An Introduction to the OSI Model

Takeaway: Instead of serving as protocol, the OSI model has become a teaching tool that shows how different tasks within a network should be handled in order to promote error-free data transmission.

The open system interconnection model, better known as the OSI model, is a network map that was originally developed as a universal standard for creating networks. But instead of serving as a model with agreed-upon protocols that would be used worldwide, the OSI model has become a teaching tool that shows how different tasks within a network should be handled in order to promote error-free data transmission.

These jobs are split into seven layers, each of which depends on the function’s “handed-off” from other layers. As a result, the OSI model also provides a guide for troubleshooting network problems by tracking them down to a specific layer. Here we’ll take a look at the layers of the OSI model and what functions they perform within a network.

1. Physical Layer

The physical layer is the actual cable, fibers, cards, switches, and other mechanical and electrical equipment that make up a network. This is the layer that transforms digital data into signals that can be sent down a wire to transmit data. These signals are often electrical but, as in the case of fiber optics, they can also be non-electrical signals such as optics or any other type of pulse that can be digitally encoded. From a networking perspective, the purpose of the physical layer is to provide the architecture for data to be sent and received. The physical layer is probably the easiest layer to troubleshoot but the most difficult to repair or construct, as this involves getting the hardware infrastructure hooked up and plugged in.

2. Data Link Layer

The data link layer is where information is converted into coherent “packets” and frames that are passed to higher layers. Essentially, the data link layer unpacks raw data coming in from the physical layer and translates information from the upper layers into raw data to be sent over the physical layer. The data link layer is also responsible for catching and compensating for any errors that occur in the physical layer.

3. Network Layer

The network layer is where the destination for incoming and outgoing data is set. If the data link layer is the highway for cars to drive on, the network layer is the GPS system telling drivers how to get there. Addressing is added to the data by tacking on information around the data packet in the form of an address header. This layer is also responsible for determining the quickest route to the destination and the handling of any problems with packet switching or network congestion. This is the layer where routers work to ensure that data is properly re-addressed before passing it on to the next leg of the packet’s journey.

4. Transport Layer

The transport layer is responsible for streaming data across the network. At this level, the data is not thought of in terms of individual packets but more in terms of a conversation. To accomplish this, protocols – which are defined as “rules of communication - are used. The protocols watch the complete transmission of many packets - checking the conversation for errors, acknowledging successful transmissions and requesting retransmission if errors are detected.

The network layer and the transport layer work together like a postal system. The network layer addresses the data, much like a person addresses an envelope. Then, the transport layer acts as the sender’s local postal branch, sorting and grouping all similarly addressed data into larger shipments bound for other local branches, where they will then be delivered.

5. Session Layer

The session layer is where connections are made, maintained and ended. This usually refers to application requests for data over the network.

Whereas the transport layer handles the actual flow of data, the session layer acts as an announcer, making sure that the programs and applications requesting and sending data know their requests are being filled. In technical terms, the session layer synchronizes data transmission.

6. Presentation Layer

The presentation layer is where received data is converted into a format that the application it is destined for can understand. The work done at this layer is best understood as a translation job. For example, data is often encrypted at the presentation layer before being passed to the other layers for sending. When data is received, it will be decrypted and passed on to the application it is intended for in the format that is expected.

7. Application Layer

The application layer coordinates network access for the software running on a particular computer or device. The protocols at the application layer handle the requests that different software applications are making to the network. If a Web browser wants to download an image, an email client wants to check the server and a file-sharing program wants to upload a movie, the protocols in the application layer will organize and execute these requests.

Putting It All Together

We’ve looked at the OSI model from the bottom layer up. A simplified summary of this process can be broken into three requirements:

1. The computer has to be hooked up to a network (physical layer), and must have a way to read data (data link layer). The network must also have a proper address (network layer) to know how to come and go.
   
2. The network itself has to have ways of efficiently delivering data to the proper recipients (transport layer) and letting those recipients know it has been delivered (session layer).
   
3. The data has to be unpacked and delivered to the application in a format it understands (presentation layer) and then must fill the requests various software applications make to the network for the user (application layer).

Sending data works in the opposite direction, starting at the top OSI layer – the application layer - and moving down through the model, finally ending when the data is received by the recipient via the physical layer.

Conclusion: Lessons From the OSI Model

The OSI model provides a conceptual viewpoint of networks by showing what tasks are handled at each level. On a practical level, however, the picture becomes much more complicated. Some devices and protocols fit neatly into a single layer, while others operate in multiple layers and carry out functions that affect every layer. As mentioned, data security in the form of encryption can be confined to the presentation layer, but network security affects all seven layers.

Real-world networks are much less defined than the OSI model suggests. That said, the model provides a conceptual framework that can be used to visualize network interactions, both for troubleshooting existing networks and for designing better networks in the future.