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The OSI Model

The OSI Model. Richard Mills Kamehameha Schools. What is the OSI Model?. The OSI Model is a conceptual model that helps to explain how information flows from one computing device to another, source to destination. The OSI model is constructed in layers. Application (Layer 7)

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The OSI Model

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  1. The OSI Model Richard Mills Kamehameha Schools

  2. What is the OSI Model? • The OSI Model is a conceptual model that helps to explain how information flows from one computing device to another, source to destination.

  3. The OSI model is constructed in layers • Application (Layer 7) • Presentation (Layer 6) • Session (Layer 5) • Transport (Layer 4) • Network (Layer 3) • Data Link (Layer 2) • Physical (Layer 1)

  4. Each layer has its own particular set of responsibilities

  5. The Application Layer • This is where you will find any application which requires network resources. Can you think of some? Protocols which directly support such applications such as: HTTP, FTP, DNS also reside at layer 7.

  6. The Presentation Layer • Making sure that data can be read correctly by applications, is the role of this layer. The format of data files (BMP, TIFF, MP3, etc) relates to this layer. Processes such as the encryption and compression of data take place at layer 6.

  7. The Session Layer • Simply, the establishment, maintenance and termination of sessions between applications. Keyword: Dialogue.

  8. The Transport Layer • To reliably deliver data from one computer to another, without any errors, is the function of the Transport layer. Windowing and flow control take place at layer 4. TCP and UDP are the main layer 4 protocols. The Transport layer is summed up in the term “Quality of Service (QoS).”

  9. The Network Layer • Best path and path determination describe layer 3. IP addresses reside at this layer. Routers forward data based on IP addresses therefore a router is known as a layer 3 device. Routable and routing protocols are found at the Network layer.

  10. The Data Link Layer • The preparation of data for transmission onto the physical medium best describes this layer. MAC is a layer 2 function. MAC addresses are found at this layer. Because bridges and switches forward information based on MAC addresses they are known as layer 2 devices. Encapsulation or type of frame is a Data-Link issue.

  11. The Physical Layer • Here resides networking Media: UTP, fibre optic cable, etc. Signals, voltages and power are all layer 1 concerns. Plugs and connectors are also layer one issues. Most networking problems occur at this layer.

  12. Protocol Data Units • The Primary concern of each layer in the model is to communicate with its direct peer in the destination device. • This exchange is done via Protocol Data Units. • Each layer in the model has its own PDU.

  13. Protocol Data Units at each layer • Application – Data • Presentation – Data • Session - Data • Transport – Segment • Network – Packet • Data Link – Frame • Physical - Bits

  14. The TCP/IP Model The TCP/IP Model is an alternative model to the OSI. Its different layers can be mapped onto the layers within the OSI Model like…..

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  16. In Summary • The OSI Model helps in the understanding of how information flows along computer networks • The OSI Model ensures greater compatibility and interoperability between various types of network technologies

  17. In Summary • Changes in one layer will not affect the other layers. This makes it easier for different layer technologies to be developed, quickly. • It standardizes network components to allow multiple-vendor development and support.

  18. In Summary • A good working knowledge of the OSI Model will aid in troubleshooting computer networking problems.

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