CPX12Networks, the Internet and Network Performance
Edexcel 1CP2 · 4.1.1
Why devices are networked
A computer network is two or more connected devices that exchange data and share resources.
Reasons to network devices
Drawbacks of networks
Share resources: one printer, scanner or internet connection serves many users.
Malware can spread from one infected device to every other device on the network.
Share files and software: everyone opens the same, up-to-date copy from a server.
An outage affects everyone: if a server or switch fails, the whole organisation may stop work.
Collaborate: several people can work on the same document or project.
Cost: servers, cabling, switches and skilled staff to run them.
Communicate: email, messaging and video calls between users.
More ways in for attackers: each connected device is a possible point of attack.
Central management: new software and updates can be rolled out to every machine at once; backups and security can be run from one place.
Exam tipMake the reason fit the organisation in the question. ‘Share files’ is fine, but ‘the box office and the marketing team both see the same seating plan, so a seat isn’t sold twice’ is what turns a 1-mark point into a 2-mark explanation.
Computer network
Two or more connected devices that exchange data and share resources.
Resource
Something useful that can be shared, such as a printer, a file, software or an internet connection.
Edexcel 1CP2 · 4.1.2
LANs and WANs
Networks are grouped by the geographical area they cover and by who owns the infrastructure (the cables and equipment).
LAN (local area network)
WAN (wide area network)
Area
A small geographical area: one building or one site
A large geographical area: towns, countries, the world
Infrastructure
Owned and managed by the organisation that uses it
Mostly owned by third parties, such as telecoms companies, and leased
Connects
Devices, usually through switches
LANs (and other networks) together
Media
Copper or fibre cable, and Wi-Fi
Fibre-optic and copper cables, satellites and microwave links
Example
A school’s network
A bank linking its branches; the internet is the largest WAN
Each site has its own LAN; a WAN links the sites
Exam tipPearson does not accept ‘a WAN is a bigger network’. Size isn’t the point: say it covers a larger geographical area, or that its infrastructure is owned by a third party.
LAN
Local area network: covers a small geographical area, such as one site; the infrastructure is owned by the organisation.
WAN
Wide area network: covers a large geographical area and connects LANs; it uses infrastructure owned by third parties.
Edexcel 1CP2 · 4.1.3
The internet and IP addresses
The internet is a global network of networks. Millions of networks are joined together, and every device on them needs an address.
An IP address is a unique identifier for a device on the internet. Packets carry the source and destination IP addresses, so they reach the right device and replies can come back.
IPv4 addresses are 32 bits long, written as four denary numbers from 0 to 255 separated by dots, e.g. 81.2.69.160. 32 bits give 232, about 4.3 billion, addresses.
IPv6 addresses are 128 bits long, written as eight groups of four hexadecimal digits separated by colons, e.g. 2001:0db8:3c4d:0015:0000:0000:1a2f:1a2b. That gives 2128 addresses: far more than will ever be needed.
Why IPv6 was introduced: the number of devices connected to the internet (phones, tablets, smart devices) grew so much that the 4.3 billion IPv4 addresses were running out.
Exam tipIf a question asks for the identifier used to send packets across the internet, the answer is the IP address. ‘Address’ on its own isn’t enough.
Internet
A global network of networks.
IP address
A unique identifier for a device on a network, used to send packets to the right place.
IPv4
32-bit IP addresses, written as four numbers from 0 to 255.
IPv6
128-bit IP addresses, written in hexadecimal; introduced because IPv4 addresses were running out.
Edexcel 1CP2 · 4.1.3
Domain names and DNS
People remember names like www.example.org; computers need IP addresses. A DNS server (domain name system) matches one to the other.
1. The user types a domain name (or clicks a link) in their web browser.
2. The browser sends the domain name to a DNS server, asking for its IP address.
3. The DNS server looks the name up and sends back the IP address of the web server. If it doesn’t have the name, it asks other DNS servers.
4. The browser sends a request for the page to the web server at that IP address.
5. The web server sends the page back to the browser, split into packets.
The browser gets the IP address first, then asks the web server for the page
Exam tipKeep the two servers apart: the DNS server only supplies the IP address; the web server stores and sends the page.
Domain name
A readable name for a web server, such as www.example.org.
DNS server
A server that matches domain names to IP addresses.
Web server
A computer that stores web pages and sends them to browsers that request them.
