Thursday, January 30, 2020

Network Typologies Essay Example for Free

Network Typologies Essay A network is a system of two or more computers that are connected in some manner. Each computer on the network has access to the files and peripheral equipment (such as printers or modems) on all the other computers on the network. The origin of local area networks can be traced, in part, to IBM terminal equipment introduced in 1974. At that time, IBM introduced a series of terminal devices designed for use in transaction-processing applications for banking and retailing. What was unique about those terminals was their method of connection: a common cable that formed a loop provided a communications path within a localized geographical area. Unfortunately, limitations in the data transfer rate, incompatibility between individual IBM loop systems, and other problems precluded the widespread adoption of this method of networking. The economics of media sharing and the ability to provide common access to a centralized resource were, however, key advantages, and they resulted in IBM and other vendors investigating the use of different techniques to provide a localized communications capability between different devices. However, Datapoint Corporation began selling its Attached Resource Computer Network (ARCNet), considered by most people to be the first commercial local area networking product. Since then, hundreds of companies have developed local area networking products, and the installed base of terminal devices connected to such networks has increased exponentially. They now number in the hundreds of millions. Designing a manageable network One of the most important considerations in designing a network to be manageable is deciding how and where to connect the network-management equipment. Is there a separate network-management center to accommodate? Do nonoperational staff members like the network designer sit in a different area? Do they require access to the network-management centers equipment through the network? In general, the design should include a separate virtual local area network (VLAN) just for network-management equipment. The management VLAN was used to access management functions on remote network equipment. This network management-equipment VLAN houses servers and workstations used to manage the network. Design Types A large-scale network design is composed of several common building blocks. Every LAN, of whatever size, has to have an access system by which the end stations connect to the network. There are several inexpensive options for LAN connections, such as Ethernet and Token Ring. As a philosophical principle, the network should be built using basic commonly available technology. The design shouldnt have to reinvent any wheels just to allow the machines to talk to one another. So, just as basic commonly available technologies exist for connecting end stations to LANs, there are common methods for interconnecting LAN segments. Once again, these technologies and methods should involve the most inexpensive yet reliable methods. But in this stage of interconnecting, aggregating, and distributing traffic between these various LAN segments, the designer may run into some serious hidden problems. There may be thousands of ways to connect things, but most of these methods result in some kind of reliability problems. Network topology The topology of a local area network is the structure or geometric layout of the cable used to connect stations on the network. Unlike conventional data communications networks, which can be configured in a variety of ways with the addition of hardware and software, most local area networks are designed to operate based on the interconnection of stations that follow a specific topology. The most common topologies used in LANs include the loop, bus, ring, star, and tree, as illustrated in the figure below Loop As previously mentioned, IBM introduced a series of transaction-processing terminals in 1974 that communicated through the use of a common controller on a cable formed into a loop. This type of topology is illustrated at the top of Figure below. Local area network topology. The five most common geometric layouts of LAN cabling form a loop, bus, ring, star, or tree structure. Because the controller employed a poll-and-select access method, terminal devices connected to the loop require a minimum of intelligence. Although this reduced the cost of terminals connected to the loop, the controller lacked the intelligence to distribute the data flow evenly among terminals. A lengthy exchange between two terminal devices or between the controller and a terminal would thus tend to weigh down this type of network structure. A second problem associated with this network structure was the centralized placement of network control in the controller. If the controller failed, the entire network would become inoperative. Due to these problems, the use of loop systems is restricted to several niche areas, and they are essentially considered a derivative of a local area network. Bus In a bus topology structure, a cable is usually laid out as one long branch, onto which o ther branches are used to connect each station on the network to the main data highway. Although this type of structure permits any station on the network to talk to any other station, rules are required for recovering from such situations as when two stations attempt to communicate at the same time. Ring