Published: 17/01/2022

OSI Model

The OSI Model (Open Systems Interconnection Model) is a conceptual model that represents how information from a webpage is transmitted throughout a network and  displayed by your browser.

The 7 layers of the OSI model

Physical layer

This layer consists of all physical components such as data cables, connectors, etc. which carry data in bits and bytes. Information passes through this layer during the transmission process.

The information then moves to this second layer where it is divided into individual packets, which are then labeled according to their destination router before being sent out across the network.

Network layer

Packets pass through this final link on their way to the recipient’s device, where they’re merged back together again using various routing protocols like RIP or OSPF.

Transport layer

This layer delivers the message into a buffer on the receiving computer’s memory storage temporarily, and then removes it once the message is received. It is also responsible for ensuring that the packets are put back together in the correct order when they reach their destination.

Session layer

This next level is where connections between two devices communicating over a network are established, maintained, and terminated as needed.

Presentation layer

This final level translates data from its raw form as bits and bytes into something we humans can understand such as letters, numbers, etc. so we can view web content on our browsers.

Application layer

These are the programs or services that we use on our computers to view information, such as email clients, web browsers, etc.

Cross-layer functions of OSI model

There are several other functions that take place across all seven layers of the OSI Model. Below are some examples:

Flow control

This process ensures that the receiving device doesn’t receive more data than it can handle. It does this by comparing a reference number with a window size to determine whether it should continue accepting packets from the transmitting device. This is typically performed on layers 2 and 3 of the OSI Model (Data Link and Network).

Error checking

This function takes place on all seven layers and provides a way for each layer to check if any errors occurred during transmission before allowing an error-free packet to be passed up to the next higher level. In some cases, it will request that the cycle be repeated using an alternate route if problems are detected.

Packet segmentation & reassembly

On layer 4 of the OSI Model (Transport), data is divided into smaller packets before being transmitted to the receiving device can reassemble them back together when they arrive.

The OSI model vs the TCP/IP model

The OSI model is used to describe how data passes through a network between two devices and the TCP/IP model is designed to help devices communicate with each other.

Even though both models come into play when discussing computer networks, they were developed for different purposes and therefore only partially overlap:

  • OSI model is used to describe the process of transferring data between computers over a network while TCP/IP model is designed to help devices communicate with one another.
  • The OSI layer numbers don’t correspond directly to the TCP/IP layers because some functions are combined into one layer in the latter whereas they’re split into multiple layers in the former.
  • SNMP, SMTP, and HTTP are protocols that belong to application, presentation, and session respectively so both models have these three protocols at their topmost level even though they sit at different levels within each model.
  • Network resources addressed by IP addresses correspond with network resources addressed by MAC addresses so devices such as routers and switches, which communicate using MAC addresses, directly map to the data link layer of the OSI model.
  • There’s overlap between routing, internetworking, and switching because they all deal with how data moves around a network so while routing is exclusively a matter for the internet layer in TCP/IP, it spans multiple layers in the OSI model.
  • The two models don’t serve exactly the same purpose. While we can think of both models as blueprints for networks, there are many ways in which real-world networks diverge from them such as hubs vs switches or packet vs cell.

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