Computer Networks and Topologies
Local and wide area networks, star and bus topologies compared properly, wired against wireless, and the three ways a wireless network is secured.
Networks
A network is two or more computers connected together so that they can share data and resources such as files, printers and an internet connection.
A LAN, or local area network, covers a small geographical area on a single site, such as a school or an office. The organisation owns and maintains the cables and equipment itself.
A WAN, or wide area network, covers a large geographical area and connects sites that may be in different towns or countries. Because the connections cross land the organisation does not own, a WAN uses infrastructure hired from a telecommunications provider. The internet is the largest WAN of all.
A LAN is on one site and the organisation owns the hardware. A WAN spans sites and uses infrastructure it rents. Size alone is not the answer.
Network hardware
- A network interface card gives a device a physical connection and its MAC address.
- A switch joins devices on a LAN and sends each frame only to the device it is addressed to.
- A router joins networks together and directs packets between them, which is what connects a LAN to the internet.
- A wireless access point allows devices to join the network without a cable.
- A LAN covers one site: one building, one campus, cabling you own.
- A second site has its own LAN, with its own switch and its own cabling.
- Joining them makes a WAN, over lines nobody at either end owns.
- That is the real difference: not size, but who owns the connection.
Star and bus topologies
A topology is the shape of the connections between devices. AQA examines two.
Star
Every device has its own cable running to a central point, normally a switch. Nothing connects directly to anything else.
- Fast, because each device has a dedicated connection that nothing else is using.
- Reliable, because if one cable fails only that one device is affected.
- Easy to add a device, since you simply run another cable to the switch.
- More expensive, because every device needs its own cable and a switch is required.
- The switch is a single point of failure: if it fails, the whole network stops.
Bus
Every device connects to a single shared cable, called the backbone, with a terminator at each end.
- Cheap, because it uses far less cable and needs no central device.
- Simple to set up for a small number of devices.
- Slow when busy, because every device shares one cable and only one can transmit at a time.
- Data collisions occur when two devices transmit at once, and both must wait and try again.
- The backbone is a single point of failure: a break in it brings down the whole network.
- Less secure, because every frame travels past every device.
A comparison question wants the two put beside each other. "A star is faster because each device has a dedicated connection, whereas on a bus every device shares one cable so they compete for it" is worth more than two separate lists of features.
- Star: every device has its own cable to a central switch.
- If one cable fails, only that one device drops off.
- Mesh: every device connects to every other, with no centre at all.
- Any one link can fail and the data simply goes a different way.
- Bus: one shared backbone, and every device taps into it.
Wired and wireless
Wired
A wired connection carries data along a cable, usually copper Ethernet or fibre optic.
- Faster and more consistent, because the connection is not shared with the air.
- More reliable, since the signal is not weakened by distance, walls or interference.
- More secure, because an attacker needs physical access to the cable to intercept data.
- Expensive and disruptive to install, since cables must be run through the building.
- Devices cannot move.
Wireless
A wireless connection carries data as radio waves through a wireless access point.
- Convenient, since devices can move freely and no cabling is needed.
- Cheap and quick to install, especially in an existing building.
- Slower and less consistent, and it degrades with distance and obstacles.
- Subject to interference from other networks and equipment.
- Less secure by default, because the signal travels through the air where anyone in range can receive it.
Securing a wireless network
AQA names three measures:
- Encryption, so that intercepted data cannot be read without the key. WPA2 and WPA3 are the current standards.
- MAC address filtering, so that only devices on an approved list may join. It is a weak measure on its own, because a MAC address can be spoofed, but it raises the effort required.
- A strong password on the access point, since a default or guessable one makes every other measure pointless.
Key terms you must be able to define
- Network
- Two or more computers connected together to share data and resources.
- LAN
- Local area network, covering a small geographical area on a single site where the organisation owns the hardware.
- WAN
- Wide area network, covering a large geographical area using infrastructure hired from a provider.
- Topology
- The arrangement of the connections between devices on a network.
- Star topology
- A layout in which every device has its own cable to a central switch.
