COMPUTER NETWORKS
A network is a set of devices (often referred to as nodes) connected by media links. A node can be a computer  printer or any or device capable of sending and receiving data  generated  by  other nodes on the network. The 1' connecting the devices are o en called communication channels.A computer network may be defined as an interconnected collection of autonomous computers. Two computers are said to be interconnected, if they are able to exchange information. statistical software is available in a PC, which has to analyze the data. A program forecasting future sales runs on the mainframe.
The sales executive has to follow the following steps to prepare the presentation: Step 1: Transfer data from files server to PC and run statistical software to get the result.
Step 2: Transfer data from Step 1 to mainframe and run forecasting program. Step 3: Transfer results from Step 2 back to PC for printing formatting.
Step 4: Send results to the laser printer.
In the above example, we have illustrated a very simple network. Many networks, however, involve many people using many PCs, each of which can access any of many printers or servers. Devices must be connected in a way that allows data to travel among many users with little or no delay.

It is important to identify the set of constraints and requirements that influence network design.
• An application programmer would list the services that his or her application needs, for example, a guarantee that each message the application sends will be delivered without error within a certain amount of time.
• A network designer would list the properties of a cost-effective design, for example, that network resources are efficiently utilized and fairly allocated to different users.
• A network provider would list the characteristics of a system that is easy to administer and manage, for example, in which faults can be easily isolated and where it is easy to account for usage.
Topology is the layout of the connections formed between computers. To some extent, the reliability and efficiency of a network is determined by its structure. Five basic topologies are possible: bus, star, ring, tree, and mesh. We will briefly describe each of them.

 Bus Topology
A network that uses a bus topology usually consists of a single, long cable to which computers are attached. Any computer attached to a bus can send a signal down the cable, and all computers receive the signal. Figure 1.3 illustrates the bus topology.
Because all computers attached to the cable can sense an electrical signal, any computer can send data to any other computer. Of course, the computers attached to a bus network must coordinate to ensure that only one computer sends a signal at any time or chaos results. In bus topology, the failure or removal of a device does not cause the network to fail.
Advantages:
Connecting a computer or peripheral to a linear bus is easy.
This topology requires least amount of cabling to connect the computers and, therefore, less expensive than other cabling arrangements.
It is easy to extend a bus since two cables can be joined into one longer cable with a connector.

Disadvantages:
• Entire network shuts down if there is a failure in the backbone.
• Heavy traffic can slow down a bus because computers on such networks do not coordinate with each other to reserve time to transmit.

Star Topology
The star topology is the oldest communications design method, with roots in telephone switching systems. However, the advances in network technology have made the star technology a good option for modern networks. A network uses a star topology if all computers attach to a central point. Figure 1.4 illustrates the star topology.
A hub is a central device that joins single cable segments or individual LANs into one network.
A typical hub consists of an electronic device that accepts data from a sending computer and delivers it to the appropriate destination. In practice, star networks seldom have a symmetric shape in which the hub is located at an equal distance from all computers. In a star topology, the failure of hub (central computer) brings down the entire network.
Advantages:
• Star topology is easy to install and wire.
• The network is not disrupted even if a node fails or is removed from the network. Disadvantages:
• It requires a longer length of cable.
• If the hub fails, nodes attached to it are disabled.
• The cost of the hubs makes the network expensive as compared to bus and ring topology.

  Ring Topology
The ring topology is a continuous path for data with no logical beginning or ending points and thus no terminators. Workstations and file servers are attached to the cable at points around the rings. Figure 1.5 illustrates the ring topology.
When data is transmitted onto the ring, it goes around the ring from node to node, finds its destination, and then continues until it ends at the source node. A ring network may be either unidirectional or bidirectional. Unidirectional means that all transmissions travel in the same direction. Thus each device can communicate with only one neighbour. Bidirectional means that data transmissions travel in either direction, and a device can communicate with both its neighbours.

The ring topology is easier to manage than the bus because the equipment used to build the ring makes it easier to locate a defective node or cable problem. Overall, the ring topology enables more reliable communications than the bus.

Advantages:
• Ring topology is easy to install and reconfigure.
• Every computer is given equal access to the ring, Hence, no single computer can monopolise the network.
Disadvantages:
• Failure in any cable or node breaks the loop and can take down the entire network.
• Maximum ring length and number of nodes are limited.

 Tree Topology

A tree topology is a variation of star. As in star, nodes in a tree are linked to a central hub that controls the traffic to the network. However, not every device plugs directly into the central hub. The majority of devices connect to a secondary hub that, in turn, is connected to the central hub as shown in Figure.

A good example of tree topology can be seen in the cable TV technology, where the main cable from the main office is divided into main branches with each branch divided into smaller branches, and so on. The hubs are used when a cable is divided. 
Advantages:
• The distance to which a signal can travel increases as the signal passes through a chain of hubs.
• Tree topology allows isolating and prioritizing communications from different nodes.
• Tree topology allows for easy expansion of an existing network, which enables organizations to configure a network to meet their needs.

 Disadvantages:
• If the backbone line breaks, the entire segment goes down.
 • It is more difficult to configure wire than other topologies.

Mesh Topology (Fully Connected Topology)
The mesh topology has a direct connection between every pair of devices in the network. This is an extreme design. Communication becomes very simple because there is no competition for common lines. If two devices want to communicate, they do so directly without involving other devices. Figure illustrates the mesh topology.

Advantages:
• The use of large number of links eliminates network congestion.
• If one link becomes unusable, it does not disable the entire system. Disadvantages:
• The amount of required cabling is very large.
• As every node is connected to the other, installation and reconfiguration is very difficult.
• The amount of hardware required in this type of topology can make it expensive to implement.
Combined Topologies
Many computer networks use combinations of the various topologies. Figure shows

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