An Introduction to the OSI Model and Layered Communications
Intro ] [ Physical ] [ Data Link ] [ Network ] [ Transport ]
Session ] [ Presentation ] [ Application ] [ Conclusion ]


Introduction

During the early years of our modern computer era, very few standards and protocols existed between various manufacturers. However, as time went on and computer technology continued to improve and become more widespread, it became apparent that standards would be necessary to ensure compatibility. This was especially true with regard to networks, and networking technology. Since the main purpose of a network is to share information, a standard that governs how this information is formatted, transmitted, received and verified would make it possible for information to be shared openly, even when dealing with dissimilar networks.

This need for a standard means of implementing open communications led the ISO and ANSI to develop the seven-layer network communications model known as Open Systems Interconnect. By providing guidelines regarding the way network equipment should be manufactured and how network operating systems communicate on a network, the OSI model became the common link that allows data to be transmitted and exchanged reliably. Although it does not actually perform any functions or do any of the actual work, the OSI model defines the way things should be done by the software and hardware on a network so that communications can take place between two computers or nodes.

In this way, the OSI model provides a universal set of rules that make it possible for various manufacturers and developers to create software and hardware that is compatible with each other. This makes for organized communications. As I thought about this, I related it to the freeways that connect the various states of the mainland U.S. Because all of these freeways were constructed with the same set of standards regarding the width of each lane, the proper side that a person should drive on, the speed at which they should travel, and so on, people can comfortably drive across the country in an organized and efficient manner and car manufacturers are able to design cars within these guidelines as well. On the other hand, if each state had devised its own set of rules, each differing from the other, not only would there be a lot more chaos on the roads, but also car manufacturers would have a hard time designing vehicles that would be compatible with each state's roads. To me, this illustrates the importance of the OSI model with respect to network communications. Not only is it the foundation for all network communications today, but also because it is such a fundamental part of these communications, it becomes very apparent to me that it is very important for a network technician to understand the OSI model in full detail.

The OSI model is made up of the following layers: the physical, data link, network, transport, session, presentation and application. Together, these seven layers are collectively referred to as a stack. As a node receives data, each layer starting with the physical layer extracts the various portions of the packet and this process works its way up to the application layer. When data is sent, it begins at the application layer and travels down to the physical layer. The information is pushed to the next layer of the stack by means of commands called primitives. Each layer uses a peer protocol to encode the information, which ensures that the same layer on the receiving node will be able to understand the information.

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Physical Layer

Beginning at the bottom, the first layer is the physical layer. It governs the actual voltages, type of electrical signals, mechanical connections and other items relating to the actual data transmission medium. This includes cabling types, distances and connectors, as well as protocols like CSMA/CD.

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Data Link Layer

The next layer is the data link layer. This is the layer that actually constructs the frames, and it also performs error checking using CRC. It ensures that the frames are sent up to the next layer in the same order that they were received, providing an error free virtual path to the network layer. The data link layer consists of two sub layers; the logical link control (LLC) and the media access control (MAC), which provide reliable communications by ensuring the data link is not broken and also by examining packet address information. A bridge is an example of a device that works at this layer. A bridge learns, forwards and filters traffic by examining the layer 2 MAC address. This helps segment network traffic. More recently, bridges have been replaced by switches, which performs the same functions as a bridge, but can do so on each port. To find out more about switches, visit the Products link on the left.

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Network Layer

Moving up to the next layer in the stack we come to the network layer. This layer actually routes packets of data, finding a path (both physical and logical) to the receiving or destination computer. It provides a unique address for each node through address resolution. One of the most common protocols for routing information at this layer is the Internet Protocol (IP). An example of hardware that can operate at this layer is a router. Although routers are often used to allow a LAN to access a WAN, layer 3 switches can also provide routing capabilities, but often at full wire-speed.

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Transport Layer

The transport layer makes sure that the data arrives without errors, in the proper sequence and in a reliable condition. It uses flow control to make sure that information is sent at the proper speed for the receiving device to be able to handle it, and it repackages large data into smaller messages and then back again at the receiving node. An example protocol at this layer is the Transmission Control Protocol (TCP). Layer 4 switches can use the port information found in the TCP header to provide QoS (Quality of Service) and load balancing. To learn more about multi-layer switches, visit the Products link.

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Session Layer

The session layer establishes the link between two nodes and ensures that the link is maintained and then disconnected. This is referred to as the session. It also makes sure the session is orderly, establishing which node transmits first, how long it can transmit, and what to do in case of an error. It also handles the security of the session.

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Presentation Layer

The presentation layer deals with the actual formatting of the data. It handles compression, encryption, as well as translation to make sure differences in formatting can be read by the receiving node. For example, data might be converted from EBCDIC to ASCII formatting so that the receiving node can understand it.

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Application Layer

This brings us to the seventh and final layer, the application layer. It allows applications access to network services, such as file and printer sharing, as well as file transfer and management services. This would be the layer that a programmer uses to allow his application to access a network service, such as linking into a database.

Although this explains the flow of data and what processes are performed by each layer starting with the physical layer and working to the top, or application, layer, the process would be the same, only reversed, for data flowing from the application layer and down to the bottom, or the physical layer.

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Conclusion

By adhering to this standard model of communications, modern networks, including the Internet, have come into existence. For anyone interested in implementing and supporting today's modern networks, an understanding of the OSI model and its various layers is crucial. Indeed, this standard of communications lays the foundation for all of todays modern network hardware and software.

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References

This page was written by Will Twiggs, an associate with ComTest Technologies, Inc. Various references were used to prepare this article. For a list of these references, or to find out where you can get more information, contact me at william@comtest.com.

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Last updated on 12/11/2000