Unit2.2
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  • 1. Computer Networks
  • 2. Computer Network A computer network is defined as the interconnection of 2 or more independent computers or/and peripherals.
  • 3. Network Source Destination
  • 4. Need of Networks – Communicate and collaborate – Share information – Share resources – Sharing computer files and disk space – Sharing high-quality printers – Access to common fax machines – Access to common modems – Multiple access to the Internet
  • 5. Classification of Networks – Local Area Networks (LANs) - a computer network covering a small geographic area, like a home, office, or group of buildings. Typically within 5-mile radius. – Metropolitan Area Networks (MANs)- are large computer networks usually spanning a city. (within 30 miles) – Wide-Area Networks (WANs) - any network whose communications links cross metropolitan, regional, or national boundaries.
  • 6. Network Topology The way in which the computers are interconnected together is known as TOPOLOGY.
  • 7. Types of topologies • Physical topology • Logical topology
  • 8. Types of physical topologies • Bus/Linear • Star • Ring • Tree • Mesh
  • 9. Linear or bus topology • Consists of a main cable, known as backbone cable, with a terminator at each end . • All nodes (file server, workstations, and peripherals) are connected to the cable. • Ethernet and LocalTalk networks use bus topology.
  • 10. Advantages of Bus Topology • Easy to connect a computer or peripheral to a linear bus. • Requires less cable length. • Easy to extend. • If one node of the N/W is faulty, the N/W can still remain working.
  • 11. Disadvantages of Bus Topology • Entire network shuts down if there is a break in the main cable. • Terminators are required at both ends of the backbone cable. • Difficult to identify the problem if the entire network shuts down. • Not meant to be used as a stand-alone solution in a large building.
  • 12. Star topology • A star topology is designed with each node (file server, workstations, and peripherals) connected directly to a central network hub. • Data on a star network passes through the hub before continuing to its destination. • The hub manages and controls all functions of the network. • It also acts as a repeater for the data flow.
  • 13. Advantages of Star Topology • Easy to install. • No disruptions to the network other than connecting or removing devices. • Easy to detect faults and to remove parts.
  • 14. Disadvantages of Star Topology • Requires more cable length than a bus topology. • If the hub fails, nodes attached are disabled. • More expensive than bus topology because of the cost of the hub.
  • 15. Tree Topology • A tree topology combines characteristics of bus and star topologies. • It consists of groups of star-configured workstations connected to a bus backbone cable. • Tree topologies allow for the expansion of an existing network.
  • 16. Advantages of a Tree Topology • Point-to-point wiring for individual segments. • Supported by several hardware and software venders.
  • 17. Disadvantages of Tree Topology • Overall length of each segment is limited by the type of cabling used. • If the backbone line breaks, the entire segment goes down. • More difficult to configure than other topologies.
  • 18. Ring topology • Is a type of computer network configuration where each network computer and device are connect to each other forming A large circle. • Data is divided into packets when transmitted. • Packet is sent around the ring until it reaches its final destination.
  • 19. Advantages of ring topology • Requires lesser amount of cable and there are not much of installation problems • All stations have equal access
  • 20. Disadvantages of ring topology • Failure of one computer may impact others • Data transfer is slow
  • 21. Mesh topology • It requires that every terminal should be attached to each other. • All the computers must have adequate number of interfaces for the connections to be made. • Because of this requirement the installations is somewhat difficult. • The length of cable required is quite higher as compared to other topologies.
  • 22. Advantages of mesh topology • Ease of troubleshooting. • Data transfer is faster.
  • 23. Disadvantages of mesh topology • uses a lot of cabling. • Complex • Most expensive topology
  • 24. Considerations When Choosing a Topology • Money. A linear bus network may be the least expensive way to install a network; you do not have to purchase concentrators. • Length of cable needed. The linear bus network uses shorter lengths of cable. • Future growth. With a star topology, expanding a network is easily done by adding another concentrator. • Cable type. The most common cable in schools is unshielded twisted pair, which is most often used with star topologies.
  • 25. Transmission Technology Two types of transmission technologies are there: 2. Broadcast Network Single communication channel is shared by all the machines in the network.
  • 26. Transmission Technology 1. Point to Point Network Data is routed directly from source machine to destination machine directly. Dedicated link
  • 27. Routing • When data is to be transmitted between two remote machines using intermediate machines, certain routing techniques have to be applied. • The intermediate machines may be: 1. Gateway 2. Hub 3. Router 4. Repeater 5. Bridge
  • 28. Gateway • A gateway is a network point that acts as an entrance to another network. • Acts as a bridge between two networks so that data can be transferred between a number of computers. • In enterprises, the gateway is the computer that routes the traffic from a workstation to the outside network that is serving the Web pages. • In homes, the gateway is the ISP that connects the user to the internet. • For example, when you send an e-mail to a friend or when you log in to a Web site, there is a gateway that allows the connection take place.
