A Guide to the OSI Model

The OSI model is one of the most important topics when it comes to computer networking. The OSI model, or Open Systems Interconnect model, helps us understand how data transfers across a network. Each of the model's seven abstraction layers plays a distinct role in the process. In this guide to OSI, learn about each of the layers and how they work together to function as one model.

What is the OSI model?

The Open Systems Interconnect (OSI) model is a conceptual framework for understanding how data flows over a network. It’s a reference model which acts as a roadmap for what’s happening across and within a network.

The OSI was developed by ISO to guide developers in understanding modern computer network technology in a connection-oriented way. Technology vendors can now create both software programs and digital communication products in line with a clear framework that describes how a network functions.

After its creation in 1984, all major network companies worldwide adopted the model. Though the modern Internet is based on the simpler TCP/IP model, the ISO model is still relevant. It moves beyond the TCP/IP model to help troubleshoot network problems when things go wrong.

Using the OSI 7-layer model, users can get a big-picture idea of how networks operate. The model divides the communication process into seven layers, each assigned a task to support the layers above and immediately below it. However, each self-contained layer performs its functions independently.

What is the purpose of the OSI model?

The primary purpose of the OSI model is to help network professionals, developers, and technology vendors create interoperable products and software. It’s also a reference guide for network architecture; by knowing the model, you can visualize the complex interactions occurring on a network.

Most telecommunications manufacturers describe a network's functions in relation to the ISO model. From a design perspective, the easiest way to have complete visibility of what is happening within a network is to split up communication systems into seven manageable layers.

All layers of the model are defined according to specific jobs that they handle. However, each of the layers works collaboratively to pass data onto the next layer. To accomplish the transmission of data from one host to the other, each module must communicate with the layer before it.

How the OSI model works

Though it can be difficult for humans to conceptualize what a computer does, the OSI reference model lets people picture what’s going on over the Internet. The model is thought to be a universal language for IT professionals.

Essentially, the OSI model breaks down data transmission processes over a series of related layers, each one stacked upon the other. The model standardizes communication from one device to another in a network.

The communication process is divided into seven layers of function. Each layer performs specific tasks and also supports the layer above and below it. For example, if you send an email, the data passes through the layers in your computer, across the network, and then compresses to arrive at the receiving device.

The 7 layers of the OSI model

The seven layers of the OSI model are as follows: the physical layer, the data link layer, the network layer, the transport layer, the session layer, the presentation layer, and the application layer.

1. Physical layer

The lowest layer of the OSI structure is called the physical layer. Its main function is to transport raw data across physical hardware such as Ethernet cables. Protocols encountered in layer 1 include RS232, ATM, and FDDI.

Most administrators use the layer to check cable connections. This includes the type of cable used, the type of connector, and the cable length.

2. Data link layer

The data link layer’s key responsibility is to transfer data frames without errors. It corrects any errors coming from the physical layer, and in turn, there’s reliable communication between two or more physically connected devices.

In this layer, data is packed into frames and transferred from one network node to another. The layer is further divided into two sublayers: the Media Access Control (MAC) sublayer, which helps track data frames using the source and destination MAC addresses, and the Logical Link Control (LLC) sublayer, which is responsible for error control, multiplexing, and identifying line protocols.

3. Network layer

Once layer 4, the transport layer, has completed its functions, the data passes to layer 3 – the network layer. The layer handles the routing of data across different networks. It takes care of packet forwarding and decides the shortest route the packets can take.

The network layer is also home to IP (Internet Protocol) addresses. It manages the mapping between physical and logical addresses. Before the data transmits, the layer identifies the IP address of both the sender and the receiver to ensure packets arrive at the correct address.

Routers also operate at this layer. The layer utilizes protocols like TCP/UP, AppleTalk DDP, IP, and IPX.

4. Transport layer

Layer 4, or the transport layer, coordinates data transfers between hosts. It offers point-to-point communication on the network. It also manages data from the fifth layer.

Typically, data collected from the session layer is divided into small units called segments, which are then sent to the third layer. The transport layer also decides if data transmission should be a single or parallel path.

This layer succeeds at end-to-end delivery of messages, and if something goes wrong during deliver, the layer checks for errors. Other transport layer tasks include flow control, error control and error recover, multiplexing and demultiplexing, and realignment of segmented data.

The layer uses TCP and UDP protocols to perform its tasks, meaning that the data transmits in either connection-oriented (TCP) or connectionless (UDP) manners.

5. Session layer

The session layer, or layer 5 of the OSI model, works to establish, maintain, and terminate connections between different applications. In other words, it creates communication channels – called sessions – between devices before it sends data over a network.

