OSI Model.pdf
- 2. Introduction ▪ The Open Systems Interconnection (OSI) model is a reference tool for understanding data communications between any two networked systems. ▪ It divides the communications processes into seven layers. ▪ Each layer both performs specific functions to support the layers above it and offers services to the layers below it. ▪ The three lowest layers focus on passing traffic through the network to an end system. ▪ The top four layers come into play in the end system to complete the process. ▪ Finally, this will draw comparisons between the theoretical OSI model and the functional TCP/IP model. ▪ Although TCP/IP has been used for network communications before the adoption of the OSI model, it supports the same functions and features in a differently layered arrangement.
- 3. An Overview of the OSI Model The Open System Interconnection model is a seven-layer structure that specifies the requirements for communications between two computers. The ISO (International Organization for Standardization) standard 7498-1 defined this model. This model allows all network elements to operate together, no matter who created the protocols and what computer vendor supports them. The main benefits of the OSI model include the following: ✓ Helps users understand the big picture of networking. ✓ Helps users understand how hardware and software elements function together. ✓ Makes troubleshooting easier by separating networks into manageable pieces. ✓ Defines terms that networking professionals can use to compare basic functional relationships on different networks. ✓ Helps users understand new technologies as they are developed. ✓ Aids in interpreting vendor explanations of product functionality.
- 5. Top to bottom All People Seem To Need Data Processing Bottom to top Please Do Not Throw Sausage Pizza Away
- 6. How Data Is Referred in the OSI Model?
- 7. Encapsulation/De-encapsulation ➢ The process of moving data between layers of the OSI Model ➢ Encapsulation: Data > segment > packet > frame > bits ➢ De-encapsulation: Bits > frame > packet > segment > data
- 8. ▪ Deals with all aspects of physically moving data from one computer to the next. ▪ Converts data from the upper layers into 1s and 0s for transmission over media . ▪ Defines how data is encoded onto the media used to transmit the data. ▪ Defined on this layer: Cable standards, wireless standards, and fiber optic standards. Device example: Hub Used to transmit data ✓ Copper wiring, fiber optic cable, radio frequencies, anything that can be used to transmit data is defined on the Physical layer of the OSI Model. The Physical Layer
- 9. The physical layer of the OSI model defines connector and interface specifications, as well as the medium (cable) requirements. Electrical, mechanical, functional, and procedural specifications are provided for sending a bit stream on a computer network.
- 10. Components of the physical layer include: ✓ Cabling system components ✓ Adapters that connect media to physical interfaces ✓ Connector design and pin assignments ✓ Hub, repeater, and patch panel specifications ✓ Wireless system components ✓ Parallel SCSI (Small Computer System Interface) ✓ Network Interface Card (NIC) In a LAN environment, Category 5e UTP (Unshielded Twisted Pair) cable is generally used for the physical layer for individual device connections. Fiber optic cabling is often used for the physical layer in a vertical or riser backbone link. The IEEE, EIA/TIA, ANSI, and other similar standards bodies developed standards for this layer. Note: The Physical Layer of the OSI model is only part of a LAN (Local Area Network).
- 11. The Data Link Layer ▪ Is responsible for moving frames from node to node or computer to computer ▪ Can move frames from one adjacent computer to another, cannot move frames across routers ▪ Encapsulation = frame ▪ Requires MAC address. or physical address ▪ Protocols defined include Ethernet Protocol and Point-to-Point Protocol (PPP) ▪ Device example: Switch ▪ Two sublayers: Logical Link Control (LLC) and the Media Access Control (MAC) Logical Link Control (LLC) –Data Link layer addressing, flow control, address notification, error correction. Media Access Control (MAC) –Determines which computer has access to the network media at any given time –Determines where one frame ends and the next one starts, called frame synchronization.
- 13. Layer 2 of the OSI model provides the following functions: • Allows a device to access the network to send and receive messages • Offers a physical address so a device’s data can be sent on the network • Works with a device’s networking software when sending and receiving messages • Provides error-detection capability Common networking components that function at layer 2 include: • Network interface cards • Ethernet and Token Ring switches • Bridges NICs have a layer 2 or MAC address. A switch uses this address to filter and forward traffic, helping relieve congestion and collisions on a network segment. Bridges and switches function in a similar fashion; however, bridging is normally a software program on a CPU, while switches use Application-Specific Integrated Circuits (ASICs) to perform the task in dedicated hardware,which is much faster.
