At some point, everyone involved with networking comes across a reference to the Open System Interconnection (OSI) seven-layer model. What is the seven-layer model, and why is it relevant?
The OSI seven-layer model describes the functions for computers to communicate with each other. The International Organization for Standardization (ISO) published this model in 1984 to describe a layered approach for providing network services using a reference set of protocols called OSI. The basis of the definition is that each of the seven layers has a particular function it must perform, and each layer needs to know how to communicate with only the layers immediately above and below it.
At the time of the definition of the OSI networking model, there was little standardization among network equipment manufacturers. Customers generally had to standardize on a particular vendor’s hardware and software to have devices communicate with each other.
As a result of the ISO’s and other standardization efforts, networking customers can mix and match hardware when running open standards protocols, such as Internet Protocol (IP).
The ISO’s efforts are an example of the constant struggle for balancing technical openness with competitive advantage. For an individual network-equipment vendor, it is to his advantage to develop technologies that other companies cannot copy or interact with. Proprietary systems let a vendor claim competitive advantage as well as collect fees from other vendors it might choose to share the technology with.
However, proprietary systems can complicate the network administrator’s work by locking him into one vendor, reducing competitiveness and allowing the vendor to charge higher prices. If the vendor goes out of business or abandons the technology, no one is left to support or enhance the technology.
The alternative is an open-systems approach in which standards bodies, such as the Institute of Electrical and Electronic Engineers (IEEE) or ISO, define technologies. Ethernet, Transmission Control Protocol/Internet Protocol (TCP/IP), and spanning tree are examples of technologies that became standards. Any network-equipment vendor can implement an open standard.
The OSI seven-layer model was an attempt to provide a model on how open network standards could interoperate with each other and proprietary technologies.
The following list outlines the seven layers of the OSI model:
Layer 7, application—. The application layer provides the networking services to a user or application. For example, when sending an e-mail, the application layer begins the process of taking the data from the e-mail program and preparing it to be put onto a network.
Layer 6, presentation—. The presentation layer provides formatting services for the application layer.
Layer 5, session—. The session layer manages connections between hosts. If the application on one host needs to talk to the application on another, the application layer sets the connection up and ensures resources are available to facilitate the connection. Networking folks tend to refer to Layers 5–7 collectively as the application layers.
Layer 4, transport—. The transport layer is responsible for taking the chunk of data from the application and preparing it for shipment onto the network. Prepping data for transport involves chopping the chunk into smaller pieces and adding a header that identifies the sending and receiving application (otherwise known as port numbers). Each piece of data and associated headers is called a packet. Content switches operate at this level.
Layer 3, network—. The network layer is responsible for adding another header to the front of the packet, which identifies the unique source and destination address. The process of routing IP packets occurs at this level.
Layer 2, data link—. The data link layer is responsible for adding another header, which identifies the particular Layer 3 protocol used and the source and destination hardware addresses (also known as Media Access Control (MAC) addresses). At this point, the packet is complete and ready to go onto the network. Ethernet switching and bridging operate at this level.
Layer 1, physical—. The physical layer is responsible for converting the packet into binary signals to be transmitted over the network. The actual physical network can be copper, fiber, or wireless radio frequency. This layer also provides a method for the receiving computer to validate that the data was not corrupted during transmission.
The combination of the seven layers is often called a stack. A transmitting workstation traverses the stack from Layer 7 down to Layer 1, converting the application data into network signals. The receiving workstation traverses the stack in the opposite direction: from Layer 1 to Layer 7. It converts the received transmission back into a chunk of data for the running application.
Although there was an attempt to make OSI a practical and implemented networking protocol, the OSI seven-layer model is mainly a reference for other protocols. Most protocols do not fit the seven layers precisely (such as TCP/IP), but the same principles apply.
At-A-Glance—OSI Model
The Open Systems Interconnection (OSI) model is a conceptual framework that defines network functions and schemes. The framework simplifies complex network interactions by breaking them into simple modular elements. This open standards approach allows many independent developers to work on separate network functions, which you can then apply in a “plug-and-play” manner.
The OSI model serves as a guideline for creating and implementing network standards, devices, and internetworking schemes. Advantages of using the OSI model include the following:
Breaking up interrelated aspects of network operation into less complex elements
Enabling companies and individual engineers to specialize with design and development efforts in modular functions
Providing standard interfaces for plug-and-play compatibility and multivendor integration
The OSI Model consists of the following layers:
Layer 7: Application
Layer 6: Presentation
Layer 5: Session
Layer 4: Transport
Layer 3: Network
Layer 2: Data Link
Layer 1: Physical
The four lower layers (referred to as the data flow layers) define connection protocols and methods for exchanging data.
The three upper layers (referred to as the application layers) define how the applications within the end stations communicate with each other and with users.
Mnemonics can help you memorize the layers and their order. Here’s an example:
Princess Di Never Tried Slapping Prince Albert
</division><division> <title>What Are the Problems to Solve?</title>An OSI layer can communicate with only the layers immediately above and below it on the stack and with its peer layer on another device. A process passes information (including data and stack instructions) down the stack, across the network, and back up the stack on the peer device.
</division><division> <title>Extra Layers?</title>Discussions among technical purists often lead to philosophic debates that can quickly derail otherwise productive meetings. You might hear these discussions referred to as Layer 8 (Political) and Layer 9 (Technical Religion) debates. Although they are not really part of the OSI model, they are usually the underlying cause of heated technology arguments.
</division><division> <title>Communicating Between Layers</title>Each layer of the OSI model uses its own protocol to communicate with its peer layer in the destination device. The OSI model specifies how each layer communicates with the layers above and below it, allowing vendors to focus on specific layers that work with any other vendors’ adjacent layers.
Layers exchange information using protocol data units (PDUs). PDUs include control information (in the form of headers and trailers) and user data. PDUs include different types of information as they go up or down the layers (referred to as the stack). To clarify where the PDU is on the stack, it gets a distinct name at each of the lower levels.
In other words, a PDU that is a packet includes network layer control information in addition to the data and control information contained at the transport layer. Similarly, a frame is a PDU that includes data link layer control information in addition to the upper-layer control information and data. The PDUs at the data link and physical layers are referred to as segments and bits, respectively.
</division><division> <title>Encapsulation</title>
The process of passing data down the stack using PDUs is data encapsulation.
Encapsulation works as follows:
When a layer receives a PDU, it encapsulates the PDU with a header and trailer and then passes the PDU down to the next layer.
The peer layer on the remote device reads the control information that is added to the PDU.
</division><division> <title>De-Encapsulation</title>De-encapsulation, the opposite of encapsulation, is the process of passing information up the stack. When a layer receives a PDU, it does the following:
First, it reads the control information provided by the peer source device.
The layer strips the control information (header) from the frame.
Finally, it processes the data (usually passing it up the stack).
Each subsequent layer performs this same de-encapsulation process.
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