In a ring topology, a single cable that forms the main data highway is shaped into a ring. As with the bus topology, branches are used to connect stations to one another via the ring. A ring topology can thus be considered to be a looped bus. Typically, the access method employed in a ring topology requires data to circulate around the ring, with a special set of rules governing when each station connected to the network can transmit data. Star The fourth major local area network topology is the star structure, illustrated in the lower portion of Figure 1. In a star network, each station on the network is connected to a network controller. Then, access from any one station on the network to any other station can be accomplished through the network controller. Here, the network controller functions like a telephone switchboard, because access from one station to another station on the network can occur only through the central device. In fact, you can consider a telephone switchboard or PBX as representing a star-structured LAN whose trunks provide connections to the wide area network telephone infrastructure. Tree A tree network structure represents a complex bus. In this topology, the common point of communications at the top of the structure is known as the head-end. From the head-end, feeder cables radiate outward to nodes, which in turn provide workstations with access to the network. There may also be a feeder cable route to additional nodes, from which workstations gain access to the network. One common example of a tree structure topology is the cable TV network many readers use on a daily basis. With the upgrade introduction to networking of many cable TV systems to two-way amplifiers and the support of digital transmission, the local cable TV infrastructure can be considered to represent an evolving type of tree-structured local area network. Mixed Topologies Some networks are a mixture of topologies. For example, a tree structure can be viewed as a series of interconnected buses. Another example of the mixture of topologies is a type of ethernet known as 10BASE-T. 10BASE-T network can actually be considered a star-bus topology, because up to 16 or 24 devices known as stations are first connected to a common device known as a hub, which in turn can be connected to other hubs to expand the network. Transmission Medium Used in LAN. The transmission medium used in a local area network can range in scope from twisted-pair wire, such as is used in conventional telephone lines, to coaxial cable, fiber-optic cable, and electromagnetic waves such as those used by FM radio and infrared. Each transmission medium has a number of advantages and disadvantages. The primary differences between media are their cost and ease of installation; the bandwidth of the cable, which may or may not permit several transmission sessions to occur simultaneously; the maximum speed of communications permitted; and the geographic scope of the network that the medium supports. Twisted-pair wire In addition to being the most inexpensive medium available for LAN installations, twisted-pair wire is very easy to install. Since this wiring uses the same RJ11 and RJ45 modular connectors as a telephone system, once a wire is cut and a connector fastened, the attachment of the connector to network devices is extremely simple. Normally, a screwdriver and perhaps a pocket knife are the only tools required for the installation of twisted-pair wire. Anyone who has hooked up a pair of speakers to a stereo set has the ability to install this transmission medium. Unshielded twisted-pair Although inexpensive and easy to install, unshielded twisted-pair (UTP) wire is very susceptible to noise generated by fluorescent light ballasts and electrical machinery. In addition, a length of twisted-pair wire acts as an antenna; however, the twists serve as a mechanism to partially counteract this antenna effect. Unfortunately, due to the law of physics, the longer the wire length, the greater the noise it gathers. At a certain length, the received noise will obliterate the signal, which attenuates or decreases in strength as it propagates along the length of the wire. This noise can affect the error rate of data transmitted on the network, although lead-shielded twisted-pair (STP) cable can be employed to provide the cable with a high degree of immunity to the line noise and enable extended transmission distances. Examining a building cabling standard and the various categories of twisted-pair that can support different transmission rates which, in turn, enable different types of Ethernet networks to be supported. Because the bandwidth of twisted-pair cable is considerably less than coaxial or fiber-optic cable, normally only one signal is transmitted on this cable at a time. Although a twisted-pair wire system can be used to transmit both voice and data, the data transmission is baseband because only one channel is normally used for data. In comparison, a broadband system on coaxial or fiber-optic cable can be designed to carry voice and several sub channels of data, as well as fax and video transmission. Other constraints of unshielded twisted-pair wire are the rate at which data can flow on the network and the distance it can flow. Although data rates up to 1 gigabit per second (Gbps) can be achieved, normally local area networks employing UTP wiring operate at a lower data rate. In addition, UTP systems normally cover a limited distance, measured