Time (s) = file size in bits ÷ rate in bits per second. 1 MiB = 1,024 × 1,024 × 8 bits.
Calculate
?A 25 MiB video is sent over a 10 Mbps connection. How long does it take, to 1 decimal place?
seconds
Mission check
?If networks spread viruses, why does every school connect its PCs?
A network lets a school share files, printers and one internet connection, and back up and manage every PC from one place. The risks, such as malware spreading, hardware costs and a central failure, are real, so networks are protected and maintained rather than avoided.
Mission 2
One box fails and a whole office goes offline. How?
Make a guess: One box fails and a whole office goes offline. How?
Networks · topologies
The star topology
A topology is the layout of how the devices on a network are connected.
In a star topology, every device has its own cable to one central device, usually a switch. All data passes through the centre, which sends it on to the right device.
Advantages
Disadvantages
If one cable fails, only that device is cut off
If the central switch fails, the whole network fails
Fast and reliable: few data collisions
Lots of cable, so it can be expensive
Easy to add devices and to find faults
The switch can become a bottleneck when traffic is heavy
Quick check
?On a star network of eight PCs, the cable to PC 3 is cut. What happens?
Networks · topologies
The mesh topology
In a mesh topology, devices connect directly to several others, so there are many paths between them.
In a full mesh every device connects to every other one. With 4 devices that is 6 links.
If a link fails, data is sent along another path: the network self-heals.
Advantages
Disadvantages
Very reliable: no single point of failure
A lot of cable and set-up: expensive
Data can take the fastest free path
Complex to set up and manage
Handles heavy traffic well
Many links are unused much of the time
Calculate
?A small office links 5 computers in a full mesh: every computer has its own cable to every other one. How many cables are needed?
cables
Networks · topologies
The bus topology
In a bus topology, every device connects to one shared main cable, the backbone, with a terminator at each end.
Data sent by one device travels along the whole backbone; only the device it is addressed to keeps it.
It is cheap and needs little cable, but if the backbone breaks the whole network fails, and it slows down as devices are added because of data collisions.
Quick check
?Why does a bus network slow down as more devices are added?
Quick check
?A hospital's patient-monitoring network must keep working even if any one cable is cut. Which topology fits best?
Mission check
?One box fails and a whole office goes offline. How?
The office used a star topology, so every PC's only link ran to one central switch. When that switch failed, no data could move at all. A mesh avoids this by giving every device more than one path.
Mission 3
How does your data find one computer among billions online?
Make a guess: How does your data find one computer among billions online?
Networks · hardware and addresses
IP addresses
An IP address identifies a device on a network, so data can be sent to it. Every packet carries a destination IP address.
An IPv4 address is 32 bits, written as four denary numbers from 0 to 255 separated by dots, e.g. 192.168.0.12. Each number is one byte (8 bits).
Quick check
?Which of these is a valid IPv4 address?
Networks · hardware and addresses
Routers
A router connects networks together, for example a home or school LAN to the internet.
The internet is a vast number of networks joined by routers.
A router reads each packet's destination IP address and sends it to the next router on a route towards it. Hop by hop, the packet reaches the right network.
Quick check
?A packet arrives at a router in the middle of the internet. What does the router use to decide where to send it next?
Your answer
?A computer stores an IPv4 address in binary: 11000000.10101000.00000001.00001010. Write it in the usual dotted denary form.
Mission check
?How does your data find one computer among billions online?
Every device online has an IP address, and every packet carries its destination IP address. The internet is many networks joined by routers; each router reads that address and forwards the packet one hop closer, until it reaches the right network and the right device.
Mission 4
How do two computers that have never met agree how to talk?
Make a guess: How do two computers that have never met agree how to talk?
Networks · protocols and layers
Protocols
A protocol is a set of rules for transferring data between devices.
It sets things like how data is formatted, how devices are addressed and how errors are spotted.
Both devices must follow the same protocol, whoever made them. Different jobs use different protocols.
Networks · protocols and layers
Protocols for the web and email
HTTP: a browser requests web pages from a web server, which sends them.
HTTPS: HTTP with encryption, so intercepted data can't be read.
FTP: transfers files between computers.
SMTP: sends email from an app to a mail server, and between mail servers.
IMAP: reads email kept on the server, so every device shows the same inbox.
POP3: downloads email to one device, usually deleting it from the server.
Networks · protocols and layers
Protocols that move the data
TCP splits data into numbered packets, checks they all arrive, has missing ones resent and puts them back in order.
IP gives each packet its source and destination IP addresses and routes it across networks.
TCP and IP work together, so they are often named as one: TCP/IP.
