The following is an assignment I did for my Networking 1 class at ECPI College of Technology. The assignment was to describe the Open Systems Interconnection model as best I could using plain language and in a manner that it would be understandable to someone who was uninitiated in networking technology. While I did receive an ‘A’ for the assignment, I offer a disclaimer: I am not an expert. I am a student and barely a novice to these concepts and I am posting this article as an attempt to reach out to the more informed masses who would be willing to offer critiques and comments. This post is not published with the intention to teach, but with the intention to learn.
The OSI model is an abstract description of the passage of data through a computer network. It is important for network administrators to understand this passage of data for a number of reasons, such as being able to competently engineer a network, troubleshoot an existing network, and connect networks to one another. The major difficulty with understanding the OSI model is that it is an abstract method of describing very concrete devices and processes. While the different layers of the OSI model are separate, the functioning devices and protocols involved frequently overlap multiple layers at once.
Another confusing aspect about the passage of date from one layer to the next is the concept of PDU’s or Protocol Data Units. A PDU is a generic term for the data being sent between layers of the OSI model. Depending on the layer, the data might be referred to in different terms, such as packets, segments, data, etc. PDU is an all encompassing description to the raw data sent through the transmission, as well as the information (headers) needed to be read by devices on each layer in order to perform required functions.
Another way to look at it might be that when you order an article of clothing from a catalog, it doesn’t arrive to your door uncovered, ironed and hanging on a hanger. You may order a shirt, but from point A to point B of the delivery of that shirt, it is referred to by different names. It may be known simply as Order #100 to the call center or Shipment #2889 to the delivery service, and from one place to the next it is packaged and labeled with different information before it arrives to your door and you open the various packaging to get to the “essential dataâ€â€”a shirt. The PDU would be not only the generic description of the shirt, the package, and the order being sent but also the order form and invoice form sent with it to request and send the shirt from place to place.
The layers of the OSI model are broken up into two categories. The upper layers (layers 7 through 4) are referred to as the host layers. These layers provide accurate delivery of data between computers. The four host layers are the Application Layer, the Presentation Layer, the Session Layer, and the Transport Layer. One way to look at the host layers would be to describe them as the white-collar management of a company. They manage, dictate policy, give orders and monitor the work of the lower layers. The lower three layers (3 through 1) are called the Network Layers (and sometimes, the Media Layers). The Network Layers consist of the Transport Layer, the Network Layer, and the Physical Layer. These could be considered the blue-collar workers of the OSI model.
The Application layer is the 7th layer of the OSI model, and provides services to application processes. Not to be confused with any actual application on a computer, the Application Layer provides service to an application (and ultimately the end-user) and a means of accessing the information on a network. Examples of Application layer protocols are FTP (File Transfer Protocol) and SMTP (Simple Mail Transfer Protocol). Notice that, as mentioned before, these protocols are used by many different applications to receive files but are not themselves applications. Returning to the before mentioned analogy, the Application layer would be the Executive of a company who dictates or read messages, but doesn’t do any of the work involved in sending or receiving it.
The 6th layer of the OSI Model is the Presentation Layer. The presentation layer is like the Executive’s Assistant, correcting errors and providing format to the information being sent. The Presentation layer puts the data into a format that is readable to the Application layer; it also determines what kind of file is being sent and sets the encoding and compression. Simply put, the Presentation layer is responsible for the translation of data from formats that can be read by the Application layer and formats that can be sent across the network, and vice versa (depending on which direction the data is traveling).
After the Presentation Layer we have the 5th layer of the OSI model, the Session Layer. The Session Layer provides the mechanism under which communication between two nodes will take place. The Session Layer establishes the endpoints between the line of communication. For instance, if 10 clients are connected to a server, the Session Layer determines that each client receives the data they request, and not the data requested by the other clients. The secretary who takes the formatted message from the executive’s assistant (Presentation Layer) and labels and addresses the message to its recipients would be a good description of the Session Layer.
Following the Session Layer is the Transport Layer, the 4th layer of the OSI Model. The Transport Layer is just barely a “white collar†worker, even though it works harder than any of the other layers. The Transport layer ensures that the data is transferred from node to node reliably and in the correct sequence and without errors. The Transport Layer also manages flow control, which is the process of finding the optimum speed data can be sent between nodes without error. The Transport Layer is kind of like the Shipping Manager that delivers and receives the messages that the Presentation Layer addresses. However, the Transport layer doesn’t actually open or package any of these shipments, he just makes sure that they go out or come in.
Which brings us to layer 3, the Network Layer. The Network Layer’s main job is routing. It makes sure the Transport layer has what it needs to get its data from node to node and meet the quality of service requirements set by the Session Layer. IP is a good example of a Network Layer protocol, as it gives each node a way of addressing itself and finding other nodes on the network. Routing devices also work at this level, as they move data through networks to assure that the data reaches its destination. The Network layer is kind of like the mail sorting system in a delivery room that ensures that each message certified by the Transport layer goes to the right vicinity of the building, and decides which delivery route a message should take to reach its destination.
The Datalink Layer, which is the 2nd layer of the OSI model, provides the functional means to transfer data across the network. This layer is where Switches and Bridges function. An example of Datalink technology would be Ethernet, a standard of network communication that can only use MAC (physical) addresses. MAC addresses only address the datalink device itself, such as a NIC card or a switch. The Datalink layer also can detect errors that may occur on the Physical Layer. At this level, the data is simply traveling from adjacent nodes and segments until reaches its ultimate destination. Once the Network Layer’s Mail Sorting System sorts the Executive’s message, it is taken to the next distribution center on the Data Link Layer. Each distribution center passes the message along to the next distribution until it reaches the center that services the address of the recipient.
The Physical Layer, or layer 1, is exactly as it sounds. It is the physical media used to carry the electrical impulses of data from one node to the next. Cables, hubs, and, to some degree, NIC cards fall into this layer. The Physical layer is dumb; it does nothing but carry along 1’s and 0’s. The Physical layer could be described as the mail truck carrying the message from one location to the next, and the hired help that takes the boxes on and off the truck. They don’t need to know where anything is going; they just need to get it there.
Finally, something that may be less than obvious to the layman would be order that the layers of the OSI Model follow as they relate to the flow of data between two nodes. The OSI model does not go in the same order for both nodes. It sends data from the highest layer to the lowest layer, and receives data from the lowest layer to the highest. To clarify, the order of the layers between two notes would be 7654321 of the sending node and 1234567 of the receiving node. I think this is easiest to visualize when you imagine two PCs connected to one another through a hub. The cable and the hub, of course, function at the Physical Layer of the OSI Model and one can easily see how that cable would sit between those two nodes just as 1 appears in the center of the figure 76543211234567.
