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Osi model


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Osi model

  1. 1. OSI Reference Model And TCP/IP Reference Model ENAM 2012/13 BWIRE SEDRICK
  2. 2. Network Model  In order for a computer to send information to another computer, and for that computer to receive and understand the information, there has to exist a set of rules or standards for this communication process.  These standards ensure that varying devices and products can communicate with each other over any network. This set of standards is called Network protocols.  The Network functions are separated in the form of Layers. 2Sep-13 BBC & BIOM III
  3. 3. 3 What is Layering? r A technique to organize a networked system into a succession of logically distinct entities, such that the service provided by one entity is solely based on the service provided by the previous (lower level) entity. Application Transport Network Datalink Physical Application Transport Network Datalink Physical Network Datalink Physical Physical medium Sep-13 BBC & BIOM III
  4. 4. Layered Tasks Example  Sender, Receiver and Carrier Sep-13 4BBC & BIOM III
  5. 5. Why divide network into layers???  Reduces complexity  Provides compatibility - It standardizes network components to allow multiple- vendor development and support.  It allows different types of network hardware and software to communicate with each other.  Accelerates evolution of technology - It prevents changes in one layer from affecting the other layers, so that they can develop more quickly. 5Sep-13 BBC & BIOM III
  6. 6. ISO (International Standards Organization)  It released a set fo specifications that describe network architecture dissimilar devices.  In 1984 the ISO released set of specifications called Open Systems Interconnection (OSI) reference model.  The OSI model has become an international standard for networking. 6Sep-13 BBC & BIOM III
  7. 7. OSI functions  Mainly, it provides a way to understand how an internetwork operates.  It also serves as a guideline or framework for creating and implementing network standards, devices, and internetworking schemes.  Helps in troubleshooting network problems 7Sep-13 BBC & BIOM III
  8. 8. OSI Layers  OSI model consists of 7 layers and each layer provides some services to the layer above it.  The principles that were applied to arrive at the seven layers are as follows:  A layer should be created where a different level of abstraction (idea, concept) is needed.  Each layer should provide some well defined functions.  Each layer should work independently from other layers.  Layer boundaries should minimize data flow across those boundaries.  The right number of layers should be there. Don't put too many functions together in a layer, and not too many layers either. 8Sep-13 BBC & BIOM III
  9. 9. OSI Layers cont’d…  The three upper layers (Application Layers) define how the applications within the end stations will communicate with each other and with users.  The four lower layers (Data Flow Layers) define ways for end stations to establish connections to each other in order to exchange data. 9Sep-13 BBC & BIOM III
  10. 10. OSI Layers cont’d… 10Sep-13 BBC & BIOM III
  11. 11. Description of each layer Physical layer  It is Layer 1, the bottom layer of the OSI model. It specifies how data is processed into bits and physically transferred over medium, such as cables.  The physical layer defines the electrical, mechanical, procedural, and functional specifications for activating, maintaining, and deactivating the physical link between communicating network systems. Physical layer specifications define characteristics such as voltage levels, timing of voltage changes, physical data rates, maximum transmission distances, and physical connectors. 11Sep-13 BBC & BIOM III
  12. 12. Data Link Layer  It is Layer 2, and sends data frames from the Network layer to the Physical layer.  On the receiving end, it packages raw bits from the Physical layer into data frames.  It combines bits into bytes and bytes into frames.  The data link layer is concerned with physical (as opposed to logical) addressing, network topology, network access, error notification (acknowledgement) and retransmission, error recovery (CRC check), ordered delivery of frames, and flow control. 12Sep-13 BBC & BIOM III
  13. 13. Network Layer  Layer 3, the Network layer, is responsible for providing end-to-end delivery of Packets.  To accomplish this the network layer defines logical addresses to identify the endpoint of the network.  It also determines the route from the source to the destination computer.  It determines which path the data should take based on network conditions, priority of service, and other factors.  It also manages traffic problems on the network, such as packet switching, routing, and controlling the congestion of data.  If the destination computer cannot receive a data chunk as large as the source computer sends, the Network layer on the source compensates by breaking the large data chunk into smaller units. 13Sep-13 BBC & BIOM III
