Networks and Topologies
LANs and WANs, the hardware that makes a network work, client server against peer to peer, star against mesh, and the factors that decide how fast it all runs.
LAN and WAN
A network is two or more computers connected together so they can share data and resources.
LAN, local area network
- Covers a small geographical area, such as one building or one site
- The hardware is owned by the organisation that uses it
- Examples: a school, an office, a home
WAN, local area network becomes wide area network
- Covers a large geographical area, potentially the whole world
- The organisation does not own all the infrastructure, it leases connections from telecommunications companies
- Examples: the internet, a bank connecting branches in different cities
The real difference is ownership as well as size. In a LAN the organisation owns the cabling and hardware. In a WAN it uses third party infrastructure that it does not own.
Advantages and disadvantages of networking
Advantages
- Share files and data easily
- Share hardware such as printers, saving money
- Share an internet connection
- Central backup of all data
- Central management of user accounts and software updates
Disadvantages
- Expensive to set up: cabling, servers and hardware
- Malware can spread quickly across the network
- If the server fails, everyone is affected
- Requires a network manager, which is a staffing cost
- Increased security risk, since data is accessible from many machines
- 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.
Network hardware
| Device | What it does |
|---|---|
| NIC | Network Interface Card. Allows a device to connect to a network, wired or wireless. Each has a unique MAC address. |
| Switch | Connects devices on a LAN. It reads the destination MAC address of each frame and sends it only to the correct device, which reduces unnecessary traffic. |
| Router | Connects different networks together and directs data between them using IP addresses. Your home router joins your LAN to the internet. |
| WAP | Wireless Access Point. Allows devices to connect to the network wirelessly using radio waves. |
| Transmission media | The cabling or radio waves that carry the data: copper twisted pair, fibre optic, or wireless. |
A switch works inside one network using MAC addresses. A router works between networks using IP addresses. Confusing these two is one of the most common lost marks in this topic.
Servers and clients
A server is a computer that provides services or resources to other computers. Common types include file servers, print servers, web servers and mail servers.
Client server and peer to peer
Client server
One or more powerful central servers provide resources, and clients request them.
Advantages
- Central backup, so data is protected in one place
- Central management of security, updates and user accounts
- Easier to add new clients
- Files are stored centrally, so any user can access their work from any machine
Disadvantages
- Expensive: servers and specialist staff cost money
- If the server fails, the whole network loses access
- The server can become a bottleneck if demand is high
Peer to peer
Every computer is equal. Each acts as both client and server, sharing files directly with the others.
Advantages
- Cheap, no server to buy
- Easy to set up, needs no specialist knowledge
- No single point of failure, if one machine goes down the rest continue
Disadvantages
- No central backup, so files must be backed up individually
- Files are spread across machines, so a computer must be switched on for its files to be available
- Difficult to manage security and updates across many machines
- Performance drops as more peers join
A home with three computers sharing a printer suits peer to peer, because it is cheap and simple. A school with 500 machines needs client server, because central backup, central account management and central security are essential at that scale.
- Client server: every device asks one central machine for what it needs.
- The server holds the files, does the work and checks who you are.
- Peer to peer: no server, and every machine is both client and server.
- Lose the server and a client server network stops. A peer network does not.
Star and mesh topologies
A topology is the layout of a network: how the devices are physically or logically arranged.
Star topology
Every device connects to a central switch or hub.
Advantages
- If one cable fails, only that one device is affected
- Easy to add or remove devices without disrupting anyone else
- Very few data collisions, because the switch directs traffic
- Good performance, since each device has its own dedicated connection
Disadvantages
- Requires a lot of cable, so it is expensive to install
- If the central switch fails, the entire network fails
Mesh topology
Every device connects to many other devices, so there are multiple routes for data.
In a full mesh every device connects to every other. In a partial mesh only some connections exist.
Advantages
- Extremely reliable: if one connection fails, data is rerouted another way
- No single point of failure
- Data can take the fastest available route
- Adding devices does not slow the network as much as other topologies
Disadvantages
- Very expensive in cabling for a wired mesh
- Complex to set up and maintain
Wireless mesh networks avoid the cabling cost, which is why mesh is now common in smart home and large scale Wi-Fi systems.
