Communication
Network topologies: bus, star, mesh, hybrid
What you need to be able to do
- 2.1Describe the bus, star, mesh and hybrid topologies, and draw them.
- 2.1Explain how packets are transmitted between two hosts in each topology.
- 2.1Justify the use of a topology for a given situation.
You need: Networks: LAN and WAN. The devices (switch, router) are described in LAN hardware and routers.
A topology is the layout of the connections between the devices in a network. Data travels as packets (small blocks of data with a header holding the source and destination addresses); write “packet”, not “package” (November 2022).
Bus topology
- All devices are connected to one central cable (the backbone), with a terminator at each end to stop signals reflecting back.
- How a packet travels: the sender puts the packet on the cable; it travels along the whole cable in both directions, so every device receives it.
- Each device checks the destination address: the intended device accepts the packet, the others ignore it.
- Only one device can transmit at a time: if two transmit together their signals collide (handled by CSMA/CD, see Ethernet and CSMA/CD).
| Advantages | Disadvantages |
|---|---|
| cheap: little cable, no central device | most likely to lose data through collisions; performance drops as traffic grows |
| easy to add a device to the cable | if the backbone fails, the whole network fails |
| every device sees every packet: less secure; hard to find faults |
Star topology
- Each device is connected directly (and only) to a central device (a switch, or a server / router acting as one).
- How a packet travels: the sending device transmits the packet to the switch.
- The switch checks the destination address on the packet (it keeps a table of the MAC addresses on each port).
- The switch forwards the packet only to the intended device, not to the others.
| Advantages | Disadvantages |
|---|---|
| fewer collisions: each device has its own link to the switch, and the switch sends packets only where they need to go | if the central switch fails, the whole network fails |
| robust: if one cable fails, only that device is affected; the rest keep full functionality | more cable than a bus, plus the cost of the switch |
| easy to add new devices (plug into the switch) and easy to find faults | |
| more secure: packets only reach their destination |
Mesh topology
- Each device is connected to at least one other device (a full mesh: to every other device), so there are multiple routes between devices.
- How a packet travels: devices act as relays, passing the packet on from device to device towards its destination; the route is chosen (by routing) and can change.
- If a link or device fails, the packet takes another route.
| Advantages | Disadvantages |
|---|---|
| robust: if one line goes down there are other routes; the network keeps full functionality | expensive: lots of cabling / connections, especially a full mesh |
| no / fewer collisions: many separate links | complex to set up and manage |
| new nodes can be added without interrupting others; more secure (no single shared line) |
The internet's core is a giant mesh of routers; wireless mesh systems link Wi-Fi points around a big house or campus.
Hybrid topology
A hybrid topology combines two or more topologies, e.g. several star networks (one per floor or classroom) joined by a bus or mesh backbone. It takes the advantages of each part where it is needed and scales to large sites; it is more complex to design and manage. Packets: inside each star, via that star's switch; between parts, along the backbone to the other switch, which forwards to the destination.
How packets travel: side by side
| Bus | Star | Mesh | |
|---|---|---|---|
| Who receives a packet? | every device on the cable | only the destination (switch forwards it) | the next relay on the route, then the destination |
| Who decides where it goes? | each device checks the address and ignores packets not for it | the switch, from the destination address | each node routes it towards the destination |
| Collisions? | most likely | few (separate links) | none / few |
| One link fails | whole network down if it's the backbone | only that device is cut off (unless it's the switch) | other routes are used |
Choosing a topology: case by case
1 A classroom of 20 computers
A new classroom has 20 computers and a switch. Why use a star instead of a bus?
Askedadvantage + reasonKnow20 devices, a switchRoutecollisions and faults
- Higher performance: fewer collisions because each computer connects only to the switch
- More resilient: no single shared cable, so one faulty cable doesn't stop the lesson
- Easier to add nodes and find faults
2 Devices round a home router
A family's devices all connect wirelessly to one router. Which topology is it?
Askedtopology + justificationKnoweverything links to one central deviceRoutecentral device → star
- Star
- every device is connected directly and independently to the router (the central device); none connect to each other
The November 2021 report: “because all the devices connect to the router” only repeats the question; say each device connects only to the router, independently.
3 A factory control network that must never stop
Machines on a production line must keep communicating even if a cable is cut. Which topology?
Askedtopology + reasonKnowno single point of failure allowedRoutemultiple routes → mesh
- Mesh
- there are multiple routes, so if one link fails packets take another route and the network keeps working
4 A small temporary network
Four computers are needed for a weekend event at minimum cost. Which topology?
