Within the definition of a network, points or nodes are connected by communication paths.These paths may vary significantly depending on the paths implemented. We cover four primary topologies: bus, star, ring, and mesh. Each topology has strengths and weaknesses, as well as different associated costs. A good network design will take each topology into consideration to determine the best solution.

Bus Topology

A bus topology is a linear LAN architecture in which transmissions from network devices or stations propagate the entire length of the medium and are received by all nodes on the medium.A common example of a bus topology is Ethernet/IEEE 802.3 networks.

Star Topology

A star topology is a LAN architecture in which the devices or stations on a network are connected to a central communications device, such as a hub or switch. Logical bus and ring topologies are often physically implemented in star topologies.

Ring Topology

A ring topology is a LAN architecture in which the devices or stations on a network are connected to each other by unidirectional transmission links to form a single closed loop. Common examples of ring topologies are Token Ring/IEEE 802.5 and FDDI networks.

Mesh Topology

A mesh topology is a LAN architecture is which every device or station on a network is connected to every other device or station. Mesh topologies are expensive to deploy and cumbersome to manage because the number of connections in the network can grow exponentially.The formula used to calculate the number of connections in a fully meshed network is as follows:

(N x (N–1))/2

where N is the number of devices on the network. Divide the result by 2 to avoid double counting the device A-to-device-B connection and the device

B-to-device-A connection.To illustrate the large numbers that a fully meshed environment can reach, review the following examples:

• A small network with 50 users wants to implement a fully meshed topology.The number of connections required to do this would be (50 × (50–1))/2, which equals 1,225.That is a lot of connections for a small LAN!
• A medium network with 500 users wants to implement a fully meshed topology.The number of connections required to do this would be (500 × (500–1))/2 which equals 124,750 connections!

Now for the reality check on fully meshed networks. Fully meshed networks are typically implemented in a small handful of situations.The most common deployment model for fully meshed networks would be in the WAN arena. Frame Relay and ATM are technologies that are well suited for fully meshed networks with high availability requirements. Figure 1.4 depicts a typical mesh network.