Network Classification and the Physical Foundation of Wired Connections
頻出~39 min7 terms19 past questions
Goals of this unit
Tell a LAN from a WAN by the area each connects and by who provides the lines.
Explain how a communication line and a transmission path, the routes signals travel, relate to the cable that physically carries them.
Name the parts and technologies that connect devices on a wired LAN, and work out a file's transfer time from the transmission speed and transmission efficiency.
A LAN is a network that connects devices within a limited area, such as a building, a floor, or a single site.
The name stands for Local Area Network. Because the area is limited, the organization or household that uses it can lay the wiring, gather the equipment, and build and manage it on its own.
AnalogyThink of the water pipes inside a house. On your own property, you can run the pipes however you like.
How to rememberLocal means "of this place", which matches the sense of "inside the building where we are".
Exam points
ExamBecause its area is limited, a LAN can be built by the organization or household that uses it, without a contract with a carrier (通信事業者). Whether a network is a LAN depends on the area it covers, not on whether it uses cable (ケーブル) or wireless.
ExamIn a LAN that wires several devices together, each device is usually connected to a central concentrator called a hub (ハブ). This layout, radiating out from the hub, is called a star topology (スター型).
A WAN is a network that spans cities and regions to link sites located far apart.
LAN at site A
Line service from a carrier
LAN at site B
The name stands for Wide Area Network. Running your own wiring over long distances is not practical, so a WAN is usually built on line services provided by a carrier.
AnalogyIt is like the railway between towns. You do not lay your own track; you ride the line the railway company runs to reach a distant town.
How to rememberWide means broad. Whether the first letter is L (Local) or W (Wide) tells you how far the network reaches.
Exam points
ExamA WAN uses services from a carrier (通信事業者) to link the LANs of distant sites.
ExamWhen the other party is at a site in another region, a WAN is needed. Communication that stays within one building or one site needs only a LAN.
Mix-upWhether a network is a LAN or a WAN depends not on how many buildings it spans but on whether it links sites in distant regions.
A communication line (通信回線) is the communication route used to send and receive data between devices or sites in separate locations.
Circuit switching
Packet switching
How the line is used
One pair holds the route for the whole call
Data is cut into small pieces and many users share the line
Line utilization
Tends to be low, since the route stays reserved even when idle
High, since other users' data fills the gaps
Guaranteed speed and quality
Easy to guarantee, as the route is exclusive
Easily affected by how busy the line is
Many lines, such as telephone and fiber-optic lines, are offered as services by carriers, though some are laid and managed by the company itself. A method in which one pair of users holds the route for the entire exchange is circuit switching (回線交換方式), while one that splits data into small units called packets (パケット) so many users can share the line is packet switching (パケット交換方式). When usage piles up all at once and traffic stalls because it exceeds what the line can handle, the state is called congestion (輻輳).
AnalogyCircuit switching is like a chartered bus, and packet switching like a regular route bus. On a charter you are sure of a seat but many seats go empty, while a route bus fills its seats without waste.
Exam points
ExamWith packet switching (パケット交換方式), packets (パケット) from many users travel mixed together on one line, so no gap in the line goes to waste.
ExamCircuit switching (回線交換方式) does not share its route with other users, so it is little affected by how busy other traffic is, and speed and quality are easy to keep steady.
ExamCongestion (輻輳) is the state in which traffic surges, the line or network cannot keep up, and communication stalls.
A transmission path (伝送路) is the route a signal actually travels from sender to receiver, covering both wired media such as wiring and the space radio waves pass through.
500 MB × 8 = 4,000 Mbit
50 Mbit/s × 0.8 = 40 Mbit/s
4,000 Mbit ÷ 40 Mbit/s = 100 s
Divide the number of bits by the actual speed to get the transfer time.
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The speed of a transmission path is expressed as its transmission speed (伝送速度, in bps), the number of bits it can send per second. Since control data and the like also flow over the path, the share of the transmission speed that can carry user data is called the transmission efficiency (伝送効率).
AnalogyIf the number of vehicles a road can pass per hour is the transmission speed, the share of them that are trucks carrying cargo is the transmission efficiency. The rest are escort vehicles that manage traffic and carry no cargo.
Exam points
ExamTo find the transfer time, convert the file size to bits and divide by the actual speed, which is the transmission speed (伝送速度) times the transmission efficiency (伝送効率). Dividing while still in bytes leaves the answer off by a factor of 8.
Mix-upA transmission path (伝送路) and a communication line (通信回線) both refer to the route signals or data take, and the two are often used almost interchangeably. Transmission path focuses on the medium itself, while communication line focuses on the route being used.
A cable (ケーブル) is the physical wiring that carries signals in wired communication, a transmission path in a form you can see.
Twisted-pair cable
Fiber-optic cable
Signal carried
Electrical
Light
Core material
Twisted copper wires
Thin fibers of glass or similar
Effect of noise
Easily affected
Hardly affected
Typical use
Connecting PCs and the like inside a building
Long-distance or high-speed backbones
The twisted-pair cable (ツイストペアケーブル) common in LANs sends electrical signals through copper wires twisted together. A fiber-optic cable (光ファイバケーブル) sends signals as light, so they weaken little even over long distances and are hardly affected by electrical noise.
AnalogyA cable is a pipe for signals. Picture copper wire as a metal pipe that conducts electricity and optical fiber as a glass pipe that lets light through, and the difference becomes easy to grasp.
How to rememberThe English word cable originally meant the thick rope used to moor a ship.
Exam points
ExamWhen a cable is plugged into a device's port and the link light (リンクランプ) comes on, all that is confirmed is the physical connection on that segment. Whether the device can reach equipment beyond it or the internet has to be checked separately.
A wired LAN (有線LAN) is a LAN in which devices are physically connected by cable.
Wired LAN
Wireless LAN
Connection
Cable
Radio waves
Stability
Tends to be stable
Easily affected by nearby radio waves and walls
Device placement
Wherever the wiring reaches
Free to move within radio range
Compared with a wireless LAN (無線LAN), which uses radio waves, it takes more effort to wire and its devices are harder to move, but communication tends to be more stable. A technology called PoE (Power over Ethernet) also sends power to devices over the LAN cable along with the data.
AnalogyIt is like a corded landline phone. The sound is steady, but you can only use it as far as the cord reaches.
How to rememberThe opposite of wired is wireless; the names split on whether there is a wire.
Exam points
ExamPoE is a technology that delivers data and power together over a LAN cable. It is used, for example, for devices mounted on ceilings or walls with no power outlet nearby.
Mix-upPoE puts power onto a LAN cable, whereas PLC puts communication signals onto indoor power lines, so the pairing runs the opposite way.
A network interface card (ネットワークインタフェースカード, NIC) is the part that connects a device such as a PC or printer to a LAN so it can communicate.
← Scroll the figure sideways →
It has a socket (port) for the cable and passes signals between the device and the network. It is also called a LAN adapter, and most current PCs have one built in from the start. Every NIC is assigned a unique number for identifying the device, its MAC address (MACアドレス), at the time of manufacture.
AnalogyIt is like the front door of a house. It is the entrance between the street and the inside, and people and parcels come and go through it.
How to rememberThe abbreviation NIC is pronounced "nick". Interface means a boundary or point of contact, so think of it as the part sitting where the device meets the network.
Exam points
ExamA NIC (ネットワークインタフェースカード) is the part that lets terminals and peripherals join a network and send and receive data.
Mix-upA NIC only covers connecting its own device to the LAN. When the destination is outside the LAN, the exit the device hands its packets to is the role called the default gateway (デフォルトゲートウェイ), and a separate device fills it.