Askedtopology + reasonKnowfew devices, low costRouteleast cable, no central device
- Bus
- cheapest: one cable, no switch; with only 4 devices and light traffic, collisions are rare
Mark-scheme focus
Explain how packets are transmitted between two devices in a star topology (2 marks)
![A design company has a LAN. (b) Part of the internal structure of the wired LAN is a star topology. Explain how packets are transmitted between two devices in a star topology. [2]](/notes-content/9618/communication/img/9618_s24_13_q05_b.png)
- Sending computer transmits packets directly to the switch / central device B1
- Switch checks the destination address of the packet and forwards it directly to that device B1
- “The data goes through the middle” (no device, no address)
- “The switch sends it to all devices” (that's a hub / a bus)
Two separate actions earn the two marks: sender → switch, then switch reads the address and sends only to the destination.
From: 9618/13/M/J/24 Q5(b)
Describe a mesh topology; two advantages over a bus (4 marks)
![(b) In one building five computers are connected in a mesh topology. (i) Describe what is meant by a mesh topology. [2] (ii) Give two advantages of using a mesh topology instead of a bus topology. [2]](/notes-content/9618/communication/img/9618_s23_13_q02_b.png)
- (i) All computers are connected to at least one other device B1; there are multiple routes between devices // computers act as relays passing packets on B1
- (ii) If one line goes down there are other routes B1; no / fewer collisions // new nodes added without interruption // more secure as not one main line B1
- (i) only the connections, nothing about routes / relaying (the report: the second mark was the hard one)
- (ii) “it is faster” with no reason
From: 9618/13/M/J/23 Q2(b)
- Draw / complete a star: in about every other paper (1-4 marks): every device connected directly to the centre, no other links, all labelled.
- How packets travel in a star: 2-3 marks: to the switch, switch checks the destination address, sends only to that device.
- Compare topologies: tick tables, star vs bus, mesh features (2-5 marks).
Worked examples
Example 1 · Which topology does each statement describe? (Paper 1, 5 marks) ★ Easy
![A business is creating a LAN. (a) Tick one or more boxes in each row to identify the topology, or topologies (bus, star, mesh), each statement describes: all devices connect to one central device; all devices connect to a central cable; multiple paths for the packets to travel along; robust against damage because if any line fails, the rest of the network retains full functionality; most likely to lose data through collisions. [5]](/notes-content/9618/communication/img/9618_s24_11_q08_a.png)
“One or more boxes”: a row can need two ticks. Test each statement against each topology.
- one or moresome rows have two answers
- any line failsone link, not the central device
- (a)five rows
- Row 4robust
- Stara broken cable cuts off only one device
- Meshother routes
- Trapticking only mesh
If one cable in a star breaks, does the rest work?
Yes: only that device loses its link.
5 Now solve it have a go on paper first, then open the worked solution
- one central device → Star B1
- central cable → Bus B1
- multiple paths → Mesh B1
- robust if any line fails → Star and Mesh B1
- most likely to lose data through collisions → Bus B1
Row 4 is the only two-tick row ✓.
Example 2 · Complete a star, then where to attach the router (Paper 1, 3 marks) ★★ Medium
![A LAN consists of four computers, one server and a switch, using a star topology. (a) Complete the diagram (switch already linked to the server) to show how the hardware is connected. [1] (b) A router is attached to one of the devices in part (a) to connect the LAN to the internet. (i) Identify the device. Give a reason for your choice. [2]](/notes-content/9618/communication/img/9618_w22_12_q10_abi.png)
(a): the switch is already joined to the server; join each computer to the switch. (b)(i): choose a device from the diagram and give a reason that isn't just “to connect to the internet” (that's in the question).
- complete the diagramadd the missing links only
- one of the devices shownswitch or server - not a new device
- (a)connections
- (b)(i)device + reason
- (b)(i)device
- Switchconnected to all computers → shared access
- Serverauthorises traffic / firewall / proxy
What did the report say about (b)(i) reasons?
“Simply saying that the device can connect the LAN to the Internet is not enough as this was stated in the question.”
5 Now solve it have a go on paper first, then open the worked solution
- (a) all four computers directly connected to the switch and no other connections B1
- (b)(i) Device: switch B1; reason: it is connected to all the computers, so all share access to the router B1
- (or: server, because it processes requests and authorises traffic / runs the firewall / acts as a proxy)
The diagram has no computer-to-computer links ✓.
Example 3 · Draw an office star with every device (Paper 1, 4 marks) ★★ Medium
![(d) The main office has four wired computers, one server, two printers, one central switch and a device providing a single point of access for the internet. The network is set up as a star topology. Draw the topology for the main office network. Label all devices. [4]](/notes-content/9618/communication/img/9618_s25_12_q06_d.png)
Ten devices to place: 4 computers, server, 2 printers, switch, and the internet device (a router or modem). Each mark is for one group connected correctly.
