Published on

  • Be the first to comment

No Downloads
Total views
On SlideShare
From Embeds
Number of Embeds
Embeds 0
No embeds

No notes for slide


  1. 1. Open System Interconnection (OSI) CMC Limited
  2. 2. Objectives • Understand OSI model • Explain the principle applied to arrive at seven layers. • Explain the advantage of dividing network into layers. • Explain the characteristics of the OSI layers. • Understand how communication done between systems using the OSI model. • Explain different networking devices used in the OSI layer. • Understand TCP/IP stack. • Explain connectionless and connection oriented services. • Understand Windowing CMC Limited
  3. 3. Introduction OSI reference model: • Describes how information from a software application in one computer moves through a network medium to a software application in anther computer. • Conceptual model composed of seven layers, each specifying particular network functions. Each layer is reasonably self-contained so that the functions assigned to each layer can be implemented independently. • Developed by the International Standards Organization (ISO) as a first step toward international standardization of the protocols used in various layers in 1983. CMC Limited
  4. 4. Introduction contd..The principles that were applied to arrive at the sevenlayers are as follows:• A layer should be created where a different level of abstraction is needed.• Each layer should perform a well-defined function.• The function of each layer should be chosen with an eye toward defining internationally standardized protocols.• The layer boundaries should be chosen to minimize the information flow across the interfaces.• The number of layers should be large enough that distinct functions need not be thrown together in the same layer out of necessity, and small enough that the architecture does not become unwieldy. CMC Limited
  5. 5. Introduction contd..The seven layers of the OSI model are numbered frombottom to top as follows: 7 Application 6 Presentation 5 Session 4 Transport 3 Network 2 Data link 1 Physical CMC Limited
  6. 6. Characteristics of the OSI Layers The main characteristics of the seven layers OSI model are: • Each layer performs a defined subset of functions for the overall communication process • To perform more primitive functions • To hide the details of the lower layer functions • Each layer provides services to the next higher layer • Modifications within a layer do not require modifications of the other layers • Dividing the communication functions into separate layers facilitates the management of the communication process. CMC Limited
  7. 7. Characteristics of the OSI Layers contd..The seven layers of the OSI reference model can be dividedinto two categories: • Upper Layers: Deal with application issues and generally implemented only in software. • Lower Layers: Handle data transport issues. CMC Limited
  8. 8. Advantages of Dividing Network intoLayers • Divides the interrelated aspects of network operation into less complex elements. • Defines standard interfaces for plug-and-play compatibility and multi-vendor integration. • Enables engineers to specialize design and promote symmetry in the different internetwork modular functions so that they interoperate. • Prevents changes in one area from affecting other areas, so each area can evolve more quickly • Divides the complexity of internetworking into discrete, more easily learned operation subsets. CMC Limited
  9. 9. The Seven OSI Reference Model LayersPhysical Layer (Layer 1) • The physical layer is concerned with transmission of unstructured bit stream over physical link; involves such parameters as signal voltage swing and bit duration. • It accepts frames of data from the upper layer, the Data Link Layer (Layer 2) and transmits the data serially, one bit at a time. CMC Limited
  10. 10. The Seven OSI Reference Model Layers contd..Data Link Layer (Layer 2) • The data link layer provides reliable transfer of data across the physical link. It sends blocks of data (frames) with the necessary synchronization, error control and flow control. • It exchanges a "frame" with Data Link Layer on another node. • The Data Link Layer provides services to the Network Layer above. It receives the data from layer 3 and adds the necessary control information, and sends it to the layer 1 as a frame. It also verifies the frame received from layer 1 and ensures the acknowledgement and sends the data up to the layer 3. CMC Limited
  11. 11. Data Link Layer (Layer 2) contd..The Data Link layer contains two sub-layers:• Logical Link Control (LLC): This sublayer is responsible for identifying Network layer protocols and then encapsulating them. An LLC header tells the Data Link layer what to do with a packet once a frame is received.• Media Access Control (MAC): This defines how packets are placed on the media. Contention media access is “first cum first served” access where everyone shares the same bandwidth. Physical addressing is defined here, as well as logical topologies. CMC Limited
  12. 12. The Seven OSI Reference Model Layers contd..Network Layer (Layer 3) The Network layer manages device addressing, tracks the location of devices on the network and determines the best way to move data. Routers (layer 3 devices) are specified at the Network layer and provide the routing services within an internetwork. Two types of packets are used at the Network layer: • Data Packets: It is used to transport user data through the internetwork. • Route Update Packets: It is used to update neighboring routers about the networks connected to all routers within the internetwork. CMC Limited