Edexcel 1CP2 · 4.1.3
Packets, routers and the backbone
Data sent over the internet is split into small units called packets. Each one finds its own way to the destination.
A router forwards packets between networks. It reads the destination IP address in the header and uses a routing table to choose where to send the packet next.
Each router passes the packet to the next router on the best route, avoiding busy (congested) or broken links. The packet hops from router to router until it reaches the destination network.
Packets from the same file can take different routes and arrive in a different order. They are put back together at the destination (see the protocols topic).
The internet backbone is the set of principal high-speed routes between the big interconnected networks. It is mostly fibre-optic cable, including cables under the sea.
Part of a packet
What it holds
Header
The source and destination IP addresses, and the packet’s number (its position in the message)
Payload
The data being sent
Footer
An error check, such as a checksum, and a marker for the end of the packet
Header, payload and footer; routers forward each packet on
Exam tipIf you’re asked how a router gets data to its destination, say what it reads (the destination IP address), what it uses (a routing table) and what it does (sends the packet to the next router on the best route). ‘It sends it to the right place’ earns nothing.
Packet
A small unit of data sent across a network, with a header, a payload and a footer.
Router
A device that forwards packets between networks, using the destination IP address and a routing table.
Routing table
A router’s list of where to send packets for each destination network.
Internet backbone
The principal high-speed data routes between interconnected networks.
Edexcel 1CP2 · 4.1.4
Wired connections: copper and fibre
Wired networks carry data along cables. There are two kinds.
Compared with wireless, a wired connection usually gives higher speeds, lower latency and a more reliable link, because each cable is shielded from most interference and isn’t shared with every device in range.
But devices are tied to a socket, and running cables through an existing building costs time and money.
Copper cable
Fibre-optic cable
How data travels
As electrical signals
As pulses of light along thin glass or plastic fibres
Bandwidth and speed
Lower
Much higher
Range
Signal weakens after about 100 m
Many kilometres before the signal needs boosting
Interference
Affected by electrical interference from nearby equipment and cables
From a wall socket to a desk; short runs in a home or office
Between buildings, across a city, the internet backbone
Exam tipDon’t say ‘fibre is faster’ and stop. Say what is better and why: ‘fibre has a higher bandwidth, so more data can be sent each second’, or ‘fibre keeps its signal over long distances, so it suits a link between two buildings’.
Copper cable
Cable that carries data as electrical signals.
Fibre-optic cable
Cable that carries data as pulses of light along glass or plastic fibres.
Interference
Unwanted signals that disrupt the data being sent.
Edexcel 1CP2 · 4.1.4
Wireless connections
Wireless connections send data through the air using radio waves. You need to know five.
Advantages of wireless: users can move around and stay connected; no cables to install; easy to add new devices.
Disadvantages of wireless: walls, distance and other devices weaken the signal (interference); bandwidth is shared, so it slows as more devices connect; signals can be picked up by anyone in range; usually higher latency than cable.
Most networks use both: cables for fixed computers, servers and links between buildings; Wi-Fi for laptops and phones that move.
Technology
Range
What it’s like
Typical use
Wi-Fi
Tens of metres from the access point
High bandwidth; shared by every device connected to the access point
Laptops, phones and tablets in homes, schools and offices
Bluetooth
About 10 m
Low power, lower bandwidth; devices are paired
Earbuds, keyboards, a phone linked to a car
Zigbee
Tens of metres; devices pass messages on to each other
Very low power and low bandwidth, so batteries last for years
Smart home sensors, smart plugs and light bulbs
RFID (radio frequency identification)
A few centimetres to a few metres
A reader’s radio waves power a small tag, which sends back its ID
Stock tags in shops, library books, animal microchips
NFC (near field communication)
About 4 cm
Very short range, so it only works when the devices touch or nearly touch
Contactless payment, tapping a phone or card to a reader
Exam tipChoose the technology from the range the scenario needs. A tap-to-pay wristband: NFC. A tag read as goods pass a door: RFID. A smart thermostat sensor that runs on a battery for years: Zigbee.
Wi-Fi
Wireless networking by radio waves between devices and an access point.
Bluetooth
Short-range, low-power wireless connection between paired devices.
Zigbee
Very low-power, low-bandwidth wireless connection used by smart home sensors and devices.
RFID
Radio frequency identification: a reader uses radio waves to read the ID stored on a tag.
NFC
Near field communication: wireless communication over about 4 cm, used for contactless payment.