A star topology fails completely if the central switch fails, whereas a mesh has no single point of failure because data can be rerouted through alternative paths.
- 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.
Factors affecting network performance
Five factors, and you need to be able to explain each.
- Bandwidth. The amount of data that can be carried per second. Higher bandwidth means more data flows at once. Bandwidth on a network is shared, so more users each get less.
- Number of users. More devices using the network at once means the available bandwidth is divided further, so each device gets less and everything slows.
- Transmission media. Fibre optic carries far more data over far greater distances than copper, and is immune to electrical interference. Copper is cheaper but slower and degrades over distance.
- Error rate and interference. Wireless signals weaken with distance and are blocked by walls, and can be interfered with by other devices. Errors mean data has to be resent, which reduces effective speed.
- Latency. The delay between sending data and it arriving. Even on a fast connection, high latency makes interactive applications such as video calls and online gaming feel unresponsive.
Bandwidth is capacity, like the number of lanes on a motorway. Latency is delay, like how long the journey takes. A satellite link can have huge bandwidth and terrible latency.
- Bandwidth is the capacity of a connection: how much data it can carry each second.
- Bandwidth is shared, so the more users there are, the less each one gets.
- Fibre carries more data further than copper, and copper fades over distance.
- Walls, distance and other devices weaken wireless signals, and lost packets are sent again.
- Latency is the delay before data arrives. A satellite link has a long delay even if its bandwidth is large.
- Bandwidth is like the number of lanes. Latency is like the length of the journey.
Key terms you must be able to define
- LAN
- Local area network. Covers a small geographical area with hardware owned by the organisation.
- WAN
- Wide area network. Covers a large geographical area using infrastructure the organisation does not own.
- NIC
- Network interface card. The hardware that allows a device to connect to a network.
- Switch
- A device that connects computers on a LAN and sends data only to the intended recipient using MAC addresses.
- Router
- A device that connects different networks together and directs data between them using IP addresses.
- WAP
- Wireless access point. Allows devices to join a network wirelessly.
- Client server
- A network model where central servers provide resources to client computers.
- Peer to peer
- A network model where all computers are equal and share resources directly with each other.
- Topology
- The layout or arrangement of devices in a network, such as star or mesh.
- Bandwidth
- The amount of data that can be transmitted over a connection in a given time.
- Latency
- The delay between data being sent and it arriving at its destination.
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
Three habits separate top answers here.
Say what fails and what still works. Reliability questions want specifics. "In a star topology, if one cable fails only the device on that cable is affected, but if the central switch fails the entire network goes down."
Attach the reason to the recommendation. Choosing client server for a school is not enough. Say that central backup protects student work, central account management lets any student log in at any machine, and central updates keep 500 machines secure without visiting each one.
Separate bandwidth from latency and from number of users. Many students blur all three into "the internet is slow". Bandwidth is capacity, latency is delay, and number of users is what divides the capacity up.
- Explain the difference between a switch and a router without hesitating
- Give three advantages and three disadvantages each for client server and peer to peer
- Compare star and mesh on reliability, cost and expandability
- Explain all five performance factors with a cause and an effect
Mistakes that cost marks here
- Saying a WAN is 'just a bigger LAN'. The ownership of the infrastructure is the key distinction.
- Confusing switches and routers. Switches work within a network using MAC addresses, routers work between networks using IP addresses.
- Saying peer to peer has 'no server'. Every peer acts as both client and server, which is the point.
- Claiming a star topology is more reliable than mesh. Mesh has no single point of failure, a star has one at the central switch.
- Using bandwidth and speed interchangeably. Bandwidth is capacity, latency is delay.
The first message ever sent over ARPANET, the ancestor of the internet, was meant to be the word LOGIN. The system crashed after two letters, so the first thing ever transmitted across the internet was LO.
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.