- 14. The Network Layer ▪ Responsible for moving packets (data) from one end of the network to the other, called end-to-end communications ▪ Requires logical addresses such as IP addresses Device example: Router Routing is the ability of various network devices and their related software to move data packets from source to destination
- 15. ✓ Layer 3, the network layer of the OSI model, provides an end-to-end logical addressing system so that a packet of data can be routed across several layer 2 networks (Ethernet, Token Ring, Frame Relay, etc.). ✓ The Internet Protocol (IP) addresses make networks easier to both set up and connect with one another. ✓ The Internet uses IP addressing to provide connectivity to millions of networks around the world. ✓ To make it easier to manage the network and control the flow of packets, many organizations separate their network layer addressing into smaller parts known as subnets. ✓ Routers use the network or subnet portion of the IP addressing to route traffic between different networks. Each router must be configured specifically for the networks or subnets that will be connected to its interfaces.
- 16. ✓ Routers communicate with one another using routing protocols, such as Routing Information Protocol (RIP) and Open version of Shortest Path First (OSPF), to learn of other networks that are present and to calculate the best way to reach each network based on a variety of criteria (such as the path with the fewest routers). ✓ Routers and other networked systems make these routing decisions at the network layer. ✓ When passing packets between different networks, it may become necessary to adjust their outbound size to one that is compatible with the layer 2 protocol that is being used. ✓ The network layer accomplishes this via a process known as fragmentation. ✓ A router’s network layer is usually responsible for doing the fragmentation. ✓ All reassembly of fragmented packets happens at the network layer of the final destination system.
- 17. ✓ Two of the additional functions of the network layer are diagnostics and the reporting of logical variations in normal network operation. ✓ While the network layer diagnostics may be initiated by any networked system, the system discovering the variation reports it to the original sender of the packet that is found to be outside normal network operation. ✓ The variation reporting exception is content validation calculations. ✓ If the calculation done by the receiving system does not match the value sent by the originating system, the receiver discards the related packet with no report to the sender. ✓ Retransmission is left to a higher layer’s protocol. ✓ Some basic security functionality can also be set up by filtering traffic using layer 3 addressing on routers or other similar devices.
- 18. The Transport Layer ✓ Takes data from higher levels of OSI Model and breaks it into segments that can be sent to lower-level layers for data transmission. ✓ Conversely, reassembles data segments into data that higher-level protocols and applications can use. ✓ Also puts segments in correct order (called sequencing ) so they can be reassembled in correct order at destination. ✓ Concerned with the reliability of the transport of sent data. ✓ May use a connection-oriented protocol such as TCP to ensure destination received segments. ✓ May use a connectionless protocol such as UDP to send segments without assurance of delivery. ✓ Uses port addressing.
- 19. Layer 4, the transport layer of the OSI model, offers end-to-end communication between end devices through a network. Depending on the application, the transport layer either offers reliable, connection- oriented or connectionless, best-effort communications. Some of the functions offered by the transport layer include: • Application identification • Client-side entity identification • Confirmation that the entire message arrived intact • Segmentation of data for network transport • Control of data flow to prevent memory overruns • Establishment and maintenance of both ends of virtual circuits • Transmission-error detection • Realignment of segmented data in the correct order on the receiving side • Multiplexing or sharing of multiple sessions over a single physical link The most common transport layer protocols are the connection-oriented TCP Transmission Control Protocol (TCP) and the connectionless UDP User Datagram Protocol (UDP).
- 20. The Session Layer ✓ Responsible for managing the dialog between networked devices ✓ Establishes, manages, and terminates connections ✓ Provides duplex, half-duplex, or simplex communications between devices ✓ Provides procedures for establishing checkpoints, adjournment, termination, and restart or recovery procedures
- 21. Layer 5, the session layer, provides various services, including tracking the number of bytes that each end of the session has acknowledged receiving from the other end of the session. This session layer allows applications functioning on devices to establish, manage, and terminate a dialog through a network. Session layer functionality includes: ✓ Virtual connection between application entities ✓ Synchronization of data flow ✓ Creation of dialog units ✓ Connection parameter negotiations ✓ Partitioning of services into functional groups ✓ Acknowledgements of data received during a session ✓ Retransmission of data if it is not received by a device
- 22. The Presentation Layer ✓ Concerned with how data is presented to the network Handles three primary tasks:
- 24. Layer 6, the presentation layer, is responsible for how an application formats the data to be sent out onto the network. The presentation layer basically allows an application to read (or understand) the message. Examples of presentation layer functionality include: ✓ Encryption and decryption of a message for security ✓ Compression and expansion of a message so that it travels efficiently ✓ Graphics formatting ✓ Content translation ✓ System-specific translation
- 25. The Application Layer Contains all services or protocols needed by application software or operating system to communicate on the network. Examples ✓ Firefox web browser uses HTTP (Hyper-Text Transport Protocol) ✓ E-mail program may use POP3 (Post Office Protocol version 3) to read e- mails and SMTP (Simple Mail Transport Protocol) to send e-mails
- 26. ✓ Layer 7, the application layer, provides an interface for the end user operating a device connected to a network. ✓ This layer is what the user sees, in terms of loading an application (such as Web browser or e-mail); that is, this application layer is the data the user views while using these applications. Examples of application layer functionality include: ▪ Support for file transfers ▪ Ability to print on a network ▪ Electronic mail ▪ Electronic messaging ▪ Browsing the World Wide Web
- 27. How Data Moves Through the OSI Model
- 28. ✓ Each layer of OSI Model except Physical adds its own header to the data that originated from the operating system. ✓ Adds own header in front of the header from the previous layer. ✓ Header contains information that describes what each layer of the OSI Model should do with the data. ✓ Data Link layer also adds a tailer. ✓ Tailer contains additional information that deals with error correction.
