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A topology is defined as the layout of the network i.e. how the nodes are connected. This describes how the network physically looks or how the network is physically designed. The concept of a topology is important because each network card is designed to work with a specific topology. Conversely, if your network cable is already installed and you want to use existing wiring, you must select your network cards based on the preexisting physical topology. Ideally, you can design your network from scratch. Then you can choose your topology, cabling, and network cards based on what best meets your needs.
Physically, a bus topology uses a linear segment of cable to connect all network devices. Devices typically connect to the bus (the cable) through T-connectors. At each end of the bus are terminators. Each terminator absorbs the signal when it reaches the end of the cable. Without a terminator, a signal would bounce back and cause network errors.
The physical bus topology uses a logical bus to transmit data on the cable in both directions. In a logical bus topology, only one transmission can occur at any given moment. Otherwise, two transmissions would collide and cause network errors. Termination ensures that the signal is removed from the cable when it reaches either end, preventing possible network errors.
Fig. 4.1 Bus Topology
Advantages of Bus Topology :
The benefits of a bus topology include the following:
1. This is less expensive topology because it requires less cable for networking
because using only one cable it is possible to connect many computers.
2. It is an easy way to network a small number of computers.
Disadvantages of Bus Topology :
The drawbacks of a bus topology include the following:
1. One break in cable cause entire failure in network.
2. It is very difficult to correct the errors because the cable is not related to only
3. On a medium-sized to large network, reconfiguration is more difficult than the
cable Management of a star topology.
The star topology looks like a star. The hub is at the center of the star, and all devices attach to the hub via a cable. Logically, the physical star topology operates as a logical bus topology by sending the data signal to all nodes at once. The hub at the center of the star works as a signal splitter, which means the signal is split and sent to all computers at the same time, with one exception-it is not sent back to the computer from which the signal sent. The signal is terminated at each network card, thereby preventing the signal from accidentally reentering the network. If this were to happen, data packets would travel the network endlessly-seriously slowing down network performance.
Fig. 4.2 Star Topology
Advantages of Star Topology :
The benefits of a star topology include the following:
1. A star topology is more fault tolerant than other topologies, because a cable
break does not bring down the entire network.
2. Reconfiguring the network, or adding nodes, is easy because each node
connects to the central hub independent of other nodes.
3. Isolating cable failures is easy because each node connects independently to the
Disadvantages of Star Topology :
The disadvantages of a star topology are:
1. If the central hub fails, the entire network becomes unavailable.
2. This topology is more expensive than others to install because of the additional
cable and equipment involved.
Physically, the ring topology is shaped in a ring. Cables pass from computer to computer until the ring is complete. When data is transmitted, each workstation receives the signal and then passes it on when the workstation is done with the data. Other than Fiber Distributed Data Interface (FDDI), no current networks use a physical ring topology, because a break in the ring makes the entire network unavailable. Logically, a ring topology works by passing the signal, traditionally called a token, from one node to another until it reaches all the way around the ring. Token-passing schemes use the logical ring topology.
Fig. 4.3 Ring Topology
Advantages of Ring Topology :
A logical ring topology ensures access to the network without the risk of collisions, which can occur in logical star or bus topologies.
Disadvantages of Ring Topology :
The drawbacks of a ring topology include the following:
1. If there is a break in the cable of a physical ring topology, the network becomes
2. Physical ring topologies are difficult to troubleshoot.
3. Physical ring topologies are hard to reconfigure.
4. There is limited support for ring networks.
5. The costs for a ring network are significantly higher than for star or bus.
Also known as a hierarchy network, The type of network topology in which a central ‘root’ node (the top level of the hierarchy) is connected to one or more other nodes that are one level lower in the hierarchy (i.e., the second level) with a point-to-point link between each of the second level nodes and the top level central ‘root’ node,
Fig. 4.4 Tree Topology
While each of the second level nodes that are connected to the top level central ‘root’ node will also have one or more other nodes that are one level lower in the hierarchy (i.e., the third level) connected to it, also with a point-to-point link, the top level central ‘root’ node being the only node that has no other node above it in the hierarchy (The hierarchy of the tree is symmetrical.) Each node in the network having a specific fixed number, of nodes connected to it at the next lower level in the hierarchy, the number, being referred to as the ‘branching factor’ of the hierarchical tree. This tree has individual peripheral nodes.
1.) A network that is based upon the physical hierarchical topology must have at least three levels in the hierarchy of the tree, since a network with a central ‘root’ node and only one hierarchical level below it would exhibit the physical topology of a star.
2.) A network that is based upon the physical hierarchical topology and with a branching factor of 1 would be classified as a physical linear topology.
3.) The branching factor, f, is independent of the total number of nodes in the network and, therefore, if the nodes in the network require ports for connection to other nodes the total number of ports per node may be kept low even though the total number of nodes is large ââ‚¬” this makes the effect of the cost of adding ports to each node totally dependent upon the branching factor and may therefore be kept as low as required without any effect upon the total number of nodes that are possible.
4.) The total number of point-to-point links in a network that is based upon the physical hierarchical topology will be one less than the total number of nodes in the network.
5.) If the nodes in a network that is based upon the physical hierarchical topology are required to perform any processing upon the data that is transmitted between nodes in the network, the nodes that are at higher levels in the hierarchy will be required to perform more processing operations on behalf of other nodes than the nodes that are lower in the hierarchy. Such a type of network topology is very useful and highly recommended
The value of fully meshed networks is proportional to the exponent of the number of subscribers, assuming that communicating groups of any two endpoints, up to and including all the endpoints, is approximated by Reed’s Law.
Fig. 4.5.1 Fully connected mesh topology
The number of connections in a full mesh = n(n – 1) / 2
Note: The physical fully connected mesh topology is generally too costly and complex for practical networks, although the topology is used when there are only a small number of nodes to be interconnected.
Fig. 4.5.2 Partially connected mesh topology
The type of network topology in which some of the nodes of the network are connected to more than one other node in the network with a point-to-point link ââ‚¬” this makes it possible to take advantage of some of the redundancy that is provided by a physical fully connected mesh topology without the expense and complexity required for a connection between every node in the network.
In most practical networks that are based upon the physical partially connected mesh topology, all of the data that is transmitted between nodes in the network takes the shortest path (or an approximation of the shortest path) between nodes, except in the case of a failure or break in one of the links, in which case the data takes an alternative path to the destination. This requires that the nodes of the network possess some type of logical ‘routing’ algorithm to determine the correct path to use at any particular time.
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