in terms of several hundred to a few thousand feet, while coaxial and fiber-optic cable–based systems may be limited in terms of miles. Extending transmission distances over twisted-pair wire requires the periodic insertion of repeaters into the cable. A repeater receives a digital signal and then regenerates it; hence, it is also known as a data regenerator. Coaxial cable At the center of a coaxial cable is a copper wire, which is covered by an insulator known as a dielectric. An overlapping woven copper mesh surrounds the dielectric, and the mesh, in turn, is covered by a protective jacket consisting of polyethylene or aluminum. The figure below illustrates the composition of a typical coaxial cable; however, it should be noted that over 100 types of coaxial cable are currently marketed. The key differences between such cables involve the number of conductors contained in the cable, the dielectric employed, and the type of protective jacket and material used to provide strength to the cable so it can be pulled through conduits without breaking. Two basic types of coaxial cable are used in local area networks. The type of cable used is based on the transmission technique employed: baseband or broadband signaling. Both cable types are much more expensive than twisted-pair wire; however, the greater frequency bandwidth of coaxial cable permits higher data rates for longer distances than you can obtain over twisted-pair wire. Normally, 50-ohm coaxial cable is used in baseband networks, while 75-ohm cable is used in broadband networks. The latter coaxial is identical to that used in cable television (CATV) applications, including the coaxial cable used in a home. Data rates on baseband networks using coaxial cable range from 50 to 100 Mbps. With broadband transmissions, data rates up to and including 400 Mbps are obtainable. A coaxial cable with a polyethylene jacket is normally used for baseband signaling. Data is transmitted from stations on the network to the baseband cable in a digital format, and the connection from each station to the cable is accomplished by the use of a simple coaxial T-connector. Because data on Coaxial cable. baseband network travels in a digital form, those signals can be easily regenerated by the use of a device known as a line driver or data regenerator. The line driver or data regenerator is a low-cost device that is constructed to look for a pulse rise, and upon detecting the occurrence of the rise, it will disregard the entire pulse and regenerate an entirely new pulse. Thus, you can install low-cost line drivers into a baseband coaxial network to extend the distance over which transmission can occur on the cable. Typically, a coaxial cable baseband system can cover an area of several miles, and may contain hundreds to thousands of stations on the network. Obtaining independent sub channels defined by separate frequencies on coaxial cable broadband transmission requires the translation of the digital signals from workstations into appropriate frequencies. This translation process is accomplished by the use of radio-frequency (RF) modems, which modulate the digital data into analog signals and then convert or demodulate received analog signals into digital signals. Because signals are transmitted at one frequency and received at a different frequency, a head-end or frequency translator is also required for broadband transmission on coaxial cable. This device is also known as a demodulator, as it simply converts the signals from one sub channel to another sub channel. Fiber-optic cable Fiber-optic cable is a transmission medium for light energy, and as such, provides a very high bandwidth, permitting data rates ranging up to billions of bits per second. The fiber-optic cable has a thin core of glass or plastic, which is surrounded by a protective shield. Several of these shielded fibers are bundled in a jacket, with a central member of aluminum or steel employed for tensile strength. Digital data represented by electrical energy must be converted into light energy for transmission on a fiber-optic cable. This is normally accomplished by a low-power laser, or through the use of a light-emitting diode and appropriate circuitry. At the receiver, light energy must be reconverted into electrical energy. Normally, a device known as a photo detector, as well as appropriate circuitry to regenerate the digital pulses and an amplifier, are used to convert the received light energy into its original digital format. The figure below provides an illustration of the cross sectio n of a single-strand fiber cable. The cladding that surrounds the core of the fiber can be considered to represent a cylindrical mirror whose job is to ensure light stays in the core as it flows along the fiber. The Kevlar fibers add strength to the cable, while the outer jacket, which is commonly colored orange, represents a polymer-based shield that protects the cable from the elements. There are two key factors that govern the manner by which light flows through a fiber-optic cable. Those factors are the diameter of the core and the light source. The first type of fiber-optic cable developed had a relatively large diameter that ranged from 50 to 140 microns, where a micron is a millionth of a meter. The original light source used to transmit information was a light-emitting diode (LED). Horizontal cross section of a single-strand fiber cable The coupling of an LED to a large-diameter optical fiber results