Draw five computers. Join them with one long cable, or through one central box, or in a circle, or with a cable between every pair, or in branches like a tree. Each drawing is a topology. The book gives you five and asks what each is good for and where it can fail.
Chapter 1 of the PECTAA Computer Science 12 book is Computer Networks. Section 1.5 covers network topologies. This note is for FSc Part 2 (2nd year, Class 12) students in Lahore, Faisalabad, Sargodha, Rawalpindi, Multan, Gujranwala and the other Punjab boards. Gujrat district comes under the Gujranwala board. PECTAA writes the books. Your own board sets your paper.
What to know
Network topology is the layout of a computer network. It explains how devices and cables are arranged. It can be physical, which shows the actual connections, or logical, which shows how data flows in the network. Topology affects network performance and maintenance.
Bus topology. All devices are connected to a single main cable. Data travels along this cable to reach devices. It is simple and low in cost. Failure of the main cable can stop the entire network. It is suitable for small networks and not good for large ones. Figure 1.8 shows terminators at both ends of the cable.
Star topology. All devices connect to a central hub or switch. Data passes through the central device. It is easy to manage and expand. Failure of one device does not affect the others, but failure of the central hub stops the network. It is widely used in modern networks.
Ring topology. Devices are connected in a circular path. Data moves in one direction around the ring. Each device receives data and passes it along. Failure of one device can affect the whole network. It is more complex than bus topology and rarely used today.
Mesh topology. Devices are connected to many other devices. Multiple paths exist for data. This increases reliability and fault tolerance, and failure of one link does not stop communication. It is expensive and complex, and it is used in large and critical networks.
Tree topology. Devices are arranged in a hierarchy like a tree. It combines features of bus and star topologies: multiple star networks are connected to a central backbone. Data flows from the root (main node) to branches and sub-branches. It is easy to expand and manage large networks, but failure of the backbone can affect the entire network. It is commonly used in large organizations.
Key points
The book's Table 1.1 compares the five topologies.
| Topology | Advantages | Disadvantages |
|---|---|---|
| Bus | Simple and easy to install; requires less cable; cost-effective | Failure of the main cable affects the whole network; difficult to troubleshoot; performance decreases with more devices |
| Star | Easy to install and manage; failure of one device does not affect others; easy to detect faults | Central device failure stops the entire network; requires more cable; higher cost |
| Ring | Data flows in an organized manner; no data collision; equal access for all devices | Failure of one device affects the whole network; difficult to add or remove devices; rarely used today |
| Mesh | Highly reliable and fault-tolerant; multiple paths for data; high security | Very expensive; complex installation; requires a large number of cables |
| Tree | Easy to expand; suitable for large networks; hierarchical management | Backbone failure affects the whole network; complex setup; more cabling required |
Single point of failure. In three topologies one part can stop everything: the main cable in bus, the central hub in star and the backbone in tree. Mesh avoids this because it has many paths.
Choosing. The book says the right topology depends on network needs. Some are simple and cheap, others are reliable but costly, and some are easy to expand.
Worked examples
Example 1. A small lab wants a low-cost network with the least cable. Which topology matches the book table?
- Bus requires less cable and is cost-effective.
- It also suits small networks.
- So bus topology fits.
Example 2. In which topology does the failure of one computer not stop the others, but the failure of the centre does?
- In star topology, all devices connect to a central hub or switch.
- A failed device does not affect others.
- A failed hub stops the network.
Example 3. A bank network must keep working when one link fails. Which topology does the book give?
- Mesh has multiple paths for data.
- Failure of one link does not stop communication.
- The cost is high, but reliability is the main need.
Example 4. Why is ring topology rarely used today?
- Failure of one device can affect the whole network.
- It is difficult to add or remove devices.
- It is also more complex than bus topology.
Common mistakes in the exam
- Mixing physical and logical topology. Physical is the actual connections. Logical is the data flow.
- Saying a star network fails if one computer fails. Only failure of the central device stops it.
- Saying mesh is cheap. The book calls it very expensive.
- Writing that tree is only a bus. The book says it combines bus and star.
- Leaving out the disadvantage. For every advantage, add a disadvantage from Table 1.1.
- Wrong direction in a ring. Data moves in one direction around the ring.
Exam technique
- For a topology question, give a definition, one figure and two points for each side.
- In a comparison, copy the book's rows into a table. Examiners look for the key words.
- Name the single point of failure for bus, star and tree.
- Check your board's notice for the sections in your paper.
Practice questions
- Define network topology. Answer: The layout of a computer network, showing how devices and cables are arranged.
- Name the topology in which all devices connect to a single main cable. Answer: Bus topology.
- Give two advantages of mesh topology. Answer: It is highly reliable and fault-tolerant, and multiple paths exist for data. High security is also listed.
- What happens in a star network when the central hub fails? Answer: The network stops.
- Which topology combines bus and star features? Answer: Tree topology.
Ready to practise this? Continue in 2nd Year Computer Science.