Network topologies describe both the physical and logical layouts of a network. A particular technology such as Token Ring might lead you to believe that the network is physically connected in one big circle of cable. In reality, the ring is formed in circuitry and the physical network appears as a star. Common topologies that you should be familiar with include Bus, Ring, Star, Hybrid, and Mesh.
A bus topology is by far the most simple. A bus is comprised of a single run of cable to which individual systems are attached. When a failure occurs on a bus network, it affects the entire network since the path for data transfer is disrupted. While you may still find Ethernet bus networks in smaller environments, they are becoming less and less common. In general, bus topologies are relatively inexpensive but tend to be more prone to failure.
As the name suggests, a ring topology is comprised of a number of systems connected in a type of loop. In most cases, systems connect to a hub-type device within which the actual ring is formed. For this reason, you’ll often see ring topologies described as being a star-ring, where data is passed through the network along the ring circuitry, but the physical layout actually appears to be a star. Logical ring / physical star layouts are commonly found in both Token Ring and FDDI environments.
By far the most common network topology found today, a physical star is created when systems connect to a central device such as a hub or switch. Systems branch out from this central device, creating the star-like appearance. The main benefit of this topology is the fact that a break in a cable only affects the particular connected system and not all others. Recognize, however, that a single point of failure still exists – if the hub or switch fails, all connected systems will not be able to communicate.
Most large networks designed today tend to be variations of star topologies. However, many networks will be comprised of a number of different topologies rather than just one. For example, a company’s network might be a star-bus hybrid, where systems connect to hubs (forming the star) and then hubs interconnect using a bus. Various hybrids are possible, including stars, rings, and buses.
Wide Area Networks (WANs) are often configured in a mesh topology for the purpose of redundancy. In a mesh, a router or switch may have more than one connection to a different site. In this way, if one link fails, another path exists. Because of this, mesh topologies are also much more expensive, and seldom found between computers on LANs (although certain systems requiring high-availability may be configured in this way). A mesh in which every system has a connection to every other system is referred to as a full mesh. If fewer connections exist, it is simply known as a partial mesh. A good example of a technology that is often configured in a mesh is Frame Relay.