Network Types and Devices
FoundationComputer networks are classified by coverage into personal, local, metropolitan and wide area networks. Offices and schools build a LAN with twisted pair or wireless links, using a switch and a router in a star topology for resource sharing and data transfer.
- Networks are classified by coverage into PAN, LAN, MAN and WAN; judge the coverage first, then the type.
- Twisted pair is cheap and common, optical fibre is fast and long-reaching, wireless is flexible but prone to interference and needs encryption.
- A switch forwards by address, a hub broadcasts, a router routes across networks — three clearly different layers.
文體小知識：科技知識說明文——講解電腦網絡的類型、傳輸介質與連網設備，比較拓撲結構的優劣，是網絡與互聯網單元的入門課。
§Text Passage
1A computer network connects two or more computers and peripheral devices so that they can share resources and exchange data. Once connected, files, printers and internet access can be shared, while users communicate and collaborate instantly. Networks are the infrastructure of the modern information society and the starting point of studying the internet.
2By coverage, networks fall into four types: a personal area network covers about ten metres, such as a Bluetooth headset linked to a phone; a local area network covers a room up to a whole building, such as a school computer room; a metropolitan area network covers a city; a wide area network spans cities or countries, and the internet is the largest WAN of all.
3Networks need transmission media to carry signals. Wired media include twisted pair, coaxial cable and optical fibre: twisted pair is cheap and easy to install, the first choice for ordinary LANs; coaxial cable resists interference better; optical fibre sends data as light pulses, fast and long-reaching, mainly used in backbones. Wireless transmission relies on radio waves, flexible but more prone to interference.
4Networking depends on devices: a network interface card lets a computer join the network; both a hub and a switch can connect many computers, but a switch forwards data only to the destination by address, far more efficient than a hub that broadcasts to every port; a router connects different networks and chooses the best path, indispensable for internet access.
5The geometric arrangement of nodes and links is called the network topology. In a star topology a central device connects all nodes; a single broken cable affects only one computer and management is easy, but failure of the central device brings down the whole network. A bus topology links all nodes along one backbone cable, saving cable, but a broken backbone stops the entire network and collisions are frequent.
1Summary 重點摘要
Computer networks are classified by coverage into personal, local, metropolitan and wide area networks. Offices and schools build a LAN with twisted pair or wireless links, using a switch and a router in a star topology for resource sharing and data transfer.
2Vocabulary cards Vocabulary
A system linking two or more computers and peripherals with transmission media so they can share resources and exchange data.
A network confined to a room, office or building, such as a school computer room network.
A network spanning cities, countries or continents, formed by interconnecting LANs through routers; the internet is the largest example.
Copper cable made of wire pairs twisted together to reduce interference; the most common wired medium in LANs.
A hair-thin glass medium that carries data as light pulses; very high bandwidth, long distance and immune to electromagnetic interference.
The component that connects a computer to a network, converting data into signals sent over the medium.
A central device connecting hosts in a star layout; it broadcasts incoming data to every port, so efficiency is low.
A central connecting device that forwards data only to the port of the destination address, beating the hub in both efficiency and security.
A device joining two or more networks and routing data by destination address; the key to linking a LAN to the internet.
The geometric arrangement of nodes and links in a network; common forms are star, bus and ring.
3Step by step Steps
- Step 1: What a computer network isA computer network links two or more computers and peripherals through transmission media so that they can communicate and share resources. Its benefits come in three forms: resource sharing, such as many computers using one network printer; data transfer, such as sending a homework file to a classmate; and centralised management, such as a school storing student files on a server. For example, home devices sharing broadband and a printer through a wireless router form the simplest home network.
- Step 2: Classifying networks by coverageBy geographic coverage networks fall into four types. A personal area network is the smallest, a few to ten metres, such as a Bluetooth headset paired with a phone. A local area network covers a room up to a building, such as a school computer room or an office. A metropolitan area network covers a whole city. A wide area network spans cities and countries, such as a bank linking its branches. The internet joins networks worldwide and is the largest WAN. For example, judge the coverage first, then pick the type, and confusion disappears.