  14. 14. Network Layer  On the destination end, the Network layer reassembles the data into the original large chunk.  Internet Protocol (IP) and Internetwork Packet Exchange (IPX) are examples of network layer protocols. 14Sep-13 BBC & BIOM III
  15. 15. Transport Layer  Layer 4, the transport layer, is responsible for breaking the data into segments and establishing an end-to-end logical connection between machines.  The transport layer ensures that packets are delivered error free, in sequence, and without losses or duplications.  At the sending computer, this layer repackages messages, dividing long messages into several packets and collecting small packets together in one package. This process ensures that packets are transmitted efficiently over the network.  At the receiving computer, the transport layer opens the packets, reassembles the original messages, and, typically, sends an acknowledgement that the message was received.  If a duplicate packet arrives, this layer will recognize the duplicate and discard it. Protocols include TCP n UDP 15Sep-13 BBC & BIOM III
  16. 16. Session Layer  Layer 5, the Session layer defines how to start, control and end the conversation (called session) between two applications running on different computers.  It provides synchronization between user tasks by placing checkpoints in the data stream.  This way, if the network fails, only the data after the last checkpoint has to be retransmitted.  This layer also implements dialog control between communicating processes by regulating which side transmits, when, for how long, and so on. 16Sep-13 BBC & BIOM III
  17. 17. Presentation Layer  The main purpose of this layer is defining the data formats. It is concerned with how data is converted and formatted for data transfer.  The Presentation Layer provides a variety of coding and conversion functions that are applied to the application layer data. These functions ensure that information sent from the application layer of one system will be readable by the application layer of another system  The Presentation layer is also responsible for protocol conversion, encrypting the data, and managing data compression.  The presentation layer also manages data compression to reduce the number of bits that need to be transmitted. 17Sep-13 BBC & BIOM III
  18. 18. Application Layer  Layer 7, the topmost layer of the OSI model, is the application layer.  The Application Layer is where the user interacts with the computer  This layer relates to the services that directly support applications, such as software for file transfers, database access, and E-mail.  A message to be sent across the network enters the OSI model at this point and exits the OSI model’s application layer on the receiving computer.  Protocols at this layer identify communication partners, determine resource availability, and synchronize communication. For example, a word processing application is serviced by file transfer services at this layer.  Application layer protocols; Http, telnet, SNMP,SMTP etc 18Sep-13 BBC & BIOM III
  19. 19. Figure 2.15 Summary of layers Summary of Layers Sep-13 19BBC & BIOM III
  20. 20. TCP/IP reference model  Although the OSI reference model is universally recognized, the historical and technical open standard of the Internet is Transmission Control Protocol/Internet Protocol (TCP/IP).  The TCP/IP reference model and the TCP/IP protocol stack make data communication possible between any two computers, anywhere in the world, at nearly the speed of light.  The U.S. Department of Defense (DoD) created the TCP/IP reference model because it wanted a network that could survive any conditions, even a nuclear war. 20Sep-13 BBC & BIOM III
  21. 21. TCP/IP reference model layers The TCP/IP model has four layers:  a) Application layer,  b) Transport layer,  c) Internet layer, and  d) Host to Network layer. NB: It is important to note that some of the layers in the TCP/IP model have the same name as layers in the OSI model. Do not confuse the layers of the two models, because the application layer has different functions in each model. 21Sep-13 BBC & BIOM III
  22. 22. TCP/IP Application layer  -Combines the functions of the OSI application, presentation, and session layers.  The TCP/IP combines all application related issues into one layer, and assumes this data is properly packaged for the next layer.  Protocols involved in this layer: HTTP, FTP, SMTP, DNS, POP3, TFTP etc. 22Sep-13 BBC & BIOM III
