By Joe Hertvik June 01, 2023
The Open System Interconnection model (OSI Model) is a foundational concept that shapes how we build digital environments. The OSI Model is a conceptual framework that describes how different computer systems communicate with each other inside network or cloud/internet environments.
Today, let’s look at how the OSI Model affects our digital lives, applications and networks.
Why we have an OSI model
The OSI model was first adopted in 1984 by the International Organization for Standardization (ISO) when networking was ruled by competing technologies such as Ethernet, Token Ring, FDDI and ARCNET.
Like today, there were many different incompatible machine technologies and operating systems from companies such as IBM, Burroughs, Univac, Hewlett Packard, and more. In the early 1980s, there were no agreed-on standards or blueprints for how two different computer systems could exchange data.
The OSI Model was created to provide a conceptual framework for how diverse computer systems using different technologies can talk to each other. The OSI Model abstracts and describes the activities, processes and standard protocols used for cross-system communication.
It helps communicate and visualize how digital communication operates for a wide variety of uses including design, engineering, marketing, documentation and more. It is also used for troubleshooting and isolating network issues.
The OSI Model today
The OSI Model isn’t the only conceptual framework used in networks and the cloud and internet. The internet also comprehensively uses the TCP/IP framework model as another foundational set of communications protocols. Many TCP and TCP/IP protocols are referenced in the OSI Model.
Even today, manufacturers, engineers, vendors, users and others still reference the model to determine what components are necessary to make their systems talk to other systems. Both the OSI Model and the TCP/IP Model have been absorbed and internalized into digital lives at such a basic level that they are both explicitly and implicitly used without even thinking about it.
Understanding the OSI Model
Shown below the OSI Model is divided into seven layers that describe the activities and processes needed for disparate computer systems to communicate with each other over networks and the cloud/internet.
Usually presented in reverse order from top to bottom (layer 7 to layer 1), the OSI Model describes the transmission path that data takes from the end-user level (layer 7: Application Layer) to the physical transmission of bits across communication links and network cards (layer 1: Physical Layer) and all layers in between.
The OSI Model for how different computer systems communicate with one another
How data is transmitted in the OSI Model
Notice that the OSI Model is bi-directional. It is used by both the data sender and data receiver, and the two sides change roles during the transmission process.
As an example, when your browser requested the URL for this article (the downward arrow), it made the request first at the Application Layer (Layer 7). The requested data (URL and transmission information) is modified and travels down the OSI Model stack until it’s received at the Physical Layer (Layer 1) where the Splunk web servers received your request.
At Layer 1, the Splunk servers processed the request, retrieved the HTML code and transmission information, and transmitted the article back to the sender (the upward arrow). The returned data starts at the Physical Layer (Layer 1) and the article is transmitted and modified back up through the OSI model until it arrives at your browser (Application Layer, Layer 7) where you’re reading it right now.
Because the model is bi-directional, both sides — your browser and the Splunk Web servers — function as sender and receiver:
- Your browser sends the URL request and receives Web page content.
- The Splunk servers receive the URL request and send the Web page content.
This data transfer framework can be modeled and used by almost all computer systems in internal networks and in the cloud/internet.
And that’s how the OSI Model defines how data is transferred.
Breaking down the OSI Model
Looking at the OSI Model, notice that the seven layers are further grouped into three higher layers. These groupings allow us to generally refer to OSI model function — software, Heart of OSI, hardware — without getting into the specific functions of each layer.
Software layers
The Application, Presentation, and Session layers (layers 7, 6, and 5) are collectively referred to as Software Layers of the model. This is where all the transmission activity associated with software apps occurs, including:
- Operating systems (OS)
- Utilities such as Web browsers, services, custom written apps, etc.
Heart of OSI
The Transport Layer (layer 4) is also referred to as the Heart of OSI. This is the layer where the actual transmission between different systems occurs.
Hardware layers
The Network, Data Link, and Physical layers (layers 3, 2 and 1) are collectively referred to as the Hardware Layers of the model. This is the journey the data takes through the physical components on each system as it is processed.
The 7 layers of the OSI Model
Now let’s look at each individual layer. The seven layers of the OSI Model reduce the design complexity of networked systems. They each describe the sub-functions of the Software, Heart of OSI, and Hardware Layers.
Software Layers
Application Layer (Layer 7)
The application layer is the closest layer to the end user. It receives information from the end user and sends results back to the user. Despite its name, Layer 7 is not where client applications live. This layer provides the protocols that allow software/apps to transmit data, including:
- HTTP and HTTPS
- FTP
- POP & SMTP
- DNS
- Telnet
- DHCP
- SNMP
Presentation Layer (Layer 6)
The presentation layer ensures the data is prepared in a usable form for the application layer (receiving side) or for the network layer (sending side). Layer 6 is responsible for:
- Data translation
- Encryption & decryption
- Compression
- Other data preparation items
Session Layer (Layer 5)
The session layer creates and maintains the sessions (connections) that two systems need in order to speak to each other. Layer 5 defines…:
- When sessions are created and opened
- How long sessions remain open to successfully exchange data
- When to close sessions
- And more
It also creates checkpoints to ensure and synchronize data transfer.
Heart of OSI Layer
Transport Layer (Layer 4)
The transport layer uses transmission protocols including Transmission Control Protocol (TCP) and User Datagram Protocol (UDP), to manage network traffic between systems to ensure correct data transfers.
- On the sending side, it takes data from the session layer and breaks it into segments for the network layer.
- On the receiving side, it reassembles the segments from the network layer and passes them on to the session layer.
Layer 4 also handles flow control and error control, regulates transmission speed and requests retransmissions if needed.
Hardware Layers
Network Layer (Layer 3)
The network layer decides which physical path the data will take. It’s responsible for breaking up transport layer segments into smaller network packets for transmission and for reassembling those packets on the receiving system. This session routes packets to their destination, mostly by using IP addressing.
Layer 3 processing is generally bypassed when the sending and receiving systems are on the same network.
Data Link Layer (Layer 2)
The data link layer defines the format of data on the network. Like the network layer, the data link layer enables data transfer between two directly connected nodes or systems on the same network. Layer 2 also corrects errors that may have occurred at the physical layer (layer 1).
It uses media access control (MAC) processing for flow control and multiplexing between two systems. It also uses logical link control (LLC) to provide flow control and error control.
Physical Layer (Layer 1)
The physical layer converts and transmits raw bit stream data (1s and 0s) over the physical medium. Layer 1 concerns the physical and electrical connections the system uses. It includes:
- Wireless frequency links, like Wi-Fi and wireless network connections
- Network cabling
- Light-speed transmission, such as fiber-optic cabling
- The physical specifications for data transmission, including voltages and pin layouts
The physical layer also discusses network components such as hubs, repeaters, modems, network adaptors, etc.
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