- Step 3: Wired transmission mediaWired media carry signals through physical cables, mainly three kinds. Twisted pair winds two conductors around each other to cancel interference; it is cheap and easy to install, the first choice for ordinary LANs. Coaxial cable has a shielding layer around the central conductor and resists interference better; it was common in early LANs and cable TV. Optical fibre sends light pulses through hair-thin glass, offering huge bandwidth, long reach and immunity to electromagnetic interference, mainly for building backbones and submarine cables. For example, the backbone from a school server room to classrooms is usually fibre.
- Step 4: Wireless transmission and bandwidthWireless transmission carries data on radio waves. The most common form is the wireless LAN, where devices join through an access point without cabling, ideal for mobile devices; Bluetooth suits short links within ten metres, such as wireless headsets and keyboards. Wireless is flexible and mobile-friendly, but speed and stability suffer from distance, walls and interference, and extra encryption is needed for security. Bandwidth means how much data a medium can carry per second; the larger it is, the faster the transfer. For example, HD video streaming demands far more bandwidth than ordinary web browsing.
- Step 5: Connecting devices — hub versus switchThe network interface card converts a computer's data into signals on the medium and is essential for every networked machine. A hub and a switch both connect many devices centrally, yet they work very differently: a hub broadcasts incoming data to all ports and lets each computer decide whether to accept it, causing collisions; a switch learns the address of the device on each port and forwards data only to the target port, so it wins in both efficiency and security. For example, modern offices and school computer rooms use switches almost exclusively, and hubs are being phased out.
- Step 6: Router and modemA router connects two or more networks and forwards each data packet along a suitable path according to its destination address, acting as the bridge between a LAN and the internet; home routers often combine a switch, an access point and firewall functions in one box. A modem modulates a computer's digital signals into forms transmittable over telephone or fibre lines and demodulates them at the far end. For example, after fibre enters a home, the modem converts the signal first, and the router then distributes connectivity to every device; the two are often merged into one unit.
- Step 7: Comparing network topologiesTopology describes the geometric arrangement of nodes and links. In a star topology one central device, usually a switch, connects all nodes: a single broken cable affects only one computer, faults are easy to isolate and expansion is easy, making it today's mainstream for LANs; the drawbacks are total failure if the central device breaks and higher cabling cost. A bus topology strings all nodes along one backbone cable, saving cable at low cost, but a broken backbone halts the whole network and collisions multiply as nodes grow. For example, a ten-computer room should pick the star layout for easy per-machine maintenance.
4Diagrams Diagrams
5Worked example Example
Design task: a ten-person small office needs a LAN. All computers must share one network printer and reach the internet, with a limited budget but stable, easy-to-manage operation. Suggest the transmission medium, the central connecting device and the topology, and justify each choice.
- Requirement analysis: ten computers in one office form a LAN; sharing a printer and internet access requires a central connecting device plus a router for external connectivity.
- Medium: the office is small and stability matters, so twisted pair is the most economical; if backbone bandwidth must rise later, upgrade to optical fibre.
- Devices: use a switch as the central device with all computers cabled in via twisted pair, beating a hub in efficiency and security; then a router connects the whole network to the internet.
- Topology: adopt a star topology so that any single cable or computer failure leaves the rest untouched, making maintenance easy and expansion a matter of adding ports.
- Answer: Recommended: connect the ten computers to one switch with twisted pair in a star-topology LAN, and attach a router for internet access. Reasons: twisted pair is cheap and stable; a switch forwards by address, outperforming a hub in efficiency and security; a star topology limits the impact of faults and is easy to manage and expand; the router routes traffic externally and is indispensable for internet connection.
6Common mistakes Mistakes
7Quick check Quick check
Q1Fifty computers are interconnected in a school computer room. Which type of network is this?
Q2In what way is a switch better than a hub?
Q3In a star topology, what happens if the central switch fails?
Q4Which medium should a building backbone use for high speed over long distance?
Q5Which device routes data between different networks by destination address?
8Recap Recap
- Networks are classified by coverage into PAN, LAN, MAN and WAN; judge the coverage first, then the type.
- Twisted pair is cheap and common, optical fibre is fast and long-reaching, wireless is flexible but prone to interference and needs encryption.
- A switch forwards by address, a hub broadcasts, a router routes across networks — three clearly different layers.
- In a star topology the central device links all nodes: one broken cable harms nothing else, but a dead centre stops everything.
- A bus topology saves cable but a broken backbone halts the whole network, and collisions grow with the number of nodes.