Open Systems Interconnection (Osi) Model

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  • Topic: OSI model, Internet Protocol Suite, Transport Layer
  • Pages : 10 (3074 words )
  • Download(s) : 133
  • Published : October 19, 2012
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OSI Reference Model

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- OSI Reference Model Network Reference Models A computer network connects two or more devices together to share information and services. Multiple networks connected together form an internetwork. Internetworking present challenges - interoperating between products from different manufacturers requires consistent standards. Network reference models were developed to address these challenges. A network reference model serves as a blueprint, detailing how communication between network devices should occur. The two most recognized network reference models are: • The Open Systems Interconnection (OSI) model • The Department of Defense (DoD) model Without the framework that network models provide, all network hardware and software would have been proprietary. Organizations would have been locked into a single vendor’s equipment, and global networks like the Internet would have been impractical, if not impossible. Network models are organized into layers, with each layer representing a specific networking function. These functions are controlled by protocols, which are rules that govern end-to-end communication between devices. Protocols on one layer will interact with protocols on the layer above and below it, forming a protocol suite or stack. The TCP/IP suite is the most prevalent protocol suite, and is the foundation of the Internet. A network model is not a physical entity – there is no OSI device. Manufacturers do not always strictly adhere to a reference model’s blueprint, and thus not every protocol fits perfectly within a single layer. Some protocols can function across multiple layers.

*** All original material copyright © 2012 by Aaron Balchunas (aaron@routeralley.com), unless otherwise noted. All other material copyright © of their respective owners. This material may be copied and used freely, but may not be altered or sold without the expressed written consent of the owner of the above copyright. Updated material may be found at http://www.routeralley.com.

OSI Reference Model v1.21 – Aaron Balchunas

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OSI Reference Model The Open Systems Interconnection (OSI) model was developed by the International Organization for Standardization (ISO), and formalized in 1984. It provided the first framework governing how information should be sent across a network. The OSI model consists of seven layers, each corresponding to a specific network function: 7 6 5 4 3 2 1 Application Presentation Session Transport Network Data-link Physical

Note that the bottom layer is Layer 1. Various mnemonics make it easier to remember the order of the OSI model’s layers: 7 6 5 4 3 2 1 Application Presentation Session Transport Network Data-link Physical All People Seem To Need Data Processing Away Pizza Sausage Throw Not Do Please

ISO further developed an entire protocol suite based on the OSI model; however, the OSI protocol suite was never widely implemented. The OSI model itself is now somewhat deprecated – modern protocol suites, such as the TCP/IP suite, are difficult to fit cleanly within the OSI model’s seven layers. This is especially true of the upper three layers. The bottom (or lower) four layers are more clearly defined, and terminology from those layers is still prevalently used. Many protocols and devices are described by which lower layer they operate at.

*** All original material copyright © 2012 by Aaron Balchunas (aaron@routeralley.com), unless otherwise noted. All other material copyright © of their respective owners. This material may be copied and used freely, but may not be altered or sold without the expressed written consent of the owner of the above copyright. Updated material may be found at http://www.routeralley.com.

OSI Reference Model v1.21 – Aaron Balchunas

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OSI Model - The Upper Layers The top three layers of the OSI model are often referred to as the upper layers: • Layer-7 - Application layer • Layer-6 - Presentation layer • Layer-5 - Session layer...
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