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Old 27th August 2012
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jggimi jggimi is offline
More noise than signal
 
Join Date: May 2008
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Default Chapter 1: Network Layers and the 7-layer OSI model

The International Organization for Standardization (ISO) has a model for describing networks, the Open System Interconnection (OSI) model. The model dates from the 1970s, and is still in use today primarily because it describes our current networking environments very well:
  • Each layer is involved only with its own traffic, or in interfaces to the layer below or above.
  • Anything used within one layer is independent of any other layer. With this model we can switch hardware or lower level protocols and not have to change our applications.
There are seven layers in the OSI model: physical, data link, network, transport, session, presentation, and application. Five of these map directly to TCP/IP.



The two layers that do not map directly to TCP/IP networking -- session and presentation -- fall into either the responsibility of the lower layer (transport) or the upper layer (application).


  1. The physical layer is hardware: coaxial cables, unshielded twisted pair wiring, fiber optic lines, hubs, switches, exchanges, and wireless transmitters and receivers of all kinds. The physical layer is responsible for moving bits of information -- ones and zeros -- between two points, or between multiple points.
  2. The data link layer organizes the bits of information into frames of data that are transmitted and received, and is responsible for communicating over the physical link. To do this, the data link uses agreed protocols -- as applicable to the the physical layer. There are many different data link protocols, such as Ethernet, ATM, DSL, and DOCSIS, just to name a few.
  3. The network layer manages communication between systems, and organizing the frames of data of layer 2 into useful messages. In TCP/IP, this also includes the routing of messages between different networks, such as between your network and mine.
  4. The transport layer conducts flow control and error handling. In TCP/IP, this may be the responsibility of the application, or the responsibility of the networking software within the OS -- the "TCP/IP protocol stack" -- or, this may a shared responsibility between the two.
  5. The session layer tracks the state of any communications traffic -- for example: whether both sides have agreed to begin communication, if communication is in progress, or if communication has completed. In TCP/IP, this is either the responsibility of the application, or of the protocol stack. There is no shared responsibility.
  6. The presentation layer is an abstraction -- such as XML -- used between different applications for data interfaces. If used in TCP/IP it is considered entirely the responsibility of the applications.
  7. The application layer is where network applications communicate with each other. For example, this is the layer where your browser and a webserver communicate with each other using the HTTP protocol.
The layer model has helped shape the independence of our networking components. Our browsers don't need to know where the web servers are, or how to route HTTP requests to them. They don't need to know if copper wires, fiber optics, or satellites are part of the communication network. Nor do the web servers need to know how the browsers are connected. Each link in the chain need only know about its own connections and how to reach the next link in the chain.

Last edited by jggimi; 2nd September 2012 at 06:28 PM.
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