Types of Network Topologies

Network topologies determine the flow of data through a network. This is essential to know when designing or troubleshooting a network, irrespective of what your role is. You need to understand the characteristics of the network topology you are working with and identify how the topology affects network performance and troubleshooting.

GLOSSARY:

The physical topology describes the placement of nodes and how they are connected. For example, in one network, nodes might be directly connected via a single cable; in another network, each node might connect to a switch via separate cables. These two networks have different physical topologies.
The logical topology describes the flow of data through the network. For example, given the two cases of physical topologies, if in both cases the nodes can send messages to one another, the logical topology is the same. The different physical implementations (directly connected via cable versus connected to the same switch) achieve the same logical layout.

POINT-TO-POINT LINKS

A single link is established between two nodes. This is called a point-to-point connection. This can be physical or logical.
For example, on a WAN or MAN, two routers might be linked to each other via intermediate networks and physical devices but still are a logical point-to-point link, where each can address only the other router.
Either a physical or logical topology, it is the 1:1 relationship that defines a point-to-point link.

BUS TOPOLOGY

A physical bus topology with more than two nodes is a shared access topology, meaning that all nodes share the bandwidth of the media. Only one node can be active at one time. All nodes attach directly to a single cable segment via cable taps. A signal travels down the bus in both directions from the source and is received by all nodes connected to the segment. The bus is terminated at both ends of the cable to absorb the signal when it has passed all connected devices.

Bus Networks are comparatively difficult to reconfigure (adding or removing nodes can disrupt the whole network), impose limitations on the maximum number of nodes, and are difficult to troubleshoot a cable fault. Most importantly, a fault anywhere in the cable means that all nodes are unable to communicate.

Physical Bus Topology

A logical bus topology is one in which nodes receive the data transmitted all at the same time, regardless of the physical wiring layout of the network. Because the transmission medium is shared, only one node can transmit at a time.

STAR TOPOLOGY

In a star topology network, each endpoint node is connected to a central forwarding node, such as a hub, switch, or router. The central node is usually a fast, self-contained computer and is responsible for routing all traffic to other nodes. The star topology is the most widely used physical topology. For example, a network that is based around a single Internet router that can connect to a cable or wirelessly.

The star topology is easy to reconfigure and easy to troubleshoot because all data goes through a central point, which can be sued to monitor and manage the network. Faults are automatically isolated to the media, network card, or the hub, switch, or router at the centre of the star.

PHYSICAL STAR-LOGICAL BUS TOPOLOGY

A physical star network can be used to implement a logical bus topology. When a device such as a hub is used at the centre of the star, transmission is still repeated to each node. Logically, the topology works like a single cable bus and the bandwidth is still shared between all nodes. This means that some of the limitations of a physical bus topology are retained.

Physical Star topology with logical bus topology implementation

PHYSICAL STAR-LOGICAL STAR TOPOLOGY

When a device such as a switch is used at the centre of the star, the bus element is reduced to the link between each node and its switch port. Taking the network as a whole, both the physical and logical topology is a star. The way the switch operates allows each node to use the full bandwidth of the network media, and it allows nodes to communicate on the network simultaneously. Faults are isolated to the link between a node and the switch or to the switch itself.

Physical and logical star topology implementation

RING TOPOLOGY

In a physical ring topology, each node is wired to its neighbour in a closed loop resulting in the creation of a circular data path. A nod receives a transmission from its upstream neighbour and passes it to its downstream neighbour until it reaches its destination.

The physical ring topology is no longer used on LANs, but it does remain a feature of many WANs.

Implementation of Ring Topology

MESH TOPOLOGY

Mesh network topologies are commonly used in WANs, especially public networks like the Internet. In theory, a mesh topology is a network setup where each computer and network device is interconnected with one another. The number of links required by a full mesh is expressed as n(n-1)/2, where n is the number of nodes. For example, a network of just five nodes would require ten links, while a network of 45 nodes would need 990 links! However, a hybrid approach is often used, with only the most important devices interconnected in the mesh, perhaps with extra links for fault tolerance and redundancy. Hence, it is referred to as a partial mesh.