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A mesh topology allows numerous access links between network elements, unlike other type of topologies. The multiplicity of access links between the network elements offers an advantage in network reliability because whenever one network element fails, the network does not cease information; it simply finds a bypass to the failed element and the network continues to function. Mesh topology is mostly applied in MAN networks. Mesh network is consistent and redundancy. If one node does not function, all the rest can still correspond. Mesh network can works well when the nodes are positioned at scattered points that do not lie near a frequent line. The major disadvantage in Mesh topology is its expense; this is because of its number of cables and connections necessary. In some cases ring network may prove beneficial and more effective in expenditure than the Mesh network and if all the nodes are near a frequent line, the bus topology could be used as one of the good choice in terms of expenses.
2. Tree topology
A more common type of network topology is the tree topology. In tree topology, network elements are put in a hierarchical formation in which the most predominant element is called the root of the tree and all other elements 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 elements fail, the network automatically ruptures and splits into two parts. The two parts communicate with each other. The functioning of the network as a unit is, therefore, fatally curtailed. Since the tree topology network is large it is difficult to configure and gets all complex after a definite point. There would be three levels of hierarchy in the tree network topology and they all toil based on the root node. The tree network topology can be extensive to function and there are no limits to how big it can be extended. Extra root nodes can be additional and they can be consistent within one single network.
3. Bus topology
A more popular topology, especially for LANs is the bus topology. Elements in a network using a bus topology always share a bus and therefore, have equal access to all LAN resources. Every network element has full duplex connections to the transmitting medium which 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 through the entire length of the medium in either direction and therefore can be received by all the 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 other element. This network must also use a mechanism that handles disputes in case two or more elements try to transmit at the same
Time. The mechanism deals with the likely collision of signals and brings a quick recovery from the collision. It is also necessary create a fairness in the network so that all other elements can transmit when they need to do so. A collision control mechanism must also improve efficiency in network using a bus topology by allowing only one element in the network to have control of the bus at any one time. This network element is called the bus master and other elements are considered to be slaves. This requirement prevents collision from occurring in the network as elements in the network try to seize the bus at the same time.
4. Ring topology
Finally another popular network topology is the ring topology. In this topology, each computing element in a network using a ring topology is directly connected to the transmitting medium via a unidirectional connection so that the information put on the transmission medium can reach all the computing elements in the network through a mechanism of taking turns in sending information around the ring. The taking of turns in passing the information is managed through a token system. A token system is a wide piece of information that guarantees the current owner to be the bus master. As long as it owns the token, no other network element is allowed to transmit on the bus. When an element currently is sending the information and holding the token has finished, it passes the token down to the nearest neighbor. The token system is a good management of collision and fairness. There are variants of a ring topology collectively called hub hybrids combining either a star with a bus or a stretched star.
Although network topologies are important in LANs, the choice of a topology depends on a number of other factors, including the type of transmission medium, reliability of the network, the size of the network, and its anticipated future growth. The most popular bus-and star- based LAN topology is the Ethernet, and the most popular ring-based LAN topology is the token ring.
B) Physical Communication Media
Coaxial cables - Coaxial cable was the dominant transmission medium for LANs for many years, lately it has lost ground to UTP and Fiber optic cable in the past.10Base2 coaxial (Thinnet cable) is installed only in networks that already contain this type of cable. Coaxial cable is composed of two conductors. Solid wire forms the inner conductor, and the outer conductor serves as a shield and is usually grounded. Between the two conductors is a plastic insulator. A plastic outer jacket surrounds the cable assembly. Coaxial cables are divided into two parts- those used for baseband transmission and those used for broadband classification.
1.1 The advantages of coaxial cables are as follows:
Coaxial cable provides excellent noise immunity compared to UTP installations. In general coaxial cable had good electromagnetic interference and radio frequency interference resistance and also in coaxial cable set-up has greater bandwidth and has low error rates and overall it has greater channel capacity.
1.2 The disadvantages of coaxial cables are as follows:
Coaxial cable is bulky and relatively difficult to install through wire ducts and other spaces within a building. In coaxial cable security tapping or intercepting a signal from a remote point is difficult because of the coaxial cabling system. Any attempt to tap into the coaxial bus will break or interrupt network traffic. Lately coaxial cables are not installed in new network implementations.
