA/CD makes both devices wait the same fixed time before retransmitting. · 冲突之后,CSMA/CD 让两台设备等待同样固定的时间再重发。
The backoff is random. If both waited the same time they would collide again the moment they retried. · 退避是随机的。如果两台等同样的时间,它们重试的那一刻又会相撞。
Marks that slip away
- A switch forwards by MAC address within one network; a router forwards by IP address between networks. Swapping them costs the mark either way.
- In a star topology the switch is the single point of failure, not the cables.
- The backoff time is random. A fixed wait would make the same two devices collide again and again.
- A hub is not a switch: it sends every frame to every port and is why it is obsolete.
容易丢掉的分
- 交换机在一个网络内按 MAC 地址转发;路由器在网络之间按 IP 地址转发。两者颠倒无论哪种都丢分。
- 星形拓扑中的单点故障是交换机,不是线缆。
- 退避时间是随机的。固定等待会让同样的两台设备一次又一次地相撞。
- 集线器不是交换机:它把每个帧发到每个端口,这正是它被淘汰的原因。
You've got it
- bus one shared cable · star one switch, most common · mesh many paths, fault-tolerant · hybrid a mix; justify by cost, reliability and how a failure spreads
- NIC/WNIC carry the 48-bit MAC address; a switch forwards a frame to one port; WAP, server, bridge, repeater each have one job
- a router joins networks and forwards packets by IP address using a routing table, plus NAT, DHCP and a firewall
- CSMA/CD: listen, send while listening, on a collision jam and wait a random backoff; switched full-duplex Ethernet has no collisions
你掌握了
- 总线一根共享线缆 · 星形一个交换机,最常见 · 网状多条路径,容错 · 混合是组合;用成本、可靠性和故障扩散来论证
- NIC/WNIC 携带 48 位 MAC 地址;交换机把帧转发到一个端口;WAP、服务器、网桥、中继器各有一项工作
- 路由器连接网络并用路由表按 IP 地址转发数据包,外加 NAT、DHCP 和防火墙
- CSMA/CD:侦听、边发边听、冲突时阻塞并等待随机退避;交换式全双工以太网没有冲突