Curs PC si internet cap 5.6 Basic Topologies Used in Networking

5.6 Basic Topologies Used in Networking 5.6.1 Network topologies Instructor Note The purpose of this target indicator is to introduce the study of topologies. Emphasize topology as a branch of mathematics, which deals with networks of nodes and links. Pose the following problem: given n nodes, how many links are required to create a fully complete network, that is one in which every node has an independent link to every other node? Give the students problem of n = 11, for which a graphical solution is very difficult. Have students write out a chart with nodes and links columns, and see if they can deduce the pattern . The correct formula is n (n-1) /2. Two types of topologies will be important in the curriculum: physical and logical. By physical topology we mean how networking devices are actually wired together. By logical topology we mean how data flows, and how access to shared media is determined. Both types of topological diagrams are crucial for the networking professional to be able to draw, read, and interpret. The word topology can be thought of as "the study of location." Topology is a subject of study in mathematics, where maps of nodes (dots) and links (lines) often contain patterns. In this chapter, you will examine the various topologies used in networking from a mathematical perspective. Then, you will learn how a physical topology describes the plan for wiring the physical devices. Finally, you will use a logical topology to learn how information flows through a network to determine where collisions may occur. A network may have one type of physical topology , and a completely different type of logical topology. For example, Ethernet 10Base-T uses an extended-star physical topology, but acts as though it uses a logical bus topology. Token Ring uses a physical star, and a logical ring. FDDI uses a physical and a logical ring. 5.6 Basic Topologies Used in Networking 5.6.2 Linear bus network topology Instructor Note The purpose of this target indicator is to introduce the linear bus topology. A good activity is to have the students draw 10 hosts physically wired in a linear bus topology. Note that each host will need some type of "tee" connection or tap to connect to the medium. Mathematical PerspectiveThe bus topology has all of its nodes connected directly to one link, and has no other connections between nodesPhysical Perspective Each host is wired to a common wire. In this topology, the key devices are those that allow the host to join or tap into the single shared medium. One advantage of this topology is that all hosts are connected to each other, and thus, can communicate directly. One disadvantage of this topology is that a break in the cable disconnects hosts from each other.Logical Perspective A bus topology enables all networking devices to see all signals from all other devices, which can be an advantage if you want all information to go to all devices. However, it can be a disadvantage because traffic problems and collisions are common. 5.6 Basic Topologies Used in Networking 5.6.3 Ring network topology Instructor Note The purpose of this target indicator is to introduce the ring network topology. A good activity is to have the students draw 10 hosts physically wired in a ring topology. Have them note what that each host would need two NICs if ring topologies were actually wired as rings (in turns out that ring topologies often work as logical rings for information flow but are actually wired as physical stars). Mathematical PerspectiveA ring topology is a single closed ring consisting of nodes and links, with each node connected to only two adjacent nodes. Physical Perspective The topology shows all devices wired directly to each other in what is called a daisy-chain, and is similar to the manner in which a mouse on an Apple PC plugs into the keyboard and then into the PC. Logical PerspectiveIn order for information to flow, each station must pass the information to its adjacent station. 5.6 Basic Topologies Used in Networking 5.6.4 Dual ring network topology Instructor Note The purpose of this target indicator is to introduce the dual ring topology. A good activity is to have the students draw 10 hosts in a dual ring topology. Note that each host will need 4 NICs or 2 dual NICs to have connections to both rings. Mathematical Perspective A dual ring topology consists of two concentric rings, each of which is linked only to its adjacent ring neighbor. The two rings are not connected. Physical Perspective A dual ring topology is the same as a ring topology, except that there is a second, redundant ring, that connects the same devices. In other words, in order to provide reliability and flexibility in the network, each networking device is part of two independent ring topologies.Logical Perspective A dual ring topology acts like two independent rings, of which, only one at a time is used. 5.6 Basic Topologies Used in Networking 5.6.6 Extended star network topology Instructor Note The purpose of this target indicator is to introduce the extended star topology. A good activity is to have the students draw 10 hosts in an extended star physical topology.Note that some kind of special connection device will be required at the middle (or hub) of the star. Mathematical PerspectiveAn extended star topology repeats a star topology, except that each node that links to the center node is, also, the center of another star. Physical Perspective An extended star topology has a core star topology, with each of the end nodes of the core topology acting as the center of its own star topology. The advantage of this is that it keeps wiring runs shorter, and limits the number of devices that need to interconnect to any one central node. Logical Perspective An extended star topology is very hierarchical, and information is encouraged to stay local. This is how the phone system is currently structured. 