  23. 23. TCP/IP Transport layer  This layer works almost similar to the Transport layer of OSI model.  Deals with the quality-of-service, issues of reliability, flow control, and error correction.  It has two major protocols TCP and UDP  The transmission control protocol (TCP), provides excellent and flexible ways to create reliable, well-flowing, low-error network communications. TCP is a connection-oriented protocol.  Note: Connection-oriented does not mean that a circuit exists between the communicating computers (that would be circuit switching). It does mean that Layer 4 segments travel back and forth between two hosts to acknowledge the connection exists logically for some period. 23Sep-13 BBC & BIOM III
  24. 24. TCP/IP Transport layer cont’d……  •The second protocol in this layer is User Datagram Protocol (UDP), its an unreliable, connectionless protocol for the applications that do not want TCP’s sequencing and flow control and where quick delivery is more important than accurate delivery like speech and video transmission. 24Sep-13 BBC & BIOM III
  25. 25. TCP/IP Internet(Network) layer  The purpose of the Internet layer is to send source packets from any network on the internet work and have them arrive at the destination independent of the path and networks they took to get there.  The specific protocol that governs this layer is called the Internet protocol (IP).  Best path determination and packet switching occur at this layer.  Think of it in terms of the postal system. When you mail a letter, you do not know how it gets there (there are various possible routes), but you do care that it arrives. 25Sep-13 BBC & BIOM III
  26. 26. TCP/IP Network Access layer  Provides the same functionality as the physical, the data link and network layers in the OSI model.  It includes the LAN and WAN technology details  The OSI model is proven to be very useful for understanding computer networks.  However the OSI Protocols have not become very popular. The opposite is the case for TCP/IP reference model. The model is practically non-existing, but two of its protocols TCP and IP are widely used in the networks. 26Sep-13 BBC & BIOM III
  27. 27. Associated TCP/IP Protocols & Services HTTP This protocol, the core of the World Wide Web, facilitates retrieval and transfer of hypertext (mixed media) documents. Stands for the HyperText Transfer protocol Telnet A remote terminal emulation protocol that enables clients to log on to remote hosts on the network. SNMP Used to remotely manage network devices. Stands for the Simple Network Management Protocol. DNS Provides meaningful names like for computers to replace numerical addresses like Stands for the Domain Name System. SLIP/ PPP SLIP (Serial Line Internet Protocol) and PPP (Point to Point Protocol) encapsulate the IP packets so that they can be sent over a dial up phone connection to an access provider’s modem. Sep-13 27BBC & BIOM III
  28. 28. Address Resolution Protocol (ARP)  The first system knows that the second is located somewhere on the network but does not know its exact location on the network.  The source will broadcast an ARP request to find the MAC address of the intended destinations MAC address.  The signal sent is a broadcast message and all the devices in the LAN will hear it.  Only the destination device will respond to the ARP request. Sep-13 BBC & BIOM III 28
  29. 29. Other Protocols  Simple Mail Transfer Protocol (SMTP) is a protocol for sending e-mail messages between servers. The messages can then be retrieved with an e-mail client using either Post Office Protocol (POP) or Internet Message Access Protocol (IMAP).  Post Office Protocol version 3 (POP3) is a common mail service protocol that is used by ISPs that provide Internet and e-mail service to home customers.  FTP is a general-purpose protocol that can be used to copy all types of files from one computer to another. FTP makes use of the TCP reliable transport services to establish a logical connection between the systems. FTP is one of the most heavily used protocols on the Internet. Sep-13 BBC & BIOM III 29
  30. 30. Comparing the OSI Model and TCP/IP Model Sep-13 BBC & BIOM III 30
  31. 31. Similarities of the OSI and TCP/IP models  Both have layers.  Both have application layers, though they include very different services.  Both have comparable transport and network layers.  Packet-switched, not circuit-switched, technology is assumed.  Networking professionals need to know both models. Sep-13 BBC & BIOM III 31
  32. 32. Differences of the OSI and TCP/IP models  TCP/IP combines the presentation and session layer into its application layer.  TCP/IP combines the OSI data link and physical layers into one layer.  TCP/IP appears simpler because it has fewer layers.  TCP/IP transport layer using UDP does not always guarantee reliable delivery of packets as the transport layer in the OSI model does. Sep-13 BBC & BIOM III 32
  33. 33.  TCP/IP protocols are considered to be standards around which the internet has been developed  The OSI model consists of 7 layers whereas the TCP/IP only has 4 layers. Sep-13 BBC & BIOM III 33