When it comes to things to be excited about in this world, learning about the Open Systems Interconnection Model (OSI) probably ranks somewhere between Tacos Tuesday (very exciting) and yeah getting a tooth pulled (very painful). If you’re reading this, you’re probably not getting a tooth pulled — so hey: life is looking up for you.
Although you may just be learning about the Open Systems Interconnection model, need a refresher, or just hearing vendors referencing it — this article will go over what the OSI Model is, the different layers within it, and the TCP/IP model (which picks up where the OSI model leaves off).
What is the OSI Model?
To define the OSI model in one sentence: the OSI model is a concept-based model that defines, and sets standards for, the way in which a computing or telecommunication system functions. The goal of the OSI model is to achieve interoperability, through the use of standards, amongst a diverse set of communications.
The OSI model was developed in the late 1970’s by the International Organization for Standardization (ISO). Before the OSI model was released, many network devices were unable to work with one another due to the lack of a set of common standards/protocols. Basically, the ISO saw the need to define networking standards so that equipment from different vendors could be more compatible.
The release of the OSI model was a watershed moment in the world of networking. Why? Having a standardized model of protocol layers opened up the gates for interoperability between software and devices in the network.
The OSI Model: A Layer-Based System
The OSI Model has 7 defined layers of networking: these layers are interdependent. What this means is that each layer within the OSI model is served by the layer below it – and each layer in this model serves the layer above it. Here’s an example of this in action:
You have a layer that takes information and breaks it into pieces, which ensures error-free communication across the network. This layer is used by the applications in the layer above it. This layer is dependant upon the layer below it to send and receive packets that comprise the contents of the path. OSI model visualization would break down like this:
Layer that breaks down data for the applications layer..
Layer that sends and receives packets.
A note about visualizing layers: if you have two different instances in the same layer, you would visualize them using a horizontal connection. Vertical connections visualize connections between layers.
The OSI Model: A Layer-Based System
There are seven distinct layers in the OSI model. They are:
7. Application Layer
Layer seven in the OSI model is known as the ‘application layer’. This is the layer that supports processes for end-users and applications. What happens in this layer? User authentication, service, privacy, data syntax are all identified and processed. Examples of what this layer provides application services for include: www browsers, http, email, transfering files, and network software. FTP and Telnet are completely contained in this layer.
6. Presentation Layer
The presentation layer (also referred to as the ‘syntax layer’) transforms data into a form the application layer can digest. What happens in this layer, typically, is formatting and encrypting data that is sent across the network. Examples of what this layer provides services for include MPEG, GIF, TIFF, ASCII, and encryption.
5. Session Layer
The session layer is responsible for initiating, and terminating, connections between different applications. This layer establishes communication connections between remote systems and allows for the flow of data between them. Protocols used in the session layer include: Telnet, SSL, SQL, SSH, and SNMP.
4. Transport Layer
The transport layer is responsible for ensuring complete data transfer. Packets must get to to their endpoints — error-free and properly sequenced. This layer is of critical importance for both securing the network and avoiding network congestion. Examples of what happens in this layer are SPX, UDP, TCP, and firewalls.
3. Network Layer
The network layer is responsible for establishing the way in which data is communicated – both in your network and in other networks – via packet forwarding and routing. This layer’s functions include internetworking, handling errors, packet sequencing, congestion control, and, routing and forwarding. Examples include IP, IPX, and AppleTalk DDP.
2. Data Link Layer
The data link layer is responsible for coding and decoding data packets — aka, ensuring communication between the network and physical layer happens. This layer can be divided into two sub-layers: MAC layer (Media Access Control) and LLC layer (Logical Link Control). The MAC sub-layer handles how the network accesses data and permissions to transmit data; the LLC sub-layer is in charge of flow control, checking errors, and synchronization of frames. Examples of what’s at work in this layer include: Frame Relay, ATM, HDLC, PPP, FDDI, and IEEE 802.5/ 802.2, IEEE 802.3/802.2,
1. Physical Layer
This is the lowest layer in the OSI model – and it’s fundamental to all the layers above it. This layer is responsible for ensuring your physical assets — hardware, routers, switches, patch panels, cabling, etc. — are powered on and able to send/receive data.
What is the TCP/IP Model?
This is a model with may names. Also referred to as the ‘Internet Protocol Suite’, it was developed by the Department of Defense in the 1960’s — so you may sometimes hear it referred to as the ‘Department of Defense (DoD) model’. The four layers in this model include:
1. Process/Application Layer
2. Host-to-Host/Transport Layer
3. Internet Layer
4. Network Access/Link Layer
The above four layers in this model cover, end-to-end, how data should be communicated across the network: packetized, addressed, transmitted, routed, and received. The TCP/IP Model (aka Transmission Control Protocol/Internet Protocol) is considered a consolidated version of the original OSI model as it only has four layers.
Congratulations: you’ve now taken a crash course on the OSI Model. How’d you do? If you’d like to explore the exciting world of the OSI model in greater detail, here’s some further reading that will help you out on your journey: