OSI Networking Model
OSI Networking Model

This lecture will focus on the OSI networking model, and will dive into the details of each of its 7 layers. The OSI model was introduced in 1984. Designed to be an abstract model and teaching tool, the OSI model remains a useful tool for learning about today's network technologies such as Ethernet and protocols like IP. The OSI is maintained as a standard by the International Standards Organization.


Let's now start with the OSI model. The open system interconnection. OSI Model defines a networking framework to implement protocols in seven layers. The OSI Model was introduced in 1984. Designed to be an abstract model and teaching tool, the OSI Model remains a useful tool for learning about today's network technologies such as Ethernet and protocols like IP. The OSI is maintained as a standard by the International Standard Organization. The OSI Model defines networking in terms of a vertical stack of seven layers. Data communication in the OSI Model starts with the top layer on the stack on the sender's side and passes down to the stack to the sender's lowest bottom layer, then traverses the physical network connection to the bottom layer on the receiving side and back up in the reverse direction. 

The seven layers are themselves grouped into two groups. The upper four layers, application, presentation, session and transport or layers seven down to four inclusive are grouped together and are referred to as the host layers. The host layers contain application level data and are responsible for accurate data delivery between computers. The tasks and/or functions of the host layers are designed to work with the upper four layers in the system being communicated with. Generally speaking, the upper four layers inter operate end to end. The lower three layers, network, data link and physical or layers three down to one inclusive are grouped together and are referred to as the media layers. The media layers mostly concern themselves with moving data around, the media layers are responsible for seeing that the information does indeed arrive at the destination for which is was intended. Generally speaking, the lower three layers inter operate pier to pier. 

Let's now move on to discuss the function of each individual layer, we'll start with the top Application layer and go do sequentially through the layers until we get to the bottom layer, the physical layer. The sequence of starting from the top layer and finishing at the bottom layer follows the process of a sender sending data and inversely, the reverse sequence of starting from the bottom layer and finishing at the top layer follows the process of a receiver receiving data. Starting with the top most layer, layer seven, the application layer, the application layer of the OSI Model refers to the highest level protocols as used by network applications. The application layer serves as the interface for users and application processes to access network services. Everything at this layer is application specific. The application layer of the model helps to provide network services to the applications. Examples of the application processes or services it offers are http, ftp, smtp and nfs. 

Next down the layer stack is layer six, the presentation layer. The presentation layer is responsible for how the data is formatted and represented. This layer operates as a data translator and is sometimes referred to as the syntax layer. Data representation may involve processes such as conversion, compression and coding and/or encryption. Presentation layer responsibilities include character code translation, converting ascii to and from other formats, data compressions reduces the number of bits that need to be transmitted on the network, data encryption or decryption, encrypting sensitive data for confidentiality purposes, graphic and auto handling, understand the different media in codings, example in codings and/or formats handled by the presentation layer are ascii, jpeg, mpeg, midi and tls. 

Next down the layer stack is layer five, the session layer. The session layer coordinates the conversation between two software application processes. The session layer sets up, manages and ends sessions enabling persistent conversations, exchanges and dialogues between two application processes running on different hosts. In more abstract terms, a session is a persistent, logical connection between two software application processes. An established session enables the exchange of data during a coordinated duration of time. Examples of session oriented protocols that operate on networks are NetBIOS and RPC. 

Next down the layer stack is layer four, the transport layer, the transport layer controls the reliability of network communications between sender and receiver end points on a network. The transport layer utilizes services such as flow control, segmentation or desegmentation and error control to provide reliable end to end transmission and error recovery. The transport layer guarantees data is delivered without error, nor duplication and in the correct sequence. Example protocols as used by the transport layer include both the transmission control protocol, TCP and the user datagram protocol, UDP. We'll just gonna see to these protocols in more detail as we go. The transport layer is last of the upper layers or last of the host layers. 

We now move onto the first of the bottom three lower layers, the network layer, layer three or first of the media layers. The network layer provides connectivity and path selection technology to move data between two network end points. The network layer provides rooting functionality. Layer three encodes data as packets and decides which network path the data should take. This is where IP source and destination addressing is defined and rooting protocols are used to carry packets from source to destination across intermediate routers. Addressing at this layer is considered logical. A router is a good example of a physical device that operates at the network layer. 

Next down the layer stack is layer two, the data link layer. The data link layer is used to convert packets into frames ready for transmission, the data link layer ensures that a connection has been established and processes acknowledgments from a receiver that the data has arrived successfully. It also ensures the incoming data has been received successfully by analyzing bit patterns at special places in the frames. The data link layer is composed to two sub layers. The data link layer's first sub layer is the media access control, MAC layer. It's used to acquire source and destination addresses which are inserted into the frame. The MAC layer allows the data link layer to provide the best data transmission vehicle and manage data flow control. The data link layers second sub layer is the logical link control, it manages error checking and data flow over a network. Ethernet which is the most popular LAN technology is a perfect example of a data link layer protocol. 

Other layer two protocols include the point to point protocol, PPP and the layer two tunneling protocol, L2TP. Finally we're at the bottom layer, layer one, the physical layer. The physical layer is responsible for the transmission and reception of bit streams over a carrier system. The physical layer defines all physical and electrical specifications for devices use to interface to the network including but not limited to voltages, cable specifications and Brookhouse frequencies. It provides the means for transmitting raw bits but is not concerned with MAC addresses, IP addresses and packets. Rather these are dealt with by higher layers. Devices that operate at the physical layer include cables, hubs, repeaters and network interface cards. Examples of protocols that use the physical layer include Ethernet, Digital Subscriber Line, Bluetooth, Universal Serial Bus. 

Let's now examine how our client and server would communicate and specifically how this communication would be described within the OSI model. Firstly, the application layer protocol packages a request into a format needed to send and receive the application content. Each message begins at the top application layer and moves down the OSI layers. This is the left hand side of the diagram. Each downwards successive layer adds a header to the message. The header is a layer specific piece of information that explains what functions a layer has carried out. 

Once the message gets to the bottom physical layer, it's encoded into raw bits and transmitted over a physical medium. When the raw bits arrive at their expected destination, the reverse process begins. At the receiving end, headers are removed and processed one by one from the message as it's passed back up through the same layers, all the way until it gets to the application layer. The application layer interfaces with a process running on the server, passing it the just received application data. Now that we've presented you with a good understanding of the OSI Model, we'll move on and introduce you to TCPI Model.

About the Author
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Jeremy is a Content Lead Architect and DevOps SME here at Cloud Academy where he specializes in developing DevOps technical training documentation.

He has a strong background in software engineering, and has been coding with various languages, frameworks, and systems for the past 25+ years. In recent times, Jeremy has been focused on DevOps, Cloud (AWS, Azure, GCP), Security, Kubernetes, and Machine Learning.

Jeremy holds professional certifications for AWS, Azure, GCP, Terraform, Kubernetes (CKA, CKAD, CKS).

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