Edexcel 1CP2 · 4.1.4
Speed, range, latency and bandwidth
Pearson uses four terms to compare how well connections perform. Learn the definitions exactly.
Speed is usually lower than bandwidth: the bandwidth is shared between users, and interference, distance and errors (packets sent again) all slow the real transfer.
High latency matters most when things happen in real time. In a video call, speech arrives late, so people talk over each other; in an online game, a player’s moves reach the server late.
What affects performance: the transmission medium (fibre, copper or wireless); the number of devices sharing the bandwidth; distance and obstacles (walls, floors); interference from other devices and networks.
Term
Definition
Measured in
Bandwidth
The capacity of a network: the maximum (theoretical) amount of data that can be transferred in a given time
Bits per second
Speed
The actual rate of data transfer in a given time
Bits per second
Range
The maximum distance a signal is able to reach
Metres
Latency
The delay between a signal being sent and it being received
Milliseconds
Exam tipLatency is not just ‘lag’ or ‘delay’: say the delay between the data being sent and being received. Bandwidth must include maximum (or capacity). And don’t mix them up with range, which is a distance in metres.
Bandwidth
The maximum amount of data that can be transferred in a given time, in bits per second.
Speed (network)
The actual rate of data transfer in a given time, in bits per second.
Range
The maximum distance a signal is able to reach, in metres.
Latency
The delay between a signal being sent and being received, in milliseconds.
Edexcel 1CP2 · 4.1.5
Network speeds: bits per second
Network speeds are measured in bits per second (bps), and the multiples go up in 1000s (SI units). This is different from file sizes, which go up in 1024s.
Unit
Bits per second
1 kilobit per second (kbps)
1000
1 megabit per second (Mbps)
1000 × 1000 = 1 000 000
1 gigabit per second (Gbps)
1000 × 1000 × 1000 = 1 000 000 000
Two ladders: ×1000 for speeds, ×1024 for storage
Worked exampleWrite 0.25 Gbps in megabits per second and in bits per second.
0.25 × 1000 = 250 MbpsEach step down the speed ladder multiplies by 1000.
250 × 1000 × 1000 = 250 000 000 bpsTwo more steps: Mbps to kbps to bps.
250 Mbps = 250 000 000 bits per second
Worked examplePut these speeds in order, slowest first: 1.2 Gbps, 900 000 kbps, 1 100 Mbps.
Change all to Mbps: 1.2 Gbps = 1200 Mbps; 900 000 kbps = 900 Mbps; 1 100 Mbps stays.Compare like with like.
900 000 kbps, 1 100 Mbps, 1.2 Gbps
Watch the bSmall b = bits (speeds: Mbps). Big B = bytes (file sizes: MiB). There are 8 bits in a byte.
Bits per second (bps)
The unit for network speed: how many bits are transferred each second.
Kilobit, megabit, gigabit
1000 bits, 1000² bits and 1000³ bits.
Edexcel 1CP2 · 4.1.5
File size, speed and time: constructing expressions
Paper 1 has no calculator, so you’re asked to construct an expression. You get a mark for each correct part; you don’t have to work out the answer.
time (seconds) = file size in bits ÷ speed in bits per second
speed (bits per second) = file size in bits ÷ time in seconds
file size in bits = speed in bits per second × time in seconds
File size in bits: change to bytes with × 1024 for each step (KiB, MiB, GiB), then × 8. Units for file sizes are in the storage topic.
Speed in bits per second: × 1000 for each step (kbps, Mbps, Gbps).
Time in seconds: minutes × 60; hours × 60 × 60.
Worked exampleConstruct an expression for the time, in seconds, to send a 45 MiB file at 12 Mbps.
File size in bits: 45 × 1024 × 1024 × 8MiB → bytes is × 1024², then × 8 for bits.
Speed in bits per second: 12 × 1000 × 1000Mbps → bps is × 1000².
Exam tipThe two mistakes examiners see most: using 1000 for the file size (or 1024 for the speed), and forgetting × 8 to turn bytes into bits. Write the units next to each number as you build the expression.
Real downloads take longerThe expression gives the time at the full stated speed. In real life the bandwidth is shared, the connection may be slower than advertised, and some packets are lost and sent again.
Transmission time
File size in bits ÷ transmission rate in bits per second.
Construct an expression
Write the calculation with every value in it; you don’t need to work out the answer.
Genious