- LA LAN covers a small geographical area, a WAN covers a large one
- LLAN hardware is owned by the organisation, WAN infrastructure is often leased from third parties
A LAN covers a small geographical area such as a single building or site, whereas a WAN covers a large geographical area and may span cities or countries. In addition, the organisation using a LAN owns all of the hardware and cabling itself, whereas a WAN relies on infrastructure leased from telecommunications companies that the organisation does not own.
- LA switch connects devices within a single network or LAN
- LA switch uses MAC addresses to send data only to the intended device
- LA router connects different networks together
- LA router uses IP addresses to direct data between networks
A switch operates inside a single network such as a LAN, connecting the devices on that network to one another. It reads the MAC address in each frame and forwards it only to the port where the intended recipient is connected, which avoids sending unnecessary traffic to every device. A router works at a higher level, connecting entirely different networks together, such as joining a home LAN to the internet. It uses IP addresses to decide which network a packet must be sent towards, and selects the appropriate route to get it there.
- LRecommends peer to peer
- LCheaper because no dedicated server hardware needs to be bought
- LSimple to set up without specialist network staff
- LWith only six computers the management overhead of client server is not justified
- LNotes the drawback that there is no central backup
- LNotes that files are unavailable if a computer is switched off, and suggests a mitigation
The business should use a peer to peer network. With only six computers, the main advantage is cost: a peer to peer network needs no dedicated server hardware and no server software licences, so the setup cost is far lower than a client server arrangement. It is also simple enough to configure and maintain without employing a network technician, which matters for a business of this size where there is unlikely to be dedicated IT staff. A client server network would bring central backup, central security and central account management, but with six machines the administrative burden those features remove is small, so the extra expense is difficult to justify. The business should be aware of the drawbacks. There is no central backup, so each computer's files must be backed up separately and it would be easy for this to be neglected until data is lost. Files are also only available while the computer holding them is switched on, which can be inconvenient. Both problems can be reduced cheaply by adding a shared network attached storage drive or a cloud storage subscription for important files, which keeps the simplicity of peer to peer while covering its weakest point.
- LStar has a single point of failure at the central switch
- LMesh has multiple paths so data can be rerouted if a connection fails
- LStar uses less cable and is cheaper to install than a full mesh
- LA wired mesh requires many connections making it expensive and complex
In terms of reliability, a mesh topology is stronger. Every device in a mesh is connected to several others, so if one link fails the data is simply rerouted along an alternative path and the network continues to function, meaning there is no single point of failure. A star topology is reliable against individual cable faults, because a broken cable only disconnects the one device attached to it, but it depends entirely on the central switch, and if that switch fails then every device loses connectivity. In terms of cost, the star topology wins clearly. It needs only one cable per device running back to the switch, whereas a full wired mesh requires a connection between every pair of devices, so the amount of cabling and the complexity of installation rise very steeply as more devices are added.
- LBandwidth is the amount of data that can be carried per second
- LHigher bandwidth allows more data to be transferred, but it is shared between users
- LThe number of users affects performance because bandwidth is divided between them
- LTransmission media matters, with fibre optic faster and less prone to interference than copper
- LInterference and errors cause data to be resent, reducing effective speed
- LLatency is the delay before data arrives and affects interactive applications
The first factor is bandwidth, which is the amount of data the connection can carry each second. A higher bandwidth connection can move more data in the same time, but bandwidth is a shared resource, so the capacity available to any one device depends on what everyone else is doing. That leads directly to the second factor, the number of users. As more devices connect and transmit at the same time, the same total bandwidth is divided between more of them, so each device receives a smaller share and everything slows down, which is why school networks feel slowest at the start of a lesson when every student logs on at once. The third factor is the transmission media. Fibre optic cable carries far more data over much greater distances than copper and is immune to electromagnetic interference because it transmits light rather than electricity, whereas copper signals degrade over distance and can be disrupted by nearby electrical equipment. Wireless is more convenient but weakens with distance and is blocked by walls, and when signal quality is poor data has to be retransmitted, which reduces the effective throughput even though the nominal bandwidth has not changed.