Twisted pair cabling- It is the most popular medium is twisted pair cabling, it consists of four to eight wires. Multi pair cable consisting of 25 pair cables is often used to consolidate many twisted pair cables for easier cable routing. Twisted pair are available with braided or foil electrostatic shielding. It comes in two varieties UTP and shielded. Twisted pairs support both analog and digital transmission using a wide variety of encoding and modulation techniques. The bandwidth and distance of twisted pairs are interrelated. They are generally used in point to point environments.
2.1 The advantages of twisted pair cabling are as follows:
Using repeaters at short intervals in a LAN environment can enhance the maximum bandwidth and enable a transmission rate in the order of 10 Mbps to greater than of 100 Mbps. Twisted pair are easy to install and in most installations, the twisted pair is routed from desktops to a central wiring closet and the number of twists per meter in the wire can give better performance.
2.2 The disadvantages of twisted pair cabling are as follows:
Twisted pair has the least secure of common LAN medium types because of its unshielded nature and also because it is easy to plug into a UTP hub where signal are broadcast. Encryption can add a higher level of data protection as opposed to limiting access to the network. It provides the least immunity to noise of all physical conductors, the medium both emits and absorbs noise and is also sensitive to electromagnetic interference and radio frequency interference.
Fiber optics- It is a technology which utilize glass (or plastic) threads (fibers) to broadcast data. A fiber optic cable consists of a bundle of glass threads, each of which is able to transmit messages modulated onto light waves. It is particularly popular technology for local-area networks and also telephone companies are steadily replacing traditional telephone lines with fiber optic cables. In future, almost all communications will be using fiber optics.
3.1 The advantages of fiber optics are as follows:
Fiber optics cables have a more amount of bandwidth than metal cables which means it can carry more data at a good speed and fiber optic cables are very thinner and also lighter in compared to metal wires and the data can be transmitted digitally rather than analogically. It is lower at installation cost, it is non-conductive and a very good point is, there are minimal chances of short circuit and sparks
3.2 The disadvantages of fiber optics are as follows:
Fiber optics cables are expensive to purchase and are also fragile then wire and it is also difficult to merge. Fiber optic transmitters and receivers are expensive in comparison to electrical interfaces. Fiber optic is not susceptible to radio frequency interference which would be a major problem for users of coaxial cabling.
4. Category-6 cable- It is also popularly known as Cat-6. It contains four twisted pairs of wires which contain copper and it is the sixth generation of twisted pairs which is utilized to the fullest. It is mainly used for Ethernet computer networking, security systems and telephone services. It is also capable of transmission of voice and data up to 155Mbps with a transmission frequency up to 550 MHz. It is backed with more rigid specifications for crosstalk and system noise than earlier cabling standards. It gives "all gigabit" network. The physical structure of the cable allows for better and efficient signal to noise ratio with minimal loss which translates to more rapidly and consistent networks for any existing application in the market place today. Commercial and residential contractors and installers turn to CAT 6 to future resistant network infrastructures.
4.1 The advantages of Cat-6 are as follows
Cat6 separator included in Category 6 cables for smooth operation of all members to contact the other, creating a bandwidth that is twice as fast, the fifth category Cat 6 will only work as fast as a slow computer or cable will - but you still get great performance from the cable. 6 Cat works best when all computers and peripherals in the networks with gigabit speeds or higher, and is ideal for use in building a new network computer.
4.2 The disadvantages of Cat6 are as follows
Users often do not realize that buying a Cat6 will give them a complete network of Gigabit Ethernet. In fact, Cat 6 cables only to return as quickly as if every item in the network can operate at gigabit speeds. If you do not connect Gigabit rating, and will run slower on the speed of network devices. Cat 6 cables are much more expensive than cable Cat 5e
C) Comparison of Different Type of Computers
NUMBER OF USERS
One individual at time and sharing is also possible.
Ranges from 7 inch to19 inch and weight is around 3kg on an average
One individual at time and sharing is also possible.
They are the small in size
Many users can use, more then 50 users can use
They are not small as micro computers. Medium size
Many users can use, thousand plus
Big size computers
MAIN FRAME COMPUTERS
Around thousand users can use.
They are large size computers.