E5124 Data Communication5.       Local Area Network (LAN)Computer networking is system consists of computer device and the...
E5124 Data Communicationtraffic for the same service. For example, a large Internet Web site is often made up of several W...
E5124 Data Communication     Fig 5.2 LAN connection with various medium like gateway, router, hub, switch, bridge, repeate...
E5124 Data Communication                                        Fig 5.3 Various topoloiesMore complex networks can be buil...
E5124 Data Communication         Fig 5.4 Bus Topology                                   Fig 5.5 Ring Topology5.1.2 Ring To...
E5124 Data Communication5.1.4 Mesh TopologyMesh topologies involve the concept of routes. Unlike each of the previous topo...
E5124 Data CommunicationEach device on a baseband network transmits bidirectional, and some can transmit and receive at th...
E5124 Data Communication5.3      Carrier Sense Multiple Access (CSMA)CSMA is a network access method used on shared networ...
E5124 Data Communication         Other computers recognize the collision and re-enforce it by talking.         Then all co...
E5124 Data Communication5.5      Differences between LAN, WAN and MANLAN         A group of computers that share a common ...
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Chapter 5

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Chapter 5

  1. 1. E5124 Data Communication5. Local Area Network (LAN)Computer networking is system consists of computer device and the others that work together to makea networking. Fig 5.1 Clients and Servers in a LANFigure 5.1, illustration shows one server for each type of service on a LAN. In practice, several functionscan be combined in one machine and, for large volumes, multiple machines can be used to balance the 1Sargunan Ainal (JKE)
  2. 2. E5124 Data Communicationtraffic for the same service. For example, a large Internet Web site is often made up of several Webservers.A Local Area Network (LAN) is a network that is confined to a relatively small area. It is generally limitedto a geographic area such as a writing lab, school, or building. Computers connected to a network are broadly categorized as servers or workstations. Servers are generally not used by humans directly, but rather run continuously to provide "services" to the other computers (and their human users) on the network. Services provided can include printing and faxing, software hosting, file storage and sharing, messaging, data storage and retrieval, complete access control (security) for the networks resources, and many others. Workstations are called such because they typically do have a human user which interacts with the network through them. Workstations were traditionally considered a desktop, consisting of a computer, keyboard, display, and mouse, or a laptop, with with integrated keyboard, display, and touchpad. With the advent of the tablet computer, and the touch screen devices such as iPad and iPhone, our definition of workstation is quickly evolving to include those devices, because of their ability to interact with the network and utilize network services.Servers tend to be more powerful than workstations, although configurations are guided by needs. Forexample, a group of servers might be located in a secure area, away from humans, and only accessedthrough the network. In such cases, it would be common for the servers to operate without a dedicateddisplay or keyboard. However, the size and speed of the servers processor(s), hard drive, and mainmemory might add dramatically to the cost of the system. On the other hand, a workstation might notneed as much storage or working memory, but might require an expensive display to accommodate theneeds of its user. Every computer on a network should be appropriately configured for its use.On a single LAN, computers and servers may be connected by cables or wirelessly. Wireless access to awired network is made possible by wireless access points (WAPs). These WAP devices provide a bridgebetween computers and networks. A typical WAP might have the theoretical capacity to connecthundreds or even thousands of wireless users to a network, although practical capacity might be farless. Nearly always servers will be connected by cables to the network, because the cable connectionsremain the fastest. Workstations which are stationary (desktops) are also usually connected by a cableto the network, although the cost of wireless adapters has dropped to the point that, when installingworkstations in an existing facility with inadequate wiring, it can be easier and less expensive to usewireless for a desktop.5.1 Topology in Network DesignThink of a topology as a networks virtual shape or structure. This shape does not necessarily correspondto the actual physical layout of the devices on the network. For example, the computers on a home LANmay be arranged in a circle in a family room, but it would be highly unlikely to find a ring topology there. 2Sargunan Ainal (JKE)
  3. 3. E5124 Data Communication Fig 5.2 LAN connection with various medium like gateway, router, hub, switch, bridge, repeaterNetwork topologies are categorized into the following basic types: Bus Ring Star Tree MeshThe physical topology of a network is the actual geometric layout of workstations. There are severalcommon physical topologies, as described below and as shown in Fig 5.3 3Sargunan Ainal (JKE)
