Exam practice

Broadband is a high-speed internet connection, always on, that can carry a large amount of data quickly (DSL, cable, fiber, satellite, wireless).

Bandwidth is the maximum amount of data a network connection can carry in a given time: the “width” of the connection. It is measured in bits per second: bps, kbps (thousand), Mbps (million) and Gbps (billion bits per second).

A data packet is a small piece of a message or file sent over a network. Data is broken into packets; each carries part of the data along with details such as the sender’s and receiver’s addresses and its number, travels on its own, and is put back in order at the destination. A lost packet can be sent again on its own.

Frequency is the number of waves of a signal sent each second, measured in hertz (Hz): kHz (thousand), MHz (million), GHz (billion). Each wireless technology uses its own range of frequencies (FM radio 88–108 MHz, Wi-Fi 2.4 and 5 GHz). Higher frequencies can carry more data but reach less far.

A repeater receives a signal that has grown weak over a long cable (or far from a Wi-Fi router), regenerates it to full strength and sends it on, so data can travel longer distances without being lost. It doesn’t read or filter the data.

A computer network is two or more computers and devices connected by wire or wireless media to share data, resources and messages. It is useful because it lets us:

• share hardware (one printer, scanner or internet connection for many computers) and software;
• communicate: email, chat, video calls;
• share and keep data in one place, and back it up together;
• reach information and services worldwide through the internet.

• Freedom to move: phones, laptops and tablets stay connected anywhere in range.
• Easy and cheap to set up: no cables to lay through walls, across hills or between buildings.
• Many more devices (phones, smart TVs, IoT sensors) can join easily.
• Mobile networks (4G, soon 5G) and Wi-Fi are now fast enough for video and online classes.

RJ-45 (Registered Jack 45) is the standard plug of Ethernet (twisted pair) cables: a clear plastic plug with eight pins, one for each of the cable’s eight wires in a fixed color order, with a clip that holds it in its socket. It is crimped onto the cable. It is used to connect computers, printers, switches and routers in wired LANs: labs, offices, homes and data centers.

A media converter is a device that joins two different kinds of cable in a network, usually copper (twisted pair) and optical fiber. Its main function is to convert the electrical signals of the copper cable into light signals for the fiber, and back, so that, for example, two buildings too far apart for copper can be joined by fiber.

Throughput is never more than the bandwidth.

Wi-Fi sends data as radio waves in the 2.4 GHz and 5 GHz bands. The device’s Wi-Fi radio turns the data into radio signals; the access point (usually in the Wi-Fi router) receives them and passes the data on to the wired network and the internet, and sends the replies back by radio. Each network uses a channel in its band, and can aim its signal at the device (beamforming) for a better connection.

The sending computer cuts the data into packets, each labeled with the receiver’s and sender’s addresses and a number. Its network card sends them, as electrical signals in a cable, light in a fiber or radio waves, to a switch or access point, which passes them to the receiving computer on the same network; for another network, they go to the router, which sends each packet on towards its address, router by router. The receiver checks every packet has arrived, asks again for any that are missing, and puts them back in order.

In a ring, each computer is joined to the next and the last to the first. Data travels round the ring in one direction, from computer to computer. Each computer receives it, keeps it if it is addressed to itself, and otherwise passes it on to the next. In a token ring, only the computer holding a special message, the token, may send, so messages never collide.

Live television broadcast to dish antennas across Nepal (direct-to-home TV). (Others: GPS navigation in phones; weather forecasting; internet and phone calls in remote mountain villages.)

• Guided (wired) media: optical fiber cable (or twisted pair CAT6, coaxial cable).
• Unguided (wireless) media: Wi-Fi radio waves (or Bluetooth, microwave, satellite, infrared).

Communication media (transmission media) are the paths or channels that carry data and information between devices in a network. They are of two types:

1. Guided (wired, bounded) media: the signal is guided along a cable.

• Twisted pair (e.g. CAT6): four pairs of twisted copper wires; cheap, used in LANs; up to 1 Gbps over 100 m.
• Coaxial cable: a copper core, insulator, braided shield and jacket with one center; used for cable TV.
• Optical fiber: glass strands carrying pulses of light; fastest, longest reach, no electrical interference; used by ISPs.

Guided media are fast, reliable and secure, but cables must be laid.

2. Unguided (wireless, unbounded) media: signals travel through the air or space.

• Radio waves: Wi-Fi, Bluetooth, mobile networks, RFID.
• Microwave: dish-to-dish links between towers in a straight line.
• Satellite: signals up to a satellite (uplink) and down (downlink), worldwide.
• Infrared: short range, line of sight, like a TV remote.

Unguided media let devices move and reach places cables can’t, but are affected by distance, walls and weather, and are easier to listen in to.

The school should build a LAN covering its campus: a hybrid network with a star in each building, the three stars joined by a fiber optic backbone, under a client–server architecture.

• In each building: computers and printers joined by CAT6 twisted pair cable to a switch (a star), with Wi-Fi access points for laptops and phones.
• Between buildings: optical fiber joins each building’s switch to the main switch (with media converters or fiber ports), since copper reaches only 100 m and fiber isn’t affected by lightning-induced interference between buildings.
• Internet access: the main switch connects to a router and the ISP’s fiber line; the router shares one public address among all (NAT) and hands out addresses (DHCP), with a firewall to block unwanted traffic.
• Communication: email and messaging between teachers and offices over the LAN; video lessons between buildings.
• File sharing: a file server in the office holds shared folders (teachers’ notes, results), backed up in one place, with each user’s access controlled by a password.