  • 29. Hub • A common connection point for devices in a network. • Hubs are commonly used to connect segments of a LAN. • A hub contains multiple ports. • When a packet arrives at one port, it is copied to the other ports so that all segments of the LAN can see all packets
  • 30. Router • A router is a device that forwards data packets along networks. • A router is connected to at least two networks. • Routers are located at gateways. • Routers use headers and forwarding tables to determine the best path for forwarding the packets. • They use protocols to communicate with each other and configure the best route between any two hosts.
  • 31. Switch • In networks, a device that filters and forwards packets between LAN segments. • LANs that use switches to join segments are called switched LANs.
  • 32. Repeater • Repeaters are used in transmission systems to regenerate signals distorted by transmission loss. • Analog repeaters frequently can only amplify the signal while digital repeaters can reconstruct a signal to near its original quality.
  • 33. Bridge • A device that connects two local-area networks (LANs), or two segments of the same LAN that use the same protocol is known as bridge.
  • 34. More Concepts… • Network protocols. • Layering. • Network/protocol architecture.
  • 35. Network Protocols • A communication protocol is a set of rules that specify the format and meaning of messages exchanged between computers across a network. • A set of related protocols that are designed for compatibility are called protocol suite.
  • 36. Human and Computer Protocols Human Protocol Computer Protocol Web client Hi open connection Hi OK Web server Got the time? send me data 2:00 <data> time
  • 37. Protocol Architecture • Task of communication broken up into modules • For example file transfer could use three modules – File transfer application – Communication service module – Network access module
  • 38. Layers • Layers are the different components that need to be designed/implemented when designing/implementing networks. • Each layer responsible for a set of functions. • Top layer relies on services provided by bottom layer. • Layer makes it service available to higher layer through an interface.
  • 39. Layering • Building complex systems is hard! – Approach: “Divide and conquer”. – Split job into smaller jobs, or layers. • Analogy to other fields. – Building a house: digging, foundation, framing, etc. – Car assembly line… • Basic idea: each step dependent on the previous step but does not need to be aware of how the previous step was done.
  • 40. Analogy: Air Travel • The problem: air travel. • Decomposed into series of steps: Arrival at airport Departure from airport Check-in Baggage claim Boarding Deplane Takeoff Landing Traveling
  • 41. More on the air travel analogy… Arrival Departure Departing airport Arriving airport Check-in Baggage claim Boarding Deplane Takeoff Landing intermediate air traffic sites Airplane routing Airplane routing Traveling
  • 42. A Three Layer Model • Application Layer • Transport Layer • Network Access Layer
  • 43. Network Access Layer • Exchange of data between the computer and the network • Sending computer provides address of destination • May invoke levels of service • Dependent on type of network used (LAN, packet switched etc.)
  • 44. Transport Layer • Reliable data exchange • Independent of network being used • Independent of application
  • 45. Application Layer • Support for different user applications • e.g. e-mail, file transfer
  • 46. Layered Protocol Design • Layering model is a solution to the problem of complexity in network protocols • The model divides the network protocols into layers, each of which solves part of the network communication problem – Each layer has its own protocol! • Each layer implements a service to the layer above – Relying on services provided by the layers below.
  • 47. Network/Protocol Architecture • Set of layers, what their functions are, the services each of them provide, and the interfaces between them. • A.k.a, protocol architecture or protocol stack. • Examples: – ISO-OSI 7 layer architecture. – TCP-IP architecture (Internet).
  • 48. Protocol Data Units (PDU) • At each layer, protocols are used to communicate. • At the source, control information is added to user data at each layer, a.k.a., encapsulation. • At the receiver, control information is stripped off at each layer going up the stack, a.k.a., decapsulation.
  • 49. Example 1: ISO OSI Architecture • ISO: International Standards Organization • OSI: Open Systems Interconnection. Application Presentation Session Transport Network Data link Physical
  • 50. ISO Model • Layer 7: Application – Application-specific protocols (e.g. ftp, http, smtp) • Layer 6: Session − establish sessions between machines. • Layer 5: Presentation − performs proper co ordination between machines • Layer 4: Transport – Delivery of data between computers (end-to-end). • Layer 3: Network – Data routing across a network. • Layer 2: Data Link – Reliable transmission over physical medium. • Layer 1: Physical - Transmission of bits between two nodes.
  • 51. Example 2: TCP/IP Architecture • Model employed by the Internet. TCP/IP Application ISO OSI Application Presentation Session Transport Transport Internet Network Network Access Data link Physical Physical
  • 52. TCP/IP Protocol Architecture
  • 53. Routing Technologies • Computer Networks may use the following routing technologies: 2. Circuit Switching 3. Packet Switching 4. Message Switching
  • 54. Circuit Switching • A physical circuit is established between two machines. • Once the connection is established, the data transfer takes place and then the connection is released. • The data transfer rate is high but error prone.
  • 55. Packet Switching • Data is divided into small, fixed size packets. • Each packet contains the address of destination. • All the intermediate machines inspect the address and route it to the right machine. • If any packet is missing, the destination machine can request the source machine to retransmit it.
  • 56. Message Switching • Message as a whole is transmitted. • Each message contains the address of the destination. • Message switching N/W is “Store and Forward” N/W. • Once the message arrives at the intermediate machine, it is stored in it completely and transmitted when the line is free.