The star topology reduces the chance of network failure by connecting all of the systems to a central node. When applied to a bus-based network, this central hub rebroadcasts all transmissions received from any peripheral node to all peripheral nodes on the network, sometimes including the originating node. All peripheral nodes may thus communicate with all others by transmitting to, and receiving from, the central node only. The failure of a transmission line linking any peripheral node to the central node will result in the isolation of that peripheral node from all others, but the rest of the systems will be unaffected.

If the star central node is passive, the originating node must be able to tolerate the reception of an echo of its own transmission, delayed by the two-way transmission time (i.e. to and from the central node) plus any delay generated in the central node. An active star network has an active central node that usually has the means to prevent echo-related problems.

A tree topology can be viewed as a collection of star networks arranged in a hierarchy. This tree has individual peripheral nodes (i.e. leaves) which are required to transmit to and receive from one other node only and are not required to act as repeaters or regenerators. Unlike the star network, the function of the central node may be distributed. As in the conventional star network, individual nodes may thus still be isolated from the network by a single-point failure of a transmission path to the node. If a link connecting a leaf fails, that leaf is isolated; if a connection to a non-leaf node fails, an entire section of the network becomes isolated from the rest.

In order to alleviate the amount of network traffic that comes from broadcasting everything everywhere, more advanced central nodes were developed that would keep track of the identities of different systems connected to the network. These network switches will “learn” the layout of the network by first broadcasting data packets everywhere, then observing where response packets come from.

Comparing Star networks to other types of network

Advantages

1. Easy to implement and extend, even in large networks
2. Well suited for temporary networks (quick setup)
3. The failure of a non-central node will not have major effects on the functionality of the network

Disadvantages

1. Limited cable length and number of stations
2. Maintenance costs may be higher in the long run
3. Failure of the central node can disable the entire network

Bus networks are the simplest way to connect multiple clients, but often have problems when two clients want to communicate at the same time on the same bus. Thus systems which use bus network architectures normally have some scheme of collision handing or collision avoidance for communication on the bus, quite often using Carrier Sense Multiple Access or the presence of a bus master which controls access to the shared bus resource.

A true bus network is passive – the computers on the bus simply listen for a signal; they are not responsible for moving the signal along. However, many active architectures can also be described as a “bus” if they provide the same logical functions as a passive bus; for example, switched Ethernet is can still be regarded as a logical bus network, if not a physical one. Indeed, the hardware may be abstracted away completely in the case of a software bus.

With the dominance of switched Ethernet over passive Ethernet, passive bus networks are uncommon in wired networks. However, almost all current wireless networks can be viewed as examples of passive bus networks, with radio propagation serving as the shared passive medium.

A topology is configuration of communication networks and is of two types, Physical and Logical. Physical topology refers to configuration of computers, cables, devices and mostly depends on various factors. A logical topology is a method of transmitting or passing data between workstations.

Types of Physical Topologies

1. Bus Network (also known as Liner Bus)
2. Star Topology (Centralization)
3. Ring Topology (also known as Star-Wired or Token Ring Network)
4. Tree
5. Mesh Topology

Bus Network

A bus network is a network architecture in which a set of clients are connected via a shared communications line, called a bus. There are several common instances of the bus architecture, including one in the motherboard of most computers, and those in some versions of Ethernet networks.

Star Topology

In a Star Topology each computer is directly connected to the centralized Hub or a Switch. In this way, when computer A sends a data packet for computer B, the data flows through the Hub or Switch to which both computer A and B are connected. Different types of cables can be used in this scenario like coaxial cable, fibre optic cable and twisted pair cable.

Token Ring / Star-Wired

A token ring topology is architecturally similar to star topology. The only difference here is that it is created of wiring that would allow transfer of data from one computer to another in a ring (or circle). A token ring network will pass information based on token system.

Tree

A tree topology combines characteristics of linear bus and star topologies. It consists of groups of star-configured workstations connected to a linear bus backbone cable. Tree topologies allow for the expansion of an existing network with ease.

Mesh Topology

A fully connected or complete topology is a network topology in which there is a direct link between all pairs of nodes. In a fully connected network with n nodes, there are n(n-1)/2 direct links. Synonym fully connected mesh network.

In a mesh topology, there are at least two nodes with two or more paths between them. A special kind of mesh, limiting the number of hops between two nodes, is a hypercube. The number of arbitrary forks in mesh networks makes them more difficult to design and implement, but their decentralized nature makes them very useful. This is similar in some ways to a grid network, where a linear or ring topology is used to connect systems in multiple directions. A multi-dimensional ring has a toroidal (torus) topology, for instance.

Considerations

Consider the following when choosing a topology:-

• Future Growth: Is the network for temporary use or will undergo lot of growth in the future. Plan it accordingly.
• Money: What is your budget? What will be the purpose of this network?
• Cable Media: Type of cable that should be used as per the standards.
• Length of the cable: How far are your systems placed in the network? Is it the same building that your systems will be placed in or if your office is in two floors which should be connected?

Summary

See the below table for a quick understanding and comparison of topologies, cable media and protocols used.