What Is The OSI Model? The 7 Layers Of OSI Explained - TechTarget

How does data flow through the OSI model

During transmission, the OSI model determines how data flows from a sending device to a receiving device. In particular, the data from the sending device travels down the seven layers of the model, i.e., from the application layer (Layer 7) to the physical layer (Layer 1). It thens travel up the seven layers on the receiving end, i.e., from the physical layer to the application layer.

Suppose User A sends an email to User B. Here's how data flows through the OSI model.

At the sending end

User A's email client, e.g., Microsoft Outlook, sits on the application layer. It uses SMTP to pass the email message to the next layer, the presentation layer.

At the presentation layer, the email message gets compressed, encrypted and formatted. It is then passed on to the session layer.

At the session layer, a session is established between User A's email server and User B's email server. This connection remains open long enough to send the email from User A to User B.

Next, the data reaches the transport layer. Here, it is divided into smaller segments, which are sent on to the network layer. At the network layer, the segments are further divided into smaller packets, and each packet is assigned source and destination IP addresses to allow routing.

These packets travel to the data link layer, where they are further broken down into frames. The layer uses MAC addresses so the packets can move across local networks. Layer 2 also corrects any errors that occur as the data moves from Layer 7 to here.

Finally, the frames are delivered to the physical layer. Here, the data is converted into a bitstream of ones and zeros and sent through a physical medium, such as a fiber optic cable.

At the receiving end

The above process is reversed at the receiving end (User B).

First, the physical medium converts the received binary bitstream into frames that are passed to and reassembled into packets at the data link layer.

The data link layer passes the packets to the network layer, which checks that the packets have arrived correctly and also assembles them into larger segments.

These segments are passed to the transport layer. Here, the segments are reassembled or reordered as required.

The session layer maintains the session until User B fully receives the email from User A. It also passes on the reassembled email to the presentation layer. This layer decompresses, formats and decrypts the email, and it passes it on to the application layer.

Finally, the application layer delivers the email in human-readable format to User B's email client -- Outlook or something else. At this point, the email appears in User B's inbox, and they can read it on a compatible device -- desktop, laptop, tablet, smartphone, etc.