in photons flowing along multiple paths through the optical fiber, resulting in the transmission referred to as multimode, which is also the same name used to reference the type of optical fiber. There are two types of multimode fiber, referred to as step-index and graded index. A step-index fiber has a core with a uniform refractive index, resulting in the different components of a light signal in the form of modes or rays flowing in a non-uniform manner through the optical cable. The top portion of the figure below illustrates the flow of light through a step-index, multimode fiber. In a graded-index multimode fiber, the refractive index is varied from the center to the edge of the core to minimize modal dispersion. The middle portion of the figure below illustrates the flow of light through a graded-index, multimode fiber. This type of fiber minimizes model dispersion and supports higher data rates than a step-index multimode optical fiber. A third type of optical fiber has a relatively small core diameter, typically between 7 and 12 microns (10−6 meters). This type of optical fiber permits only one path for the flow of light due to the small diameter of the core. As a result of the lack of modal dispersion, single mode supports a much higher data rate than multimode fiber. Because of the small diameter of single-mode fiber, lasers are used as the light source instead of LEDs. Both the core thickness and the cladding of an optical fiber are measured in microns. The three major core thicknesses used in optical fiber are 50, 62 and 100 microns. The associated claddings for those core diameters are 125 and 140 microns, respectively. Light flow in multimode and single-mode optical fiber. Computer networks are everywhere; from a simple two-node home setup to the vast number of computers on the internet. However, any computer network includes certain basic components, regardless of which operating system one is running. Network scope refers to the extent to which a network provides coverage. There are two major divisions of network scope; Local Area Networks and Wide Area Networks. †¢ A Local Area Network (LAN) consists of any number of computers that are linked directly together and are housed in a clearly defined geographic area, such as in a single building or campus. A LAN can only be as large as the physical limitations of the cabling you use which also depends on the cabling type. Usually the computers linked together in a LAN are workstations that can access data on computers on the same LAN, and use devices like printers that are connected to the LAN. †¢ A Wide Area Network (WAN) can span large geographic areas like countries and continents. WANs often contain two or more LANs. At least some of the connections used in WAN rely on long distance communications media such satellite links, long distance fiber optic cable, or specialized high speed telephone lines. WAN technology is essentially used to link all the computers in a multi-site or multinational enterprise in a reliable way Note that the key characteristic of a LAN or WAN is not how big it is, but rather the technologies used to connect the computers. There are other more specialized scopes used to describe networks. These include: †¢ Metropolitan Area Networks (MAN): this is a mini-wan or a giant LAN that is confined to a single municipality. A company might use a private MAN to link different offices together within the same compound. Computers on a MAN are linked using high-speed media like fiber optic or dedicated digital lines. This is the typical description of the IITA Ibadan network as will be discussed later. †¢ Storage Area Network (SAN): A specialized LAN linking several network servers that are dedicated to storing large amounts of data in a centralized secure repository. †¢ Personal Area Network (PAN): This is a connection you personally have with the technology that is around you (within your body) e.g. the way your cell phone communicates with your Bluetooth headset and your laptop. Bluetooth and infrared are currently the major types of PAN. Network Topology A network topology refers to the layout of the transmission medium and devices on a network. Topologies use either a point to point or multipoint connection scheme. A connection scheme indicates how many devices are connected to a transmission media segment or an individual cable. An example of point-to-point connection scheme is a printer or modem connected to your computer. Another is two computers connected directly to each other to use file transfer software like windows i.e. the network computer communicates with other network devices via direct cable connection between them. An example of a multi point connection scheme is a star or bus topology network. The entire physical structure of the network is called its physical topology. Star topology: This is a local area network topology where all the nodes are connected individually to a central connecting device called a hub. Signals travel from the nodes to the hub which then sends signals to other nodes on the network. A star topology network is easily scaleable – nodes can be added and removed fairly easily- and if a computer fails, none of the other nodes are affected. However if the hub fails the entire network fails. A hub does not perform any type of filtering or routing of the data. It is simply a junction that joins all the different nodes together.

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