The layer opens sessions, ensures they function properly, and then closes them when the data transfer successfully completes. Synchronization of data flow also happens at this layer. It establishes checkpoints in the communication to avoid data loss. Layer 5 protocols include NetBios, RPC, SQL, and NFS (Network File System).

6. Presentation layer

The presentation layer is responsible for preparing data in a format understood by applications, devices, and networks. It converts incoming data from one format to another based on the syntax or semantics that the application accepts.

In general, the layer prioritizes three tasks: translation, data compression, and encryption and decryption. It translates data sent over the network, then compresses to reduce the data size without loss. The layer reduces the number of bits transmitted over the network when performing data compression. Layer 6 also manages data encoding, encryption, and decryption for security.

7. Application layer

Most end users are familiar with the application layer. Layer 7 is what you see when you use your computer application programs, meaning the application layer is the most visible. Every time you perform network-related tasks like sending emails or reading files and messages, you’re active on Layer 7 of the OSI model.

The layer’s main functions include identifying resources, identifying communication partners, and synchronizing communication. Web browsers, apps like Skype, and email clients like Outlook all make their homes on this layer. Many protocols – including HTTP/HTTPS, SMTP, POP3, and FTP – also function on the seventh layer.

Advantages and disadvantages of the OSI model

Like all procedures and models, the functioning of the OSI model has both advantages and disadvantages.

Advantages

There are several reasons why the OSI model still matters today. It helps with connections across networks and, furthermore, it helps users understand how networks work. Here are just a few of the advantages of the OSI model.

• All layers are self-contained and can function independently, meaning any changes in one layer won’t affect the others.
• The model helps managers narrow down network problems by isolating the source of trouble.
• Using the OSI model allows you to standardize the motherboard, switch, router, and other hardware.
• Vendors and developers use the model to create interoperable technology.
• The model supports both connection-oriented and connectionless services.
• It facilitates modular engineering.
• It provides a big picture of how hardware and software elements work together.

Disadvantages

While the OSI model is vital in computer networking, it has a few disadvantages.

• OSI is theoretical, so it has a restricted practical implementation.
• Some of the OSI layers, like the session layer and presentation layer, aren’t useful when practically implemented.
• The complexity of the OSI structure potentially leads to confusion.
• It lacks a protocol definition.
• The model doesn’t support a parallel process.

In addition, the model is ultimately less important than the TCP/IP model since most of the Internet functions after that model.

How is the OSI model used?

Users utilize the OSI model in several ways. It’s a useful conceptual model for troubleshooting network problems, for one; security teams use the OSI seven-layer model to identify and solve technical errors. For example, if a website does down, the OSI model helps identify the potential problem quickly.

It also serves as a unified standard for manufacturers. Manufacturers recognize the model, allowing them to build products that work together based off of it.

Finally, the model allows users of all levels to understand how a network operates. Using a layered model like OSI allows users to comprehend, differentiate between, and pinpoint essential network processes.

Comparing the OSI model to the TCP/IP model

The OSI model and the TCP/IP model are the two most broadly-used networking models for communication. Both define sets of standards for networking, simplify the network communication process, and enable vendors to make network components that work together. The two are similar models, but the layers are slightly different. Though the TCP/IP model is typically held as the standard over the OSI model, the OSI model still helps improve networking knowledge.

Parameter	OSI Model	TCP/IP Model
Acronyms	OSI stands for Open Systems Interconnection	TCP/IP stands for Transmission Control Protocol/Internet Protocol
Developed by	ISO in 1984	The Department of Defense in 1974
Number of layers	Seven layers	Four layers
Layer separation	The model has a separate session layer and presentation layer	The model does not have separate session or presentation layers
Model concept	Based on three concepts: service, interface, and protocol	The model doesn't distinguish between service, interface, or protocol
Header size	5 bytes	20 bytes

Frequently asked questions

What are the layers of the OSI model?

The OSI model layers are the physical layer, the data link layer, the network layer, the transport layer, the session layer, the presentation layer, and the application layer. Because the layers technically work in inverse order, many remember the layers by the mnemonic device “All People Seem to Need Data Processing,” which represents layers 7 through 1 in that order.

Which came first, the TCP/IP model or the OSI model?

The TCP/IP model preceded the OSI model. Scientists published it in 1974, whereas the International Standards Organization (ISO) published the OSI model in 1984. The former has four layers, but the latter has seven.

When was OSI proposed and finalized?

The OSI network model was developed by the ISO in 1984 to provide design standards for communication equipment. Since then, the American Institute of Electrical Engineers and the International Telecommunications Union have since collaborated with the ISO to advance the OSI model template for communication systems.

What layers of the OSI model do firewalls operate at?

Firewalls typically operate at the network layer or transport layer in the model.

How many layers are in the OSI model?

There are seven OSI network model layers.