Network topology is the physical placement of the network nodes and other
devices so that an efficient network infrastructure is achieved. This physical
layout is determined by a number of issues including physical obstacles, distances, number of nodes to be connected, and cabling materials to be used. The topology of a network is essential for the security of the resulting network infrastructure. Computer networks, whether LANs, MANs, or WANs, are constructed based on the following popular topologies.
Bus
This is the oldest, and at one time the most popular, network topology. In the bus topology, the computers and all other devices seeking connection are connected together by a long bus cable, which must be properly terminated on both ends and at least one end must be earthed. The bus may be made up of a coax ot UTP. Every network element has full-duplex connections to the shared transmitting medium that allows every element on the bus to send and receive data. Because each computing element is directly attached to the transmitting medium, a transmission from any one element propagates the whole length of the medium in either direction and, therefore, can be received by all elements in the network. Because of this, precautions need to be taken to make sure that transmissions intended for one element can be received by that element and no one else; and there is a method for constructing this access. Channel distribution of messages via this physical medium may be governed by a contention control protocol, like the CSMA/CD for Ethernet or CSMA/CA for LocalTalk. It also can be controlled by a token (see Figure 5). We will discuss these methods in the coming sections.
Laptop Server Frewall Laptop Laptop Laptop Figure.5..Bus.topology
Star
A star topology is characterized by a central prominent node that connects to every other element in the network, so all elements in the network are connected to a central element. Every network element in a star topology is pair-wise connected in a point-to-point manner through the central element by its own UTP link segment. Communication between any pair of elements must go through this central element. The central element or node can operate either in a broadcast fashion, in which case information from one element is broadcast to all connected elements, or as a switching device in which the incoming data is transmitted only to one element, the nearest element enroute to the destination. Few early networks actually used this topology. The advantages of a star topology include its ability to automatically parti- tion itself and isolate a faulty node; it is easy to add and delete nodes; and it is simple and has a modular design. The biggest disadvantage to the star topology in networks is that the failure of the central element results in the failure of the entire network. Figure 6 shows a star topology.
Ring.
The ring topology uses a mechanism in which all the elements in the network must take turns in sending information around the ring. Figure 7 shows a ring
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topology network. The taking of turns in passing information is managed through a token system. A token, such as IEEE 802.5 or the ANSI X3T12, is a systemwide piece of information that guarantees the current owner to be the bus master. As long as it still owns the token, no other network ele- ment is allowed to transmit on the bus. When an element currently sending
information and holding the token is finished, it passes the token downstream
to its nearest neighbor. The token system is a good management system of collision and fairness. Two of the earliest networks were the Zurich ring and the Cambridge ring.
There are variants of a ring topology collectively called hub hybrids combin- ing either a star with a bus or a stretched star as shown in Figure 10.
Rng Laptop Laptop Laptop Laptop Server Frewall Laptop Token-rng Workstaton Laptop Server Frewall Workstaton Token Intene t Workstaton Figure.7..Ring.topology.network Figure.10..Token.ring.hub
Mesh
A mesh topology allows multiple access links between network elements, unlike other types of topologies. The multiplicity of access links between network elements offers an advantage in network reliability, because whenever one network element fails, the network does not cease operations, it simply
finds a bypass to the failed element and the network continues to function.
Mesh topology is most often applied in MAN networks. Figure 8 shows a mesh network.
Tree.
A more common type of network topology is the tree topology. In the tree topology, network elements are put in a hierarchical structure in which the most predominant element is called the root of the tree, and all other ele- ments in the network share a child-parent relationship. As in ordinary, though inverted trees, there are no closed loops, so dealing with failures of network elements presents complications depending on the position of the failed element in the structure. For example, in a deeply rooted tree, if the root element fails, the network is automatically ruptured and split into two parts. The two parts cannot communicate with each other. The functioning of the network as a unit, therefore, is fatally curtailed. Figure 9 shows a network using a tree topology.
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