Network Topologies

What is a network topology?

A network topology is a schematic description of a network arrangement that connects various nodes (devices) through links (communication lines). It describes the arrangement of a network and the flow of data between devices.

Two types of topology are distinguished:

• Physical topology: This refers to the actual physical layout of devices and cables, which can be determined from the network wiring.
• Logical topology: This describes the path taken by data signals, which can differ from the physical layout. A network can be physically wired as a star and operate logically as a bus.

Full mesh topology

In a full mesh topology, also called a fully connected or polygonal topology, every node is directly connected to every other node in the network. This creates a resilient and direct communication structure.

Characteristics

• Advantages: Its main advantage is high redundancy. If a single link fails, traffic can be routed around the failure. Direct links avoid a shared intermediary node, which can reduce contention for its resources.
• Disadvantages: The primary drawback is the large number of connections required. The number of links grows quadratically with the number of nodes, making the topology expensive and complex to cable and manage.
• Number of links: For $N$ nodes, the total number of required links is calculated by the formula: $\frac{N(N-1)}{2}$.

Partial mesh topology

A partial mesh topology is a practical approach where only some nodes are fully interconnected. Central nodes might be connected to one another, while peripheral nodes are connected to only one or two other nodes.

This topology balances the high cost of a full mesh with the need for redundancy. It can provide less redundancy than a full mesh and more than topologies such as star or bus. The Internet backbone is a large-scale example of a partial mesh network.

Ring topology

In a ring topology, each node is connected to exactly two other nodes, forming a continuous signal path, or a closed loop. Data travels sequentially from node to node, typically in one direction.

Characteristics

• Advantages: A simple structure is easy to install and can control access to bandwidth, which can limit collisions. More complex configurations, such as dual rings, can combine features of mesh and star topologies.
• Disadvantages: In an unprotected ring, the failure of a single node or a cable break can disrupt the entire network. Adding or removing nodes requires breaking the loop, which can cause network downtime.
• Related problems: Finding the shortest route that visits every node in a ring or mesh network exactly once and returns to the origin is a computational problem known as the Traveling Salesman Problem or finding a shortest Hamiltonian Cycle.

Tree topology

A tree topology combines characteristics of star and bus topologies. It has a hierarchical structure where nodes form branches of a tree, with a single root node at the top. Each node, except the root, has exactly one parent node.

Characteristics

• Advantages: This topology is scalable, relatively easy to configure, and simple to manage. Fault isolation is easier because a problem in a lower-level branch can remain limited to that branch.
• Disadvantages: Its primary weakness is the reliance on higher-level nodes. If a node in the upper hierarchy fails, the entire sub-tree of nodes beneath it can be cut off from the rest of the network.
• Minimal connections: A tree with $N$ nodes always has $N-1$ links. In a connected weighted graph, an algorithm can find a Minimum Spanning Tree, a tree that connects all nodes with the lowest total edge weight. Prim's and Kruskal's algorithms are used for this task.