- Bus topology
- A layout in which every device connects to one shared backbone cable.
- Switch
- A device that joins computers on a LAN and forwards each frame only to its destination.
- Router
- A device that joins networks together and directs packets between them.
- MAC address filtering
- Allowing only devices whose hardware addresses appear on an approved list to join a network.
- WPA2
- A wireless encryption standard that protects data travelling over a Wi-Fi network.
Flashcards
Say the definition out loud before you flip the card. Retrieving it from memory is what builds recall, reading it again is not.
Reach the top grade on this topic
Marks here come from causal links rather than lists.
Give the reason inside the claim. "A bus network slows down as devices are added, because they all share one cable and only one can transmit at a time" is a full answer. "Bus networks are slow" is a fragment.
Be honest about weaknesses. MAC address filtering can be defeated by spoofing a permitted address, and saying so shows genuine understanding rather than a memorised list.
Answer the situation in the question. A room of fixed desktops, a building that cannot be rewired and a cafe offering guest access all have different right answers, and the marks for evaluation come from choosing on the evidence given.
Mistakes that cost marks here
- Defining a LAN purely by size. Ownership of the infrastructure is part of the distinction.
- Confusing a switch with a router. A switch connects devices within a network, a router connects networks together.
- Saying a star network has no single point of failure. The switch is one.
- Claiming MAC address filtering makes a network secure. It can be bypassed by spoofing.
- Saying wireless is always slower. A modern wireless standard can beat an old wired one, so the answer is about consistency and interference as much as raw speed.
Wi-Fi is not short for anything. The name was invented by a branding company in 1999 because the standard's real name, IEEE 802.11b Direct Sequence, was never going to appear on a box in a shop.
Knowledge check
Exam-style questions
Five written questions in the style of the real paper. Write a full answer, then let the marker check it against the mark scheme.
- LAn advantage such as each device having a dedicated connection, so it is faster and a cable failure affects one device only
- LA disadvantage such as needing more cable and a switch, so it costs more
An advantage is that every device has its own dedicated cable to the switch, so devices do not compete for the medium and a single cable failure disconnects only the device on it. A disadvantage is cost: a star needs a separate cable run for every device plus a central switch, which is considerably more expensive than one shared backbone.
- LAll devices share a single backbone cable
- LOnly one device can transmit successfully at a time
- LMore devices means more collisions, and after a collision data must be resent
Every device on a bus network is attached to the same backbone cable, so they all share one transmission medium and only one device can be transmitting successfully at any moment. As devices are added, the chance that two of them try to transmit at the same instant rises, and when that happens the signals interfere and both transmissions are lost. Each device then has to wait and send again, which adds further traffic to the same cable, so throughput falls away noticeably once the network becomes busy.
- LWireless needs no cabling, so it can be installed without altering the building
- LIt allows devices to move around the building
- LWireless is generally slower and less consistent than a wired connection
- LThe signal is weakened by thick walls and by distance from the access point
- LWireless is less secure by default because the signal travels through the air
- LReaches a justified conclusion, including measures such as encryption and multiple access points
Wireless is the obvious fit for the constraint. No cabling has to be run, so the fabric of a listed building is untouched, installation is quick and cheap, and staff can move around with laptops and tablets rather than being tied to a desk. There are three real costs to weigh. Wireless is generally slower and less consistent than a wired connection, so a task that moves large files will take longer. The signal is weakened by distance and by obstacles, and the thick masonry typical of a listed building is exactly the kind of obstacle that causes dead spots. And the signal travels through the air, so anyone within range can receive it, which makes the network easier to attack than one where an intruder would have to reach a cable. None of those is decisive against wireless here, because the alternative is effectively unavailable. The company should install wireless, but should plan for it properly: several access points positioned to give overlapping coverage rather than one central unit, WPA2 or WPA3 encryption so that intercepted traffic cannot be read, a strong non default password on every access point, and a separate guest network so that visitors never reach company systems. If any single task genuinely needs sustained high bandwidth, a small number of surface mounted cable runs to those specific machines would be a sensible exception.