  13. 13. The Seven OSI Reference Model Layers contd..Transport Layer (Layer 4) The transport layer accepts data from the session layer and segments the data for transport across the network. This layer is responsible for making sure that the data is delivered error-free and in the proper sequence. Flow Control: Data integrity is ensured at the Transport layer by maintaining flow control and by allowing users to request reliable data transport between systems. Multiplexing enables data from several applications to be transmitted onto a single physical link CMC Limited
  14. 14. Transport Layer (Layer 4) contd..Reliable data transport employs a connection-orientedcommunications session between systems, and theprotocols involved ensure that the following will beachieved:• The segments delivered are acknowledged back to the sender upon their reception.• Any segments not acknowledged are retransmitted.• Segments are sequenced back into their proper order upon arrival at their destination.• A manageable data flow is maintained in order to avoid congestion, overloading, and data loss. CMC Limited
  15. 15. The Seven OSI Reference Model Layers contd..Session Layer (Layer 5) The main function of the OSI models session layer is to control "sessions", which are logical connections between network devices. This layer provides dialogue control between devices, or nodes. It coordinates communication between systems and serves to organize their communication by offering three different modes: • Simplex • Half duplex • Full duplex CMC Limited
  16. 16. The Seven OSI Reference Model Layers contd..Presentation Layer (Layer 6) It presents data to the Application layer. Its basically a translator and provides coding and conversion functions. By providing translation services, the Presentation layer ensures that data transferred from the Application layer of one system can be read by the Application layer of another host. Tasks like data compression, decompression, encryption and decryption are associated with this layer. CMC Limited
  17. 17. Presentation Layer (Layer 6) contd..Some Presentation layer standards are involved inmultimedia operations. The following serve to direct graphicand visual image presentation:• JPEG: The Joint Photographic Experts Group brings these photo standards.• MIDI: The Musical Instrument Digital Interface is used for digitized music.• MPEG: The Moving Pictures Experts Groups standard for the compression and coding of motion video for CDs is very popular.• QuickTime: This is for use with Machintosh or Power PC programs, it manages audio and video applications. CMC Limited
  18. 18. The Seven OSI Reference Model Layers contd..Application Layer (Layer 7) The application layer is the OSI layer closest to the end user, which means that both the OSI application layer and the user interact directly with the software application. Application layer functions typically include identifying communication partners, determining resource availability and synchronizing communication. Some examples of application layer implementations include Telnet, File Transfer Protocol (FTP) and Simple Mail Transfer Protocol (SMTP). CMC Limited
  19. 19. Communication between Systemsusing the OSI ModelInformation being transferred from a software application inone computer system to a software application in anothermust pass through the OSI layers.A given layer in the OSI model generally communicates withthree other OSI layers: System X System Y• layer directly above it Application Application• layer directly below it Presentation Presentation• its peer layer in other Session Session networked computer Transport Transport systems Network Network Data link Data link Physical Physical CMC Limited
  20. 20. Communication between Systems using the OSI Model contd..How data is transmitted using the OSI model System X System Y Sending Receiving Process Process Data Application Application Application Protocol AH Data Layer Layer Presentation Presentation Layer Presentation Protocol PH Data Unit Layer Session Layer Session Layer Session Protocol SH Data Unit Transport Layer Transport Layer Transport Protocol TH Data Unit Network Layer Network Network Layer NH Data Unit Protocol Data link Layer DH Data Unit DT Data link Layer Physical Layer Physical Layer Bits Network Actual data transmission path CMC Limited
  21. 21. Communication between Systems using the OSI Model contd..EncapsulationAs data travels down the layer stack at the sending system,each layer adds a header to the front of it and gives theresulting item to the lower layer. Some layer also adds a trailerto the rear of the data unit. This addition of a header/trailer tothe higher layer data unit is called encapsulation.De-encapsulationAs data travels upward the layer stack at the receiving system,the layer reads the header from its peer layer, strips it off, andpasses the remaining information unit to the next highest layer.This is known as de-encapsulation. CMC Limited
  22. 22. More on Encapsulation There are five steps of encapsulation: 1. User information is converted into data. 2. Data is converted into segments for transport across the network. 3. Segments are converted into packets. 4. Packets and datagrams are converted into frames and the Data Link header is added. 5. The data in the frames is converted into bits for transmission over the physical media. CMC Limited
