A Brief Understanding Of How Computer Networks Function:

The OSI Model

Derrick Rice

A computer network is an important tool in today’s world either for business purposes or in the home for personal use. Computer networks grant us ease of communication, the ability to share files, pictures, and videos, or the ability to remotely access another device. Computer networks can be complex and difficult to work with, however a model has been designed to help us understand how a computer network operates and to simplify any complicated concepts.

The 7-Layer OSI (Open Systems Interconnection) Model gives us a linear guide that describes the methods and technologies that allow computer networks to operate. Let us review these seven layers to hopefully give you a better understanding of what goes on behind the scenes when interacting with a computer network.

Layer 1: Physical

The first layer in the OSI model is the Physical layer. I’m sure we’ve all noticed a cable like the one pictured here running from our computer to a special plug located in a wall. Or maybe this wire plugged into a box with blinking lights somewhere in your home? This wire represents the Physical layer of OSI. It is the physical medium (in this case, electrical signals in the wire) by which data is transported out to the network.

In recent times, however, we have all grown accustomed to having a wireless connection for our networks. This wireless connection, made by radio waves, has been integrated into the Physical layer of OSI as these radio waves are the physical medium that carries your data back and forth on a network. Regardless of what medium is used to transport our data, it all must be generated by our next layer, the Data Link.

Layer 2: Data Link

Our next layer of the OSI Model is the Data Link. This is where the actual transmission of data occurs onto a network. A special device called the Network Interface Controller (NIC) will take any data needing to be sent out and package it into a format for transmisson, or if receiving data prepare it for interpretation. The NIC is also responsible for locating the intended recepient of our data package and “listening” for any incoming data to our device. All devices have a NIC of some form, either with a cable attaching to it or as part of a wireless network card.

However, the Data Link layer and our NIC are only good for local networks, like inside your home or even an office. To access larger networks, like the Internet, we need the third layer of the OSI Model: the Network layer.

Layer 3: Network

While the third layer is technically labled Network, it can alternatively be called Routing. This is due to the fact that while our second layer does indeed work over networks, it is really only useful for more local connections. To really get around large networks like the Internet we need another device to send our data out over the web, and that device is the router. Routers use Internet Protocol (IP) addresses to find other devices and send or receive traffic out over the Internet to users around the world.

You may already be familiar with IP addresses if you’ve ever seen something like this: 192.168.1.0. This string of numbers is the unique identity of a device on a larger network like the Internet that a router will look for and send your data too. But how exactly is data transmitted over the Internet, you might ask? This leads us to the next layer (layer four) of the OSI Model: Transport.

Layer 4: Transport

Data is transported over the Internet by two primary methods: Transmission Control Protocol (TCP) and User Datagram Protocol (UDP).

TCP

When TCP is used, data is sent in a smooth, controlled process to make sure everything is received perfectly. Examples of TCP in action include web pages, emails, or sending pictures. Each of these examples need every bit of data to provide an accurate representation of what someone is trying to send or access, otherwise the results might appear scrambled and useless.

UDP

UDP, however, is the polar opposite to TCP. Data is streamed as fast as possible without regard as to whether the receiver is getting it all correctly. If you’ve ever seen a YouTube video stutter while streaming it, you are seeing UDP in action. Some of the video data was lost somewhere, but the video stream is going to continue regardless. Both TCP and UDP require the next layer of the OSI Model to begin, the Session layer.

Layer 5: Sessions

Before any transmission of data can begin the two (or more) devices need to acknowledge one another and prepare the transmission process. This is accomplished through the creation of sessions. A session is basically when devices recognize that they need to transmit data between each other and essentially create a link together using some of the previously mentioned OSI layer methods. The link is kept open while the data is transmitted, yet once the transmission is complete and everything checks out, the session is terminated. Sessions can be opened and closed as needed by a device when appropriate. Sessions left continually open can present a security risk, so it is important for sessions to be open and closed when needed. Once a session is complete, the Presentation layer goes into action.

Layer 6: Presentation

Once the transmission of data is completed and our session concluded, we might notice a problem. The data is more than likely in a raw form such as code or binary that humans can’t read or interpret easily, if at all.

The sixth layer, Presentation, is designed to help us with this problem. The Presentation layer will take the raw data and process it into a format the next layer of the OSI Model, Application, can use to make this data readable and understandable for humans. The Presentation layer is also responsible for encrypting data to make it secure during transmission and only readable for the proper recipient.

Layer 7: Application

Finally, we have arrived at the final layer of the OSI Model, the Application layer. This layer comprises the actual programs a user will use to initiate or request a transmission of data. It is also the layer that takes the formated data from the previous layer and displays the data in the appropriate form depending on the program (documents for a word processor, web pages for a browser, email messages for an email client, etc.).

To conclude, one might notice that the OSI Model works in both directions. An application sending data will begin at Layer 7 and work down to Layer 1 for the transmission process. On the other hand, an application receiving data will first get the data from Layer 1 and work back up to Layer 7 to present the requested data. This makes the OSI Model a useful tool to remember when viewing the networking process or having to troubleshoot a problem.

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