Ethernet is the set of rules for wired LANs; Wi-Fi (802.11) for wireless LANs.
Quick check
?Ama clicks Send in her email app. Which protocol carries the message to her mail server?
Networks · protocols and layers
Why use layers?
Sending data is split into layers. Each layer does one part of the job and each protocol belongs to one layer.
Sent data passes down the layers, each adding its own information. Received data passes back up.
Benefits: each layer is a smaller problem; one layer can change without changing the others (swap a cable for Wi-Fi and the browser still works); makers can build for one layer.
Networks · protocols and layers
The 4-layer TCP/IP model
Layer
What it does
Protocols and parts
Application
Where network applications, such as web browsers and email programs, work
HTTP, HTTPS, FTP, SMTP, POP3, IMAP
Transport
Sets up communication between the two hosts, which agree settings such as packet size; splits data into packets
TCP
Internet
Addresses and packages data; routes the packets across the network
IP
Link
The network hardware, such as the NIC, and the operating system's device drivers
Ethernet, Wi-Fi
Quick check
?A router reads a packet's destination IP address to choose where to send it next. Which layer of the TCP/IP model is this?
Mission check
?How do two computers that have never met agree how to talk?
The two computers have never met, but they follow the same protocols, such as SMTP for email and TCP and IP for moving the data. The work is split into layers, each with its own protocols, so every device that follows the rules can talk to every other.
Mission 5
How does a video arrive in order if its pieces take different routes?
Make a guess: How does a video arrive in order if its pieces take different routes?
Networks · packets and packet switching
Splitting data into packets
Before data is sent across a network, it is broken into small pieces called packets.
Each packet is sent on its own and the receiving computer joins them back together.
Small packets share the network's links fairly: no single huge file blocks a link. If one packet is lost or damaged, only that packet needs to be sent again.
Networks · packets and packet switching
Inside a packet
The header holds the destination address, the source address (where it came from) and the packet number, used to put the data back in order.
The payload is the actual data being carried.
Quick check
?A web page is sent as 40 packets. Which part of each packet lets the receiving computer put the page back together in the right order?
Networks · packets and packet switching
Packet switching
In packet switching, each packet is sent across the network separately.
Routers read each packet's destination address and send it on towards its destination, one hop at a time.
Each packet may take a different route, depending on which links are free or working.
So packets can arrive out of order. Once the last packet arrives, the receiver uses the packet numbers to reorder them.
Your answer
?Four packets arrive in this order. Packet 3: "E GA" · Packet 1: "MEET" · Packet 4: "TE" · Packet 2: " AT TH". Type the original message.
Quick check
?A cable between two routers is cut while a file is being sent. What happens to the packets still to be sent?
Mission check
?How does a video arrive in order if its pieces take different routes?
The video is split into packets, and each header includes a packet number. Routers send each packet on whatever route works best, so they can arrive jumbled. When the last packet arrives, the receiving computer uses the numbers to put them back in order.
Recap
Big ideas
A network connects devices so they can share data and resources. It brings gains (sharing, central backup and management) and costs (hardware, maintenance, security risks).
A LAN covers a small area and is owned by one organisation; a WAN covers a wide area and links LANs. The internet is the biggest WAN.
Speeds are measured in bits per second: time = file size in bits ÷ rate in bits per second.
A star links every device to a central switch: a cut cable affects one device, but if the switch fails the whole network fails.
A mesh gives devices many paths, so it self-heals, but it needs a lot of cable and is complex to set up.
A bus shares one backbone cable: cheap, but a break stops everything and collisions grow as devices are added.
An IP address identifies a device on a network. Routers read each packet's destination IP address and forward it hop by hop.
An IPv4 address is 32 bits, written as four numbers from 0 to 255.
A protocol is a set of rules for transferring data. Both devices must follow the same one, and different jobs use different protocols.
In the 4-layer TCP/IP model, HTTP, HTTPS and SMTP work at the application layer, TCP at the transport layer and IP at the internet layer.
Data is split into packets; each has a header (destination address, source address, packet number) and a payload.
In packet switching, routers send each packet separately, so packets may take different routes and arrive out of order; the receiver reorders them using the packet numbers.
🏆
Mini-lesson complete!
⭐⭐⭐
You've worked through Networks for Pearson Edexcel GCSE Computer Science (1CP2). 🎉
Mission 1If networks spread viruses, why does every school connect its PCs?
Mission 2One box fails and a whole office goes offline. How?
Mission 3How does your data find one computer among billions online?
Mission 4How do two computers that have never met agree how to talk?
Mission 5How does a video arrive in order if its pieces take different routes?