- 30. TCP/IP Model Overview Built around the TCP/IP protocol suite A protocol suite is a large number of related protocols that work together to allow networked computers to communicate. Layers with same names as OSI Model don’t function exactly the same.
- 31. ✓ The TCP/IP model uses four layers to perform the functions of the seven- layer OSI model. ✓ The network interface layer is functionally equal to a combination of OSI physical and data link layers (1 and 2). ✓ The Internet layer performs the same functions as the OSI network layer (3). ✓ Things get a bit more complicated at the host-to-host layer of the TCP/IP model. If the host-to-host protocol is TCP, the matching functionality is found in the OSI transport and session layers (4 and 5). ✓ Using UDP equates to the functions of only the transport layer of the OSI model. ✓ The TCP/IP process layer, when used with TCP, provides the functions of the OSI model’s presentation and application layers (6 and 7). ✓ When the TCP/IP transport layer protocol is UDP, the process layer’s functions are equivalent to OSI session, presentation, and application layers (5, 6, and 7).
- 32. ✓ Some of the layers use equipment to support the identified functions. ✓ Hub related activity is “Layer One”. ✓ The naming of some devices designates the functional layer such as “Layer Two Switch” or “Layer Three Switch”. ✓ Router functions focus on “Layer Three”. ✓ User workstations and servers are often identified with “Layer Seven”.
- 33. Application Layer of TCP/IP Model ✓ Encompasses same functions as these OSI Model layers Application Presentation Session Transport Layer of TCP/IP Model ✓ Functions the same as the Transport layer in OSI Model and part of Session layer. ✓ TCP and other similar protocols take on some of the function of the Session layer. ✓ Synchronize source and destination computers to set up the session between the respective computers.
- 34. Internet Layer of TCP/IP Model ✓ Performs same functions as OSI Model Network Layer. ✓ Many of the functions of the Logical Link Control sublayer of the OSI Model’s Data Link layer. ✓ Primary protocol is Internet Protocol (IP). ✓ Also uses Address Resolution Protocol (ARP), which performs much of the LLC sublayer’s job in the area of physical addressing. Interface Layer of TCP/IP Model ✓ Performs much of the job of the MAC portion of the Data Link and Physical layers of the OSI Model. ✓ TCP/IP Protocol does not dictate what happens on Network Interface layer. ✓ TCP/IP protocol suite relies on standards created by the various standards organizations concerning how to encode bits onto media to do the work on this layer.
- 35. Summary ✓ The OSI Model is a framework and reference model to explain how different networking technologies work together and interact. ✓ The Physical layer of the OSI Model deals with all aspects of physically moving data from one computer to the next. ✓ The Data Link layer of the OSI Model is responsible for moving frames from node to node or computer to computer. ✓ The two sublayers of the Data Link layer are the LLC and MAC. ✓ The Network layer of the OSI Model is responsible for moving packets (data) from one end of the network to the other, called end-to- end communications. ✓ The TCP/IP Model is built around the TCP/IP protocol suite.
- 36. ✓ The Application layer of the TCP/IP Model encompasses the same functions as the Application, Presentation, and Session layers of the OSI Model. ✓ The Transport layer of the TCP/IP Model functions the same as the Transport layer in OSI Model and part of Session layer. ✓ The Internet of layer of the TCP/IP Model Performs the same functions as the OSI Model Network layer and many of the functions of the LLC sublayer of the OSI Model Data Link layer. ✓ The Network Interface layer of the TCP/IP Model performs much of the job of the MAC portion of the Data Link and Physical layers of the OSI Model.
- 38. Watch the video for better understanding of the OSI MODEL