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6.3.4 Star Topology

            The star topology has a central controller known as a hub. All the nodes are connected to the hub. So, in a network
            with one hub device, and n nodes, there will be n connections, each connecting the hub to a node. Communication
            between any two devices must pass through the hub. So, no two devices can communicate directly in a star network. A
            hub may be of one of the two types: a broadcasting hub or a unicast hub. On receiving a message, a broadcasting hub
            will broadcast it to all nodes. In contrast, on receiving a message, a unicast device  forwards it to the destination node
            on an appropriate outgoing line. In Figure 6.8, we show a network laid using star topology.














                                                      Figure 6.8 Star Topology

            Advantages of Star Topology
            1.  Since each device in star topology requires only one link point (I/O port) for connecting it to a hub, it can be easily
               installed and reconfigured.
            2.  Addition and removal of devices in an existing network are easy.
            3.  Failure of a link will only affect communication with one node. The rest of the network will continue to function as
               before.
            4.  A faulty node can be easily detected and isolated from the network.

            Disadvantages of Star Topology

            1.  There is no point to point communication between devices. Any communication must pass through the hub.
            2.  Failure of the hub will disrupt the functioning of the entire network.
            6.3.5 Tree Topology

            The tree topology exhibits a hierarchical structure. The central hub at the root of the tree may be connected to other
            nodes or secondary hubs. Devices in tree topology are connected in a hierarchical structure with one root and several
            intermediate nodes (devices). The central hub is called an active hub because it includes a repeater that regenerates
            the signal. However, the secondary hub simply forwards a  message to the appropriate outgoing line. Figure 6.9 shows a
            network that uses the tree topology. When a node wants to communicate with another node, the  messages are relayed
            via central and secondary hubs, as required. For example, in Figure 6.9, to send a message from node A to node C, the
            secondary hub (Hub2) will forward it directly to node C without involving the central hub (Hub1). However, to send a
            message from node A to node F, node A will send the message to the secondary hub (Hub2), which will forward it to
            the central hub (Hub1). Then the central hub will deliver it to the secondary hub (Hub3), which will forward it to node F.

                                 A                          Hub 1
                                           Hub 2                             Hub 3            H





                                 B         C            D          E           F         G



                                                      Figure 6.9 Tree Topology

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