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A 3 Pages Term Paper on OSI and TCP/IP Models

     Modern computer networks are designed in a highly structured way. To reduce their design complexity, most networks are organized as a series of layers, each one built upon its predecessor. To discuss network communication it is often necessary to use terminology that is unique to the networking world. A model developed by the International Standards Organization (ISO) is normally used when discussing data communications protocols. It is called the Open Systems Interconnect (OSI) Reference Model. This model defines seven distinct layers, each with its own specific functions. TCP/IP architecture is based on the OSI model, but as is often the case when applying theory to a practical application, TCP/IP does not follow the OSI model exactly. TCP/IP is usually described as having fewer layers than the OSI model, and many of the functions that are distinct to a layer in OSI cross layer boundaries in TCP/IP.

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OSI and TCP/IP Models- Layered Approach

     Each layer defines a different set of the protocols necessary for communication, ranging from connectors and wires, to how to identify the reply address in an email.

     Information always flows down through the layers from an application at the Application Layer (or above), across a real physical connection between two (or more) Physical Layer objects, and back up through all seven layers The standards are concerned with interfaces between layers rather than what goes on within layers These interfaces may be described as services provided by a lower level layer to a higher level one. The nature of these services is summarized in the diagram. They are described in more detail in the reading. In the highest (application orientated) and lowest layers (network hardware dependent) layers of the model there are many standards are supported. That is the whole point. The model is, however, dependent on using essentially a single transport layer protocol to get all the different applications working with all the different low-level networks. This also means proportionally there are fewer protocols generally in the middle layers. (W. Richard, Introduction)

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OSI Model-Why a Seven-Layered Approach

     The OSI Reference Model is based on a proposal developed by the International Standards Organization (ISO). The model is called ISO OSI (Open systems Interconnection) Reference Model because it deals with connecting open systems - that is, systems that are open for communication with other systems. The OSI model has seven layers. The principles that were applied to arrive at the seven layers are as follows:

  1. A layer should be created where a different level of abstraction is needed.
  2. Each layer should perform a well-defined function.
  3. The function of each layer should be chosen with an eye toward defining internationally standardized protocols.
  4. The layer boundaries should be chosen to minimize the information flow across the interfaces.
  5. 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.

The ISO/OSI protocol with seven layers is the usual reference model. Since TCP/IP was designed before the ISO model was developed it has five layers; however the differences between the two are mostly minor.

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TCP/IP Model-Why a Five-Layered Approach

     The TCP/IP model does not exactly match the OSI model. There is no universal agreement regarding how to describe TCP/IP with a layered model but it is generally agreed that there are fewer levels than the seven layers of the OSI model. Most differences are obvious from third to fifth layer. The five-layer structure of TCP/IP is built as information is passed down from applications to the physical network layer. When data is sent, each layer treats all of the information it receives from the layer above as data and adds control information to the front of that data. This control information is called a header, and the addition of a header is called encapsulation. When data is received, the opposite procedure takes place as each layer removes its header before passing the data to the layer above. The TCP/IP protocol suite has always had an applied, "get the job done" orientation. Over the years it has handled most challenges by growing to meet the needs, and it is now the de-facto standard for internetworking for several reasons, including:

  1. It is relatively simple and robust compared to alternatives such as OSI;
  2. It is available on virtually every hardware and operating system platform--often free;
  3. It is the protocol suite on which the Internet depends.

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OSI and TCP/IP Models- A Comparison

     The OSI reference model was devised before the protocols were invented. This ordering means that the model was not biased toward one particular set of protocols, which made it quite general. The down side of this ordering is that the designers did not have much experience with the subject and did not have a good idea of which functionality to put in which layer. With the TCP/IP the reverse was true, the protocols came first, and the model was really just a description of the existing protocols. There was no problem with the protocols fitting the model, but it is hardly possible to be used to describe other models.

Focus of Reliability Control

     Implementation of the OSI model places emphasis on providing a reliable data transfer service, while the TCP/IP model treats reliability as an end-to-end problem. Each layer of the OSI model detects and handles errors; all data transmitted includes checksums. The transport layer of the OSI model checks source-to-destination reliability.

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     In the TCP/IP model, reliability control is concentrated at the transport layer. The transport layer handles all error detection and recovery. Individual hosts or links can lose data without making any attempt at recovery. Corrupted datagrams can be discarded at internal gateways, and datagrams can be rerouted or dropped if network line problems occur. The TCP/IP transport layer uses checksums, acknowledgments, and timeouts to control transmissions and provides end-to-end verification.

Roles of Host Systems

     Another contrast between the OSI model and the TCP/IP model is the role of the host system. Hosts on OSI implementations do not handle network operations (simple terminal), but TCP/IP hosts participate in most network protocols. TCP/IP hosts carry out such functions as end-to-end verification, routing, and network control. The TCP/IP Internet can be viewed as a data stream delivery system involving intelligent hosts.

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De-jure vs. De-facto Standards

     In practice, what we are discussing here is the difference between a dejure standard, OSI, and a de facto standard, TCP/IP. The focus in the TCP/IP world is on agreeing on a protocol standard which can be made to work in diverse heterogeneous networks. The focus in the OSI world has always been more on the standard than the implementation of the standard. (Douglas)

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Works Cited

Douglas Comer, Internetworking with TCP/IP Vol. I: Principles, Protocols, and Architecture

The Protocols (TCP/IP Illustrated, Volume 1) by W. Richard Stevens


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