  23. 23. More on Encapsulation contd..The following table describes data encapsulation by OSIlayer: OSI Layer Encapsulation Transport Segment Network Packet Data Link Frame Physical Bits CMC Limited
  24. 24. Networking Devices to their OSI LayerPhysical Layer: Repeater, Hub, Network interface card (NICcard) • Repeater: A repeater is a network device. The main function of repeater is to regenerate a single to allow it to travel greater distances along a network. CMC Limited
  25. 25. Networking Devices to their OSI Layer contd..• HUB: It operates at the Physical layer of the OSI model. HUB does not have any processing power, its just a box where you plug cables. CMC Limited
  26. 26. Networking Devices to their OSI Layer contd..• NIC: Puts the data into packets and transmits packet onto the network. Every NIC card has a unique address burnt into the card. This address is in a flat hexadecimal number. CMC Limited
  27. 27. Networking Devices to their OSI Layer contd..Data Link Layer: Switches, Bridges• Switch: Switch acts like a hub, but unlike the hub it examines the destination MAC (Media Access Control) addresses of the packet to decide where the packet should be forwarded.• Bridge: Bridges segments a network and reduce network traffic by examining the source and destination hardware address of the packet. CMC Limited
  28. 28. Networking Devices to their OSI Layer contd..Network Layer: Routers• Routers: Devices at the Network layer are concerned with two main things – network addressing and routing. Routers reduce broadcast storms because they dont route broadcast packets. Cisco 1600 and 2500 Series Router CMC Limited
  29. 29. Differences between Data Link andNetwork Addresses Data Link addresses (Data-link layer) are for the most part referred to as flat address space unique addresses. We mostly relate them to the term physical or hardware addressing. Example Address: 00-AA-BB-CC-DD-EE (MAC Address) CMC Limited
  30. 30. Differences between Data Link and Network Addresses contd.. Network Addresses (Network Layer) are logical addresses that are used for path selection, route determination and selection. Example Address: where 192.168.100 is the network and 2 is the individual host ID CMC Limited
  31. 31. OSI Model and TCP/IP Stack OSI Model Layers TCP/IP Protocol Architecture Layers TCP/IP Protocol Suite Application Layer FTP HTTP Presentation Layer Application Layer SMTP SNMP TELNET Session Layer Transport Layer Host-to-Host Transport Layer TCP UDP Network Layer Internet Layer IP ARP RARP ICMP Data link Layer Ethernet Fast Ethernet Network Interface Layer Token Ring Physical Layer FDDI CMC Limited
  32. 32. OSI Model and TCP/IP Stack contd..TCP/IP Protocol SuiteTCP/IP makes it possible for two computers, which are partof different networks (connected by routers or gateways) toexchange data.Different protocols of TCP/IP suite: • Application protocols occupy the highest protocol layers and provide specific services. • TCP (Transfer Control Protocol) and UDP (User Datagram Protocol) facilitate the transmission of data streams between applications running on different hosts. • IP (Internet Protocol), a lower-level protocol than TCP or UDP, governs the transmission of data packets throughout a computer network. CMC Limited
  33. 33. TCP/IP Protocol Suite contd..Application Protocols: • FTP (File Transfer Protocol) • HTTP (Hypertext Transfer Protocol) • SMTP (Simple Mail Transfer Protocol) • SNMP (Simple Network Management Protocol) • NNTP (Network News Transfer Protocol) • Telnet • WAP (Wireless Application Protocol)Network layer protocols, that are less visible but playequally important roles in TCP/IP networks, include: • ARP (Address Resolution Protocol) • RARP (Reverse Address Resolution Protocol) • ICMP (Internet Control Message Protocol) CMC Limited
  34. 34. Connection-Oriented and ConnectionlessServices Connection-oriented means that a connection (a virtual link) must be established before data can be exchanged. In connection-oriented service the source first informs the network it wishes to start a conversation with destination, the network sends its request to the destination that accepts or rejects the request. If the destination refuses, connection fails, otherwise connection is established. The communication proceeds through three well-defined phases: • Connection establishment • Data transfer • Connection release CMC Limited
  35. 35. Connection-Oriented and Connectionless Services contd..A typical reliable session taking place between sending andreceiving systems. Sender Receiver Synchronize Negotiate Connection Synchronize Acknowledge Connection Established Data Transfer (Send segments) CMC Limited
  36. 36. Connection-Oriented and Connectionless Services contd..Connectionless is the opposite of connection-oriented.The sender does not establish a connection before it sendsdata, it just sends without guaranteeing delivery. Packetssent between two hosts may take different routes. UDP isan example of a connectionless transport protocol. CMC Limited
  37. 37. Windowing The quantity of data segments (measured in bytes) the transmitting machine is allowed to send without receiving an acknowledgment for them is called a window. CMC Limited
  38. 38. Windowing contd..Sender Receiver Window size of 1 Send 1 Receive 1 Ack 2 Send 2 Receive 2 Ack 3 Window size of 3 Send 1 Send 2 Send 3 Ack 4 Send 4 CMC Limited