- single point of access for the interneta router / modem
- label all devicesunlabelled boxes score nothing
- (d)the drawn star
- Marks
- 1server → switch
- 2four computers → switch
- 3printers → switch (or server / a computer)
- 4router → switch (or server)
5 Now solve it have a go on paper first, then open the worked solution
- Server directly connected to the switch B1
- Four labelled computers all connected directly to the switch B1
- Two printers each directly connected to the switch (or server / a computer) B1
- Router / modem directly connected to the switch (or server) for internet access B1
Every device has exactly one line, to the centre ✓.
Example 4 · One advantage of star over bus, explained (Paper 1, 2 marks) ★ Easy
![(d) A new classroom is being set up with 20 computers and a switch. Explain one advantage of implementing a star topology instead of a bus topology in the new classroom. [2]](/notes-content/9618/communication/img/9618_w23_11_q02_d.png)
“Explain one advantage”: the advantage (1) and why it happens / what it means in the classroom (1).
- explain oneadvantage + expansion
- (d)1 advantage, explained
- Advantage
- Faultsno single cable → less disruption
- Collisionseach device only on the switch
5 Now solve it have a go on paper first, then open the worked solution
- Star topology is more resilient to faults B1
- … because there is no single shared cable, so a fault causes less disruption to teaching B1
Also credited: higher performance as fewer collisions … because each device is connected only to the switch ✓.
Summary
- Bus: one shared cable, every device receives every packet, most collisions, backbone failure stops everything.
- Star: each device only to a central switch; switch reads the destination and forwards only there; switch failure stops everything.
- Mesh: multiple routes, devices relay packets, robust, expensive. Hybrid: a mix (e.g. stars joined by a backbone).
| If you see… | do this |
|---|---|
| “draw a star” | every device → centre only, all labelled |
| “how are packets transmitted” (star) | sender → switch → reads destination address → only to that device |
| “most likely to have collisions” | bus |
| “multiple routes / robust” | mesh (and star for one-cable failures) |
| “justify the topology” | reason linked to the situation (cost, reliability, collisions, expansion) |
Can I do this?
- I can describe a bus and how a packet travels on it.
- I can draw a star and explain how a switch delivers a packet.
- I can describe a mesh (and hybrid) and how packets are relayed.
- I can compare collisions, robustness and cost across the topologies.
- I can justify a topology for a given situation.
Anything unticked: revise that section first. All ticked: take the quiz.
Quiz
The multiple-choice and guided questions need JavaScript. The concept checks and exam practice below work without it.
Concept checks
Answer each one in your head first, then open it.
1Why does a star have fewer collisions than a bus?Show answer
Each device has its own link to the switch and the switch forwards each packet only along the link it needs, so devices aren't competing for one shared cable.
2Is a star with a hub the same as a star with a switch?Show answer
Same layout, different behaviour: a hub repeats every packet to every port (like a bus), a switch sends it only to the destination.
3Why don't we build every network as a full mesh?Show answer
The number of links grows very fast (n devices need n(n−1)/2 links): 20 devices would need 190 cables.
Exam-style practice
New questions, not used above. Do them on paper, then mark your work against the official mark scheme.
![(c) One of the company's offices has one server and four computers connected in a star topology. Draw a diagram to show the layout of the office's star topology. [2]](/notes-content/9618/communication/img/9618_s23_12_q01_c.png)
Show the mark scheme
| Part | Answer / working | Marks | Guidance |
|---|---|---|---|
| (c) | Each computer directly connected only to the server | B1 | a switch in the middle is also fine |
| All components correctly labelled | B1 | ||
| Total: 2 marks | |||
![A LAN contains a router, two laptop computers and a server. (b)(i) The LAN is set up as a star topology. Draw a diagram of the topology of the LAN. [2] (ii) Explain how data is transmitted between the two laptops in the LAN. [2]](/notes-content/9618/communication/img/9618_w23_12_q07_bibii.png)
Show the mark scheme
| Part | Answer / working | Marks | Guidance |
|---|---|---|---|
| (b)(i) | Server connected only to the router | B1 | |
| Two laptops connected only to the router | B1 | ||
| (b)(ii) | The data from the sending laptop is transmitted to the router, with the address of the recipient | B1 | |
| The router determines the destination (routing table) and transmits the data directly / only to the recipient | B1 | ||
| Total: 4 marks | |||
![Andy plays games over the internet; several devices connect wirelessly to the router in his house. (b)(i) Identify the topology of Andy's home network. Justify your choice. [2]](/notes-content/9618/communication/img/9618_w21_12_q03_bi.png)
Show the mark scheme
| Part | Answer / working | Marks | Guidance |
|---|---|---|---|
| (b)(i) | Star | B1 | |
| Devices are connected directly to the router independently // all devices are only connected to the router | B1 | ||
| Total: 2 marks | |||
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