Cambridge A-Level Computer Science 9618 – Networks: Topologies2.1 Networks – The Internet and LAN Topologies
Learning Objective
Show understanding of the bus, star, mesh and hybrid topologies.
Key Concepts
- Definition of a network topology.
- Physical vs logical topology.
- Advantages and disadvantages of each common topology.
Bus Topology
A bus topology connects all devices to a single continuous cable, called the backbone.
- Structure: One central cable; each node taps into the bus.
- Transmission: Data travels in both directions; only one device can transmit at a time (using CSMA/CD).
- Advantages:
- Simple and inexpensive to implement.
- Requires less cable than star.
- Disadvantages:
- Failure of the backbone disables the whole network.
- Performance degrades as more devices are added.
- Limited cable length and number of nodes.
Star Topology
In a star topology each node is connected directly to a central device (hub, switch, or router).
- Structure: Central hub/switch with individual links to each node.
- Transmission: Data from a source node passes through the central device to the destination.
- Advantages:
- Failure of a single link affects only that node.
- Easy to add or remove devices.
- Better performance; each link can operate simultaneously.
- Disadvantages:
- Requires more cable than bus.
- Central device is a single point of failure.
Mesh Topology
A mesh topology provides a dedicated point‑to‑point link between every pair of nodes.
- Structure: Fully connected network; each node has a direct link to every other node.
- Transmission: Data can take multiple paths; routing algorithms choose the optimal path.
- Advantages:
- Highly reliable – multiple redundant paths.
- Scalable bandwidth; simultaneous transmissions.
- Disadvantages:
- Expensive – number of links grows as \$ \frac{n(n-1)}{2} \$ for \$n\$ nodes.
- Complex cabling and management.
Hybrid Topology
A hybrid topology combines two or more basic topologies to meet specific requirements.
- Common examples:
- Star‑bus: multiple star segments linked by a bus backbone.
- Star‑mesh: central star with some nodes interconnected as a mesh.
- Advantages:
- Flexibility – can optimise cost, performance, and reliability.
- Tailored to organisational layout.
- Disadvantages:
- Design and troubleshooting can be more complex.
- Cost depends on the constituent topologies.
Comparison Table
| Topology | Typical Use | Advantages | Disadvantages | Scalability (nodes) |
|---|
| Bus | Small LANs, early Ethernet | Low cabling cost, simple | Backbone failure stops network; performance degrades with traffic | Limited – up to \overline{30} nodes before performance drops |
| Star | Modern Ethernet LANs, Wi‑Fi access points | Easy to manage, fault isolation, supports high traffic | Central device failure; more cable than bus | Highly scalable – limited mainly by hub/switch ports |
| Mesh | Backbone of the Internet, mission‑critical systems | Redundant paths, high reliability, simultaneous transmissions | Expensive, complex cabling, difficult to maintain | Scales poorly in cost; theoretical \$ \frac{n(n-1)}{2} \$ links |
| Hybrid | Large campuses, corporate networks | Customisable, balances cost and reliability | Design complexity, mixed failure points | Depends on chosen combination |
Key Formulas
Number of links in a full mesh:
\$ L = \frac{n(n-1)}{2} \$
Maximum cable length for a 10 Base‑T Ethernet segment (star): 100 m per link.
Suggested diagram: Bus, Star, Mesh, and a simple Hybrid (Star‑bus) layout showing how devices connect.
Summary
- Bus topology is simple but vulnerable to backbone failure.
- Star topology offers easy management and fault isolation, with a central point of failure.
- Mesh topology provides the highest reliability at a high cost.
- Hybrid topologies combine strengths of basic types to suit specific network requirements.