This is fast (1 Gbps or more inside, fiber between buildings), reliable (one building’s fault doesn’t stop the others), secure (central control) and easy to grow (add a switch or access point).

Devices: the ISP’s fiber box (ONT/modem), a Wi-Fi router (router, 4-port switch and access point in one), three PCs, one printer, one mobile phone.
Connections:

• ISP fiber → ONT → router’s internet (WAN) port, by a short Ethernet cable.
• PC 1 and PC 2 (near the router): CAT6 cables with RJ-45 plugs to the router’s switch ports: wired, fastest and steadiest.
• PC 3 (another room): Wi-Fi, or a cable if one can be laid.
• Printer: Wi-Fi (or a cable to the third port), shared by all the PCs and the phone.
• Mobile phone: Wi-Fi.

Technologies: the router gives every device a private IP address by DHCP and shares the one public address by NAT; its switch joins the wired devices in a star; its access point (2.4 GHz for range, 5 GHz for speed) joins the wireless ones. Secure the Wi-Fi with WPA2 or WPA3 and a strong password, and change the router’s admin password.
It is a LAN with a star topology, joined to the internet (a WAN) through the router.

For a small office (say 3–10 computers and a printer), peer-to-peer is usually more suitable:

• Cost: no server computer, server software or administrator to pay for; peer-to-peer is built into Windows, macOS and Linux.
• Management: simple; each user shares folders and the printer themselves.
• Security: weaker than client–server, but acceptable for a few trusted staff with good passwords.
• Scalability: fine for a few computers; beyond about ten it becomes hard to manage.

If the office keeps sensitive data (accounts, customers’ details), expects to grow, or needs central backup, client–server is better: central security and backup, easy to add users, but at a higher cost and with someone to manage the server. A common middle way is peer-to-peer with shared files in a cloud service that keeps backups.

Layout (star topology): a 16-port switch in the middle of the lab; each of the 10 computers joined to it by its own CAT6 cable with RJ-45 plugs; a shared printer on the switch; the switch joined to a router, which is joined to the ISP’s fiber box (ONT); an access point on the switch for the teacher’s laptop.

Internet (ISP fiber)        |   fiber box (ONT)        |     router  (DHCP, NAT, firewall)        |   16-port switch ---- printer   /  |  |  ...  \  \ PC1 PC2 PC3 ... PC10  access point ))) teacher's laptop

Why:

• Star with a switch: one faulty cable or computer affects only itself; easy to find faults; the switch sends each message only where it is going, so all computers get full speed.
• CAT6 cable: 1 Gbps, cheap, reliable within a room (under 100 m).
• Spare switch ports for more computers later.

How they communicate: the router gives every computer an IP address by DHCP (192.168.1.x). A computer printing sends packets to the printer’s address; the switch delivers them only to the printer’s port. A computer opening a website sends its packets to its default gateway, the router, which forwards them to the internet through NAT and sends the replies back to the right computer.

A star topology LAN: every computer joined by its own CAT6 Ethernet cable to a switch (a 48-port switch, or two 24-port switches joined together), the switch joined to the router and the internet.

• Stable: a broken cable or a faulty computer affects only that computer; the other 35 keep working.
• Easy to maintain: each cable runs to one port, so a fault is found quickly; computers can be added or moved without stopping the network.
• Fast file sharing: the switch sends each message only to its destination, so many pairs share files at once at up to 1 Gbps.
• Internet: one router shares the connection with all 36.

A bus would fail completely with one break and slow down with 36 computers; a ring would stop if one computer is off. The only weak point is the switch, so keep a spare.

Possible reasons:

• The router is in a corner of the ground floor, and the signal weakens with distance and through the concrete floor and walls (5 GHz Wi-Fi fades fastest).
• Interference: neighbors’ Wi-Fi on the same channel, a microwave oven, or Bluetooth devices in the 2.4 GHz band.
• With a weak signal, packets are lost and sent again, so the throughput falls even though the plan’s bandwidth is the same.
• Many devices sharing the connection at once.

Solutions:

• Move the router to a central, high place, such as near the stairs, away from metal and thick walls.
• Add a Wi-Fi repeater (extender) or, better, a second access point or a mesh Wi-Fi system on the first floor, joined by a cable where possible.
• Change to a less crowded channel; use 2.4 GHz upstairs for range, 5 GHz near the router for speed.
• Use a cable for a desktop computer upstairs.

Upgrades:

• A 5G router (or 5G mobile broadband device) where 5G is available, with 4G as a fallback; until 5G is on sale in Birgunj, 4G LTE or a fiber broadband line.
• Replace old hubs with a gigabit switch and CAT6 cables, and old Wi-Fi routers with dual-band (Wi-Fi 6) access points, so the inside of the network doesn’t become the bottleneck.
• New devices: phones and laptops that support 5G and Wi-Fi 6.

Benefits of 5G over 3G:

• Much higher throughput (hundreds of Mbps instead of a few): fast uploads and downloads to cloud storage, smooth video meetings.
• Very low latency (about 10 ms instead of about 100): cloud apps feel as quick as local ones; calls don’t lag.
• Many more devices at once: room for IoT devices, such as security cameras, sensors and smart meters.
• Reliable mobile broadband for staff working on the move.

The office should also secure the new network (firewall, strong passwords) and check that its cloud services and internal network are fast enough to use the new speed.