5.6 Basic Topologies Used in Networking 5.6.7 Tree network topology Instructor Note The purpose of this target indicator is to introduce the tree topology. A good activity is to have the students draw 10 hosts in a star physical topology. Note that some kind of special connection device will be required every time the tree branches. Mathematical PerspectiveThe tree topology is similar to the extended star topology, the primary difference is that it does not use one central node. Instead, it uses a trunk node from which it, then, branches to other nodes. There are two types of tree topologies: the binary tree (each node splits into two links); and the backbone tree (a backbone trunk has branch nodes with links hanging from it). Physical PerspectiveThe trunk is a wire that has several layers of branches.Logical PerspectiveThe flow of information is hierarchical. 5.6 Basic Topologies Used in Networking 5.6.8 Irregular network topology Instructor Note The purpose of this target indicator is to introduce the irregular topology. A good activity is to have the students draw 10 hosts in an irregular physical topology. Note that multiple NICs would be required for each device. Mathematical PerspectiveIn the irregular network topology there is no obvious pattern to the links and nodes. Physical PerspectiveThe wiring is inconsistent; the nodes have varying numbers of wires leading from them. This is how networks that are in the early stages of construction, or poorly planned, are often wired.Logical PerspectiveThere is no obvious pattern to the links and nodes. 5.6 Basic Topologies Used in Networking 5.6.9 Complete (mesh) network topology Instructor Note The purpose of this target indicator is to introduce the complete (mesh) topology. A good activity is to have the students draw 10 hosts in a complete (mesh) topology. Note that a number of NICs required becomes huge as you move past a few hosts being connected. Mathematical PerspectiveIn a complete, or mesh topology, every node is linked directly to every other node. Physical Perspective This wiring has very distinct advantages and disadvantages. The advantages are that, because every node is physically connected to every other node (creating a redundant connection), should any link fail to function, information can flow through any number of other links to reach its destination. Also, this topology allows information to flow along many paths on its way through the network. The primary physical disadvantage is that for anything more than a small number of nodes, the amount of media for the links, and the amount of connections to the links becomes overwhelming. Logical Perspective The behavior of a complete, or mesh topology depends greatly on the devices used. 5.6 Basic Topologies Used in Networking 5.6.10 Cellular network topology Instructor Note The purpose of this target indicator is to introduce the cellular topology. A good activity is to have the students draw 10 nodes in a cellular topology. Mathematical PerspectiveThe cellular topology consists of circular or hexagonal areas, each of which has an individual node at its center. Physical PerspectiveThe cellular topology is a geographic area that is divided into regions (cells) for the purposes of wireless technology a technology that becomes increasingly more important each day. There are no physical links in a cellular topology, only electromagnetic waves. Sometimes the receiving nodes move (e.g. car cell phone), and sometimes the sending nodes move (e.g. satellite communication links).The obvious advantage of a cellular (wireless) topology is that there are no tangible media other than the earth's atmosphere or the vacuum of off-planet space (and satellites). The disadvantages are that signals are present everywhere in a cell and, thus, are susceptible to disruptions (man-made and environmental) and to security violations (i.e. electronic monitoring and theft of service).Logical PerspectiveCellular technologies communicate with each other directly (though distance limitations and interference sometimes make it extremely difficult), or communicate only with their adjacent cells, which is extremely inefficient. As a rule, cellular-based topologies are integrated with other topologies, whether they use the atmosphere or satellites. Summary In this chapter, you learned that the function of the physical layer is to transmit data. In addition you learned that the following types of networking media may be used to connect computers: Coaxial cable consists of a hollow outer cylindrical conductor that surrounds a single inner wire conductor. UTP cable is a four-pair wire medium used in a variety of networks. STP cable combines the techniques of shielding, cancellation, and twisting of wires. Fiber-optic cable is a networking medium capable of conducting modulated light transmissions. This chapter discussed various criteria, such as rate of data transfer and expense, help determine which types of networking media should be used. You learned that TIA/EIA-568-A and TIA/EIA-569 are the most widely used standards for technical performance of networking media.Additionally, this chapter described how bits propagating at the same time on the same network results in a collision. Lastly, you learned that a network may have one type of physical topology, and a completely different type of logical topology. In the next chapter, you will about LAN media and the IEEE model and how the data link layer provides reliable transit of data across a physical link by using the Media Access Control (MAC) addresses.