  4. 4. E5124 Data Communication Fig 5.3 Various topoloiesMore complex networks can be built as hybrids of two or more of the above basic topologies.5.1.1 Bus TopologyBus networks (not to be confused with the system bus of a computer) use a common backbone toconnect all devices. A single cable, the backbone functions as a shared communication medium thatdevices attach or tap into with an interface connector. A device wanting to communicate with anotherdevice on the network sends a broadcast message onto the wire that all other devices see, but only theintended recipient actually accepts and processes the message.Ethernet bus topologies are relatively easy to install and dont require much cabling compared to thealternatives. 10Base-2 ("ThinNet") and 10Base-5 ("ThickNet") both were popular Ethernet cablingoptions many years ago for bus topologies. However, bus networks work best with a limited number ofdevices. If more than a few dozen computers are added to a network bus, performance problems willlikely result. In addition, if the backbone cable fails, the entire network effectively becomes unusable,refer Fig 5.4 4Sargunan Ainal (JKE)
  5. 5. E5124 Data Communication Fig 5.4 Bus Topology Fig 5.5 Ring Topology5.1.2 Ring TopologyIn a ring network, every device has exactly two neighbors for communication purposes. All messagestravel through a ring in the same direction (either "clockwise" or "counterclockwise"). A failure in anycable or device breaks the loop and can take down the entire network.To implement a ring network, one typically uses FDDI, SONET, or Token Ring technology. Ring topologiesare found in some office buildings or school campuses.5.1.3. Star TopologyMany home networks use the star topology. A star network features a central connection point called a"hub" that may be a hub, switch or router. Devices typically connect to the hub with Unshielded TwistedPair (UTP) Ethernet.Compared to the bus topology, a star network generally requires more cable, but a failure in any starnetwork cable will only take down one computers network access and not the entire LAN. (If the hubfails, however, the entire network also fails.) Fig 5.6 Star Topology Fig 5.7 Mesh Topology 5Sargunan Ainal (JKE)
  6. 6. E5124 Data Communication5.1.4 Mesh TopologyMesh topologies involve the concept of routes. Unlike each of the previous topologies, messages senton a mesh network can take any of several possible paths from source to destination. (Recall that evenin a ring, although two cable paths exist, messages can only travel in one direction.) Some WANs, mostnotably the Internet, employ mesh routing.A mesh network in which every device connects to every other is called a full mesh. As shown in Fig 5.7,partial mesh networks also exist in which some devices connect only indirectly to others.5.1.5 Tree TopologyTree topologies integrate multiple star topologies together onto a bus. In its simplest form, only hubdevices connect directly to the tree bus, and each hub functions as the "root" of a tree of devices. Thisbus/star hybrid approach supports future expandability of the network much better than a bus (limitedin the number of devices due to the broadcast traffic it generates) or a star (limited by the number ofhub connection points) alone. Fig 5.8 Tree Topology5.2 Signal TransmissionTwo techniques can be used to transmit the encoded signals over cable baseband and broadbandtransmission.5.2.1 Baseband TransmissionBaseband systems use digital signaling over a single frequency. Signals flow in the form of discretepulses of electricity or light. With baseband transmission, the entire communication channel capacity isused to transmit a single data signal. The digital signal uses the complete bandwidth of the cable, whichconstitutes a single channel. A cables total bandwidth is the difference between the highest and lowestfrequencies that are carried over that cable. 6Sargunan Ainal (JKE)
  7. 7. E5124 Data CommunicationEach device on a baseband network transmits bidirectional, and some can transmit and receive at thesame time. Figure 5.9: Baseband transmission showing bidirectional digital waveAs the signal travels along the network cable, it gradually decreases in strength and can becomedistorted. If the cable length is too long, the result is a signal that is weak or distorted. The receivedsignal may be unrecognizable or misinterpreted. As a safeguard, baseband systems sometimes userepeaters to receive an incoming signal and retransmit it at its original strength and definition toincrease the practical length of a cable.5.2.2 Broadband TransmissionBroadband systems use analog signaling and a range of frequencies. With analog transmission, thesignals are continuous and non-discrete. Signals flow across the physical medium in the form ofelectromagnetic or optical waves. With broadband transmission, signal flow is unidirectional. Figure 5.10: Broadband transmission showing unidirectional analog waveIf sufficient total bandwidth is available, multiple analog transmission systems such as cable televisionand network transmissions can be supported simultaneously on the same cable. Each transmissionsystem is allocated a part of the total bandwidth. All devices associated with a given transmissionsystem, such as all computers using a LAN cable, must then be tuned so that they use only thefrequencies that are within the allocated range. While baseband systems use repeaters, broadbandsystems use amplifiers to regenerate analog signals at their original strength. Because of broadbandtransmission signal flow is unidirectional, there must be two paths for data flow in order for a signal toreach all devices. There are two common ways to do this: Mid-split broadband configuration divides the bandwidth into two channels, each using a different frequency or range of frequencies. One channel is used to transmit signals, the other to receive signals. In dual-cable broadband configuration, each device is attached to two cables. One cable is used to send and the other is used to receive. 7Sargunan Ainal (JKE)
