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A short class with Sarah and her students. The other ways are under More.
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Telephone and cable lines were built to carry analog signals, but a computer speaks in digital. What device solves this problem?
A modem. It modulates digital data into analog signals and demodulates them back at the other end.
Right, its name is short for modulator-demodulator. Now, what lets a computer join a network in the first place?
A network interface card, either with an Ethernet port or a Wi-Fi radio.
And every NIC carries an address that the manufacturer sets. What is it called?
A MAC address. It is 48 bits long and written as 12 hexadecimal digits.
Exactly. Keep that address in mind, because switches use it.
Imagine a hub with four computers. Computer 1 sends a message to computer 3. Who receives it?
Computers 2, 3 and 4, because a hub copies the message to every other port.
And with a switch?
Only computer 3. The switch has learnt which MAC address is on which port.
Good. So what is left for a router to do?
It connects different networks, like the school network and the internet, and forwards packets using IP addresses.
Exactly. Switches work inside a network with MAC addresses; routers work between networks with IP addresses.
A 300 MB video is downloaded on a 40 Mbps connection. Someone says it takes 7.5 seconds. What went wrong?
They divided 300 by 40 without changing megabytes into megabits.
Right. So what is the correct time?
300 times 8 is 2400 megabits, and 2400 divided by 40 is 60 seconds.
Correct. Now, a twisted-pair Ethernet cable should not run more than about 100 metres. What if the next building is farther?
Use repeaters to regenerate the signal, or use optical fibre, which carries light much further.
Both are good answers.
Hardware that lets computers send and receive data over networks.
Converts digital signals to analog (modulation) and back (demodulation) for telephone and cable lines.
Every network card connects one computer to a network and has a 48-bit hardware address.
A hub copies data to every port; a switch sends it only to the destination port.
Connects different networks and directs packets by IP address, for example a school network to the internet.
Measured in bits per second; divide the file size in bits (bytes × 8) by the speed to find the time.
A typical home internet box combines a modem, a router, a small switch and a Wi-Fi access point, so one device connects every phone and laptop in the house to the ISP.
This program finds how long a 300 MB file takes to download on a 40 Mbps connection by first changing megabytes into megabits (it prints 2400.0 megabits and 60.0 seconds):
public class DownloadTime {
public static void main(String[] args) {
double fileSizeMB = 300; // file size in megabytes (MB)
double speedMbps = 40; // connection speed in megabits per second (Mbps)
double fileSizeMb = fileSizeMB * 8; // 1 byte = 8 bits
double seconds = fileSizeMb / speedMbps;
System.out.println("File size in megabits: " + fileSizeMb);
System.out.println("Time needed: " + seconds + " seconds");
}
}This program imitates a hub and a switch receiving the same frame on port 1 for the device AA-03: the hub sends it out of ports 2, 3 and 4, while the switch sends it only out of port 3:
public class SwitchVsHub {
public static void main(String[] args) {
// The table a switch has learnt: which MAC address is on which port
String[] mac = { "AA-01", "AA-02", "AA-03", "AA-04" };
int[] port = { 1, 2, 3, 4 };
int fromPort = 1; // the frame arrives on port 1
String destination = "AA-03";
// A hub repeats the frame on every other port
String hubPorts = "";
for (int i = 0; i < port.length; i++) {
if (port[i] != fromPort) {
hubPorts = hubPorts + port[i] + " ";
}
}
System.out.println("Hub sends the frame out of ports: " + hubPorts.trim());
// A switch looks up the destination and uses only that port
for (int i = 0; i < mac.length; i++) {
if (mac[i].equals(destination)) {
System.out.println("Switch sends the frame out of port: " + port[i]);
}
}
}
}The hub shouts to all, the switch whispers to one, the router finds the road till the journey is done.
A letter leaves a student's computer through its network card, the switch in the lab hands it to the router at the school gate, the modem translates it for the outside line, and routers across the internet pass it from city to city until it reaches the website.
Bytes are Bigger: multiply megabytes by 8 to get megabits before dividing by the speed.
N.S.R.M.: NIC, Switch, Router, Modem, the order in which a school computer's request leaves for the internet.
Term
What does a modem do?
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Term
How long is a MAC address, and who sets it?
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Hub or switch: which one sends a message only to its destination?
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Which device connects a school network to the internet?
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How long does a 300 MB file take to download at 40 Mbps?
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A modulator-demodulator that converts digital signals to analog signals for transmission and converts them back when receiving.
The hardware that connects a computer to a wired or wireless network.
A 48-bit physical address set by the manufacturer of a network card, written as 12 hexadecimal digits.
A device that joins computers in a LAN and copies every incoming signal to all its other ports.
A device that joins computers in a LAN and forwards each frame only to the port of its destination, using MAC addresses.
A device that connects different networks and forwards packets between them using IP addresses.
A device that regenerates a weakened signal so that it can travel further.
A device that connects networks using different protocols and translates between them.
Communication devices let computers exchange data with one another and with networks such as the internet; the main ones are the modem, the network interface card, the hub, the switch, the router, the repeater, the access point, the bridge and the gateway.
A single computer becomes far more useful once it can share data. This lesson explains the wired and wireless media that carry data, then the job of each communication device: the modem that converts between digital and analog signals, the network interface card with its MAC address, hubs and switches that join computers in a local network, routers that join networks and direct packets by IP address, and repeaters, access points, bridges and gateways. It ends with how data transfer rate is measured and how to work out download times.
Communication devices are hardware that allow computers and other devices to send and receive data over a network, whether across a room, a building, a city or the whole world through the internet. Every message, video call and web page you use travels through several of them.
Data travels along a medium:
A modem (MOdulator-DEModulator) converts the computer's digital signals into analog signals that can travel over telephone or cable lines (modulation), and converts the analog signals it receives back into digital form (demodulation). Types include dial-up modems (now rarely used), DSL modems that work over telephone lines and cable modems that work over cable TV lines.
A NIC, or network adapter, connects a computer to a network. A wired NIC has an Ethernet port that takes an RJ-45 connector, and a wireless NIC uses radio waves to join a Wi-Fi network. Most computers today have a NIC built into the motherboard. Every NIC has a MAC address, a 48-bit physical address set by the manufacturer and written as 12 hexadecimal digits, such as 00:1A:2B:3C:4D:5E.
The speed of a connection, often called its bandwidth or data transfer rate, is measured in bits per second: Kbps (thousand), Mbps (million) and Gbps (billion). File sizes are measured in bytes, and 1 byte = 8 bits. For example, a 300 MB file on a 40 Mbps connection needs 300 × 8 = 2400 megabits, which takes 2400 ÷ 40 = 60 seconds, ignoring delays and overheads.
When a school computer opens a website, the request leaves through the computer's NIC, passes through the lab's switch to the router, goes through the modem to the ISP, and is passed from router to router across the internet to the web server. The reply returns along a similar path, and the switch delivers it only to the computer that asked for it.