Network Topologies
Each computer in a network is called a node. There are ways in which you can connect the various computers for networking. This physical arrangement is called Topology. Following are the different types of Network Topologies:
1. Bus Network
In a bus network each computer is connected to a single communication cable via an interface and every computer can directly communicate with every other computer or device in the network as shown in the figure. This topology is commonly used alongwith the local area networks.
Advantages
i. It has short cable length and simple wiring layout.
ii. I is easy to extend bus network. Additional nodes can be connected to an existing bus network at any point along its length.
Disadvatages
i. Fault detection is difficult in this case. In most LANs based on a bus, control of the network is not centralized in any particular node. This means that detection of a fault may be performed from many points in the network.
ii. There is a limit on central cable length and number of nodes that can be connected.
iii. Dependency on central cable in this topology has its disadvantages.If the main cable encounters some problem, whole network breaks down.
2. Ring Network
In a ring network, several devices or computers are connected to each other in a closed loop by a single communication cable as shown in the figure. A ring network is also called loop network. In the ring network, data must travel around the ring to each station in turn until they arrive at the required station.
Advantages
i. Ring network works well where there is no central node for making routing decisions.
ii. It is more reliable than a star network because communication is not dependent on a single central node. If a link between any two nodes fails, or if one of the nodes fails, alternate routing is possible.
Disadvantages
i. In a ring network, communication delay is directly proportional to number of nodes in the network. Hence, addition of new nodes in a network increases communication delay.
ii. requires more complicated control software than star network.
3. Star Network
In this network all the computers and the main server form a star like shape. All the computers are connected to a single main host. All communication first goes through the centralized computer allowing it to control the operation, work load and resource allocation of the other computers in the network.
Advantages
i. Star topology has minimal line cost because only n-lines are required for connecting n nodes.
ii. Transmission delays between two nodes do not increase by adding new nodes to network, because any two nodes are connected via two links only.
iii. If any node other than the host node fails, remaining nodes are unaffected.
Disadvantages
i. The system crucially depends on host node. If it fails, entire network.
4. Tree Network
In a tree network, several devices or computers are linked in a hierarchical fashion as shown in the figure. Tree network is also known as hierarchical network. This type of distribution system is commonly used in the organization where headquarters communicated with regional offices and regional offices communicate with district offices and so on.
Advantages
i. It is easier to add new nodes or branches to it.
ii. It is possible to disconnect whole branches of the network from the main structure. This makes it easier to isolate a defective node.
Disadvantages
i. If the ‘headend’ device fails to operate, the entire network is rendered inoperable. In this respect, the tree suffers from the same reliability problems as the star.
5. Completely Connected Network
It has a separate physical link for connecting each node to any other node. Hence, each node has a direct link, point-to-point link, with all other nodes in the network. The control is distributed with each node deciding its communication priorities.
Advantages
i. It is very reliable because any link failure will affect only direct communication between the nodes connected by that link.
ii. Each node of the network need not have individual routing capability.
iii. Communication is very fast between any two nodes.
Disadvantage
i. It is the most expensive network from the point of view of link cost. If there are n nodes in a network, n(n-1)/2 links are required. Hence, cost of linking the system grows with square of the number of nodes.
6. Hybrid Network
Different network topologies have their own advantages and limitations. Hence, in reality, a pure star or ring or completely connected network is rarely used. Instead an organization normally uses a hybrid network that IS a combination of two or more different network topologies, Exact configuration of a network depends on needs and structure of the organization. In some cases, hybrid network may have components of star, ring, and completely connected networks.
Advantages
i. Unlike other networks, fault detection and troubleshooting is easy in this type of topology. The part in which fault is detected can be isolated from the rest of network and required corrective measures can be taken, WITHOUT affecting the functioning of rest of the network.
ii. Its easy to increase the size of network by adding new components, without disturbing existing architecture.
iii. Hybrid Network can be designed according to the requirements of the organization and by optimizing the available resources. Special care can be given to nodes where traffic is high as well as where chances of fault are high.
iv. Hybrid topology is the combination of two or more topologies, so we can design it in such a way that strengths of constituent topologies are maximized while there weaknesses are neutralized. For example we saw Ring Topology has good data reliability (achieved by use of tokens) and Star topology has high tolerance capability (as each node is not directly connected to other but through central device), so these two can be used effectively in hybrid star-ring topology.
Disadvantages
i. One of the biggest drawback of hybrid topology is its design. Its not easy to design this type of architecture and its a tough job for designers. Configuration and installation process needs to be very efficient.
ii. The hubs used to connect two distinct networks, are very expensive. These hubs are different from usual hubs as they need to be intelligent enough to work with different architectures and should be function even if a part of network is down.
iii. As hybrid architectures are usually larger in scale, they require a lot of cables, cooling systems, sophisticate network devices, etc.