  8. 8. E5124 Data Communication5.3 Carrier Sense Multiple Access (CSMA)CSMA is a network access method used on shared network topologies such as Ethernet to control accessto the network. Devices attached to the network cable listen (carrier sense) before transmitting. If thechannel is in use, devices wait before transmitting. MA (multiple access) indicates that many devices canconnect to and share the same network. All devices have equal access to use the network when it isclear. Even though devices attempt to sense whether the network is in use, there is a good chance thattwo stations will attempt to access it at the same time. On large networks, the transmission timebetween one end of the cable and another is enough that one station may access the cable even thoughanother has already just accessed it. There are two methods for avoiding these so-called collisions, listedhere:CSMA/CD (carrier sense multiple access/collision detection)CD (collision detection) defines what happens when two devices sense a clear channel, then attempt totransmit at the same time. A collision occurs, and both devices stop transmission, wait for a randomamount of time, then retransmit. This is the technique used to access the 802.3 Ethernet networkchannel. This method handles collisions as they occur, but if the bus is constantly busy, collisions canoccur so often that performance drops drastically. It is estimated that network traffic must be less than40 percent of the bus capacity for the network to operate efficiently. If distances are long, time lagsoccur that may result in inappropriate carrier sensing, and hence collisions. Fig 5.11 CSMA/CD medium accessBase on Fig 5.11, Here are the rules that make CSMA/CD work. All computers stay quiet if another computer is talking. Any computer that wants to talk must listen to see if the network is silent, they must wait a predetermined time based on the time it takes for a message to travel the length of the network. If after waiting, if the network is silent, then a computer may talk. If two computers wait and both talk together, a collision is said to occur. 8Sargunan Ainal (JKE)
  9. 9. E5124 Data Communication Other computers recognize the collision and re-enforce it by talking. Then all computers stop talking. Each computer picks a random number and counts backward to zero. Hopefully one of the two computers that wanted to talk finishes first and begins talking causing the other computers to wait.CSMA/CA (carrier sense multiple access/collision avoidance).In CA (collision avoidance), collisions are avoided because each node signals its intent to transmit beforeactually doing so. This method is not popular because it requires excessive overhead that reducesperformance.5.4 Token PassingToken passing is an orderly access method and is shown in Fig5.12. Each workstation passes on theopportunity to transmit to its closest neighbor until a station is found with a message to send. Thispermission to transmit is called a token. When a workstation with data to send is handed a token, partof the token is changed, indicating it is carrying a message, and then data is transmitted with the token.The token is then passed around the network, and every station checks whether the message isintended for it. The receiving station copies the message from the token, but then passes the unchangedtoken along the network. When the transmitting station receives the same token, it knows the messagehas been passed around the network. The transmitting station erases the message and puts the emptytoken back into circulation on the network. The amount of information that maybe transmitted duringpossession of the token is limited so that all workstations can share the cable equally. Network PC Server 1 Token is here PC Nodes PC 5 Nodes PC 2 PC 5 data to send has data to 2 to node PC1 send to node PC4 PC PC 4 3 Fig 5.12 A ring network using the token passing access method 9Sargunan Ainal (JKE)
  10. 10. E5124 Data Communication5.5 Differences between LAN, WAN and MANLAN A group of computers that share a common connection and are usually in a small area or even in the same building. For example an office or home network. They are usually connected by Ethernet cables and have high speed connections. If it was a wireless setup it would be called a WLAN, which would have a lower connection speed.MAN (Metropolitan Area Network) This is a larger network that connects computer users in a particular geographic area or region. For example a large university may have a network so large that it may be classified as a MAN. The MAN network usually exist to provide connectivity to local ISPs, cable tv, or large corporations. It is far larger than a LAN and smaller than a WAN. Also large cities like London and Sydney, Australia have metropolitan area networks.WAN (Wide Area Network) This is the largest network and can interconnect networks throughout the world and is not restricted to a geographical location. The Internet is an example of a worldwide public WAN. Most WANs exist to connect LANs that are not in the same geographical area. This technology is high speed and very expensive to setup. Fig 5.13 View of LAN, MAN and WAN by area coverage 10Sargunan Ainal (JKE)

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