lesson-12-information systems
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© Copy Right: Rai University 17C.102 43 NETWORKING LESSON 12 DESCRIPTION OF NETWORK TOPOLOGIES Introduction The physical layout of cabling in LANs has seen significant change in the last decade. Basic topologies still exist to support the individual approach that the various network standards offer. However, these basic topologies frequently exist in a physical structure that looks somewhat different from the logical structure that the standard indicates. Those changes have largely occurred as LANs have grown larger and the need to find an efficient method of managing the installation has become a necessity. In this chapter we will discuss the basic layouts and the structured layouts that are now being imple-mented. Objectives On completion of the Lesson, students should be able to: Understand the technology involved in network topologies. Understand the different topologies used in network architec- ture. Topologies In connecting devices together to form a network Fe must consider the structure or layout of the cabling. The purpose of a LAN is to bring order and management control to what could otherwise become a difficult problem. Consider the problem of adding another node or PC to either of the layouts. It is fairly Clear which of the two layouts will give us the most difficulty in establishing an additional connection to all the other existing stations. Cable structure is very important, and its logical layout is a key element in supporting the different LAN approaches such as Ethernet and token ring implementations. The physical installation of the cable which-represents the logical layout has been changing quite significantly over recent years. A structured cabling approach is now commonly implemented which may not resemble the logical layout we are fundamentally trying to create. Since it is important that the fundamental structures are understood, we’ll use those as a starting point in describing the topologies. Bus Topology A bus topology consists of a ‘single’ communications channel. Each connected device is attached to the medium at an interface point and has its own unique hardware address. In fact, the hardware address is provided on the network interface card, whether the system is an Ethernet network, or some other network implementation. Data transfers between the interfaces or nodes take place using these hardware addresses. The bus topology connects workstations using a single cable. Each workstation is connected to the next workstation in a point-to-point fashion. All workstations connect to the same cable. Figure : Bus Topology If one workstation goes faulty, all workstations are affected. Workstations share the same cable for the sending and receiving of information. The cabling costs of bus systems is the least of all the different topology’s. Each end of the cable is terminated using a special terminator. The common implementation of this topology is Ethernet. A message transmitted by one workstation is heard by all the other workstations. Ring Topology The ring topology connects workstations in a closed loop. Each terminal is connected to TWO other terminals (the next and the previous), with the last terminal being connected to the first. Data is transmitted around the ring in one direction only, each station passing on the data to the next station till it reaches its destination. In contrast to the bus topology, it is normal in a ring configura- tion for the data transmission to be unidirectional. Of course, there are exceptions when dual cable rings are operating, but the simple arrangement is unidirectional. Data transmissions are received by each station’s interface as the data passes through the interface connection. Each network interface connection has its own hardware address for identification. As in the bus topol- ogy, each interface only copies the data from the network and passes it to the device it connects to the network when it recognises the packet’s destination address as its own. When a station is not the destination address, the interface normally acts as a simple repeater and regenerates the signal to its ‘full strength’. There are several local area network standards that are based on a ring topology. Dominant in the UK during the 1980s was the Cambridge Ring implementation, but this has largely disap- peared. Currently, both token ring networks and larger networks such as fibre distributed data interface (FDDI) networks are based on a ring topology.
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Transcript of lesson-12-information systems
© Copy Right: Rai University17C.102 43
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LESSON 12DESCRIPTION OF NETWORK TOPOLOGIES
IntroductionThe physical layout of cabling in LANs has seen significantchange in the last decade. Basic topologies still exist to supportthe individual approach that the various network standardsoffer. However, these basic topologies frequently exist in aphysical structure that looks somewhat different from thelogical structure that the standard indicates. Those changes havelargely occurred as LANs have grown larger and the need to findan efficient method of managing the installation has become anecessity. In this chapter we will discuss the basic layouts andthe structured layouts that are now being imple-mented.
ObjectivesOn completion of the Lesson, students should be able to:Understand the technology involved in network topologies.Understand the different topologies used in network architec-ture.
TopologiesIn connecting devices together to form a network Fe mustconsider the structure or layout of the cabling. The purpose of aLAN is to bring order and management control to what couldotherwise become a difficult problem. Consider the problem ofadding another node or PC to either of the layouts. It is fairlyClear which of the two layouts will give us the most difficulty inestablishing an additional connection to all the other existingstations.Cable structure is very important, and its logical layout is a keyelement in supporting the different LAN approaches such asEthernet and token ring implementations. The physicalinstallation of the cable which-represents the logical layout hasbeen changing quite significantly over recent years. A structuredcabling approach is now commonly implemented which maynot resemble the logical layout we are fundamentally trying tocreate. Since it is important that the fundamental structures areunderstood, we’ll use those as a starting point in describing thetopologies.
Bus TopologyA bus topology consists of a ‘single’ communications channel.Each connected device is attached to the medium at an interfacepoint and has its own unique hardware address. In fact, thehardware address is provided on the network interface card,whether the system is an Ethernet network, or some othernetwork implementation. Data transfers between the interfacesor nodes take place using these hardware addresses.The bus topology connects workstations using a single cable.Each workstation is connected to the next workstation in apoint-to-point fashion. All workstations connect to the samecable.
Figure : Bus TopologyIf one workstation goes faulty, all workstations are affected.Workstations share the same cable for the sending and receivingof information. The cabling costs of bus systems is the least ofall the different topology’s. Each end of the cable is terminatedusing a special terminator.The common implementation of this topology is Ethernet. Amessage transmitted by one workstation is heard by all theother workstations.
Ring TopologyThe ring topology connects workstations in a closed loop. Eachterminal is connected to TWO other terminals (the next and theprevious), with the last terminal being connected to the first.Data is transmitted around the ring in one direction only, eachstation passing on the data to the next station till it reaches itsdestination.In contrast to the bus topology, it is normal in a ring configura-tion for the data transmission to be unidirectional. Of course,there are exceptions when dual cable rings are operating, but thesimple arrangement is unidirectional. Data transmissions arereceived by each station’s interface as the data passes through theinterface connection. Each network interface connection has itsown hardware address for identification. As in the bus topol-ogy, each interface only copies the data from the network andpasses it to the device it connects to the network when itrecognises the packet’s destination address as its own. When astation is not the destination address, the interface normally actsas a simple repeater and regenerates the signal to its ‘fullstrength’.There are several local area network standards that are based on aring topology. Dominant in the UK during the 1980s was theCambridge Ring implementation, but this has largely disap-peared. Currently, both token ring networks and larger networkssuch as fibre distributed data interface (FDDI) networks arebased on a ring topology.
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Figure : Ring TopologyThere is one important concept, which applies to most ringimplementations. Each inter-face station allows data transmis-sions to pass through the interface with only a few bits delay.So, for example, in a data packet structure of, say, 64 bytes, thestart of the data packet may well have circulated around the ringand arrived back at the originating station before the end of thepacket has left it. In this mode, there is only one circulating datapacket on the ring.This will not be true in all ring based implementations, and verylarge rings may support one or more complete packets incirculation. This area is quite complex, with the FDDI topologyimposing different criteria from the token ring topology.However, for the ubiquitous token ring topology, the conceptthat any station is only ‘holding’ a few of the circulating bits atany instant is an important one to recognise. The token ringtopology is so named because a station cannot talk (or transmit)until it notionally holds a token, which is circulating around thering.
Star TopologyThe star topology uses a central hub through which all compo-nents are connected. In a computer network, the central hub isthe host computer, and at the end of each connection is aterminal.
Figure : Star Topology
A star network uses a significant amount of cable (each terminalis wired back to the central hub, even if two terminals are sideby side several hundred meters away from the host). All routingdecisions are made by the central hub, and all other worksta-tions can be simple.An advantage of the star topology is failure in one of theterminals does not affect any other terminal, however, failure ofthe central hub affects all terminals.This type of topology is frequently used to connect terminals toa large time-sharing host computer.
Mesh TopologyThe mesh topology connects all computers to each other. Thecable requirements are high, but there are redundant paths builtin. Any failure of one computer allows all others to continue, asthey have alternative paths to other computers.
Fig: Mesh Topology Mesh topology’s are used in critical connection of hostcomputers (typically telephone exchanges). Alternate paths alloweach computer to balance the load to other computer systems inthe network by using more than one of the connection pathsavailable.
