Basic Concepts of OSI Layer

The seven layers refer to the Open Systems Interconnection (OSI) model, a conceptual framework that characterizes and standardizes the communication functions of a telecommunication or computing system without regard to its underlying internal structure and technology. Its goal is the interoperability of diverse communication systems with standard protocols. To be precise, the OSI model is a tool used by IT professionals to model or trace the actual flow of how data transfers in networks. So, the OSI model is a logical model/representation of how the network systems are supposed to send data (or, communicate) to each other.

The model uses layers to help in giving a visual description of what is going on with a particular networking system. This can help network managers narrow down problems (Is it a physical issue or something with the application?), as well as computer programmers (when developing an application, which other layers does it need to work with?). Tech vendors selling new products will often refer to the OSI model to help customers understand which layer their products work with or whether it works “across the stack”.

Layer 7 - Application

The Application Layer is the one at the top - it’s what most users see. In the OSI model, this is the layer that is the “closest to the end-user”. Applications that work at Layer 7 are the ones that users interact with directly. A web browser (Google Chrome, Firefox, Safari, etc.) or other apps - Skype, Outlook, Office - are examples of Layer 7 applications.

Layer 6 - Presentation

The Presentation Layer represents the area that is independent of data representation at the application layer - in general, it represents the preparation or translation of application format to network format, or from network formatting to application format. In other words, the layer “presents” data for the application or the network. A good example of this is encryption and decryption of data for secure transmission - this happens at Layer 6.

Layer 5 - Session

When two devices, computers or servers need to “speak” with one another, a session needs to be created, and this is done at the Session Layer. Functions at this layer involve setup, coordination (how long should a system wait for a response, for example) and termination between the applications at each end of the session.

Layer 4 – Transport

The Transport Layer deals with the coordination of the data transfer between end systems and hosts. How much data to send, at what rate, where it goes, etc. The best-known example of the Transport Layer is the Transmission Control Protocol (TCP), which is built on top of the Internet Protocol (IP), commonly known as TCP/IP. TCP and UDP port numbers work at Layer 4, while IP addresses work at Layer 3, the Network Layer.

Layer 3 - Network

At the Network Layer, you’ll find most of the router functionality that most networking professionals care about. In its most basic sense, this layer is responsible for packet forwarding, including routing through different routers. You might know that your Boston computer wants to connect to a server in California, but there are millions of different paths to take. Routers at this layer help do this efficiently.

Layer 2 – Data Link

The Data Link Layer provides node-to-node data transfer (between two directly connected nodes) and also handles error correction from the physical layer. Two sublayers exist here as well - the Media Access Control (MAC) layer and the Logical Link Control (LLC) layer. In the networking world, most switches operate at Layer 2.

Layer 1 - Physical

At the bottom,we have the Physical Layer, which represents the electrical and physical representation of the system. This can include everything from the cable type, radio frequency link (as in 802.11 wireless systems), as well as the layout of pins, voltages and other physical requirements. When a networking problem occurs, many networking pros go right to the physical layer to check that all of the cables are properly connected and that the power plug hasn’t been pulled from the router, switch or computer, for example

How Data Flows through the OSI Layers

Each layer adds (or encapsulates) some form of header or trailer. (Layer 2, the Data Link layer, is responsible for adding a trailer.) The figure below shows the data flow from Client A to Client B. When the end system receives the unstructured bitstream from the physical wire, each layer removes the header information applicable to it until the application receives the data. The following diagram depicts what occurs in the OSI model's layers when an email is sent from Client A to Client B.

1. An application, such as an email program, creates data that will be sent by an end-user, such as an email message. The Application layer (layer 7) places a header (encapsulation) field that contains information such as screen size and fonts and passes the data to the Presentation layer (layer 6).

2. The Presentation layer places layer 6 header information. For example, the text in the message might be converted to ASCII. The Presentation layer will then pass the new data to the Session layer (layer 5).

3. The Session layer follows the same process by adding layer 5 header information, such as information that the Session layer will manage the data flow, and passes this data to the Transport layer (layer 4).

4. The Transport layer places layer 4 information, such as an acknowledgment that the segment was received in the header, and passes it to the Network layer (layer 3).

5. The Network layer places layer 3 header information, such as the source and destination address so the Network layer can determine the best delivery path for the packets, and passes this data to the Data Link layer (layer 2).

6. The Data Link layer places layer 2 header and trailer information, such as a Frame Check Sequence (FCS) to ensure that the information is not corrupt, and passes this new data to the Physical layer (layer 1) for transmission across the media.

7. The bitstream is then transmitted as ones and zeros on the Physical layer. It is at this point that the Physical layer ensures bit synchronization. Bit synchronization will ensure the end-user data is assembled in the correct order it was sent.

8. Steps 1 through 7 occurs in reverse order on the destination device. Client B collects the raw bits from the physical wire and passes them up to the Data Link layer. The Data Link layer removes the headers and trailers and passes the remaining information to the Network layer and so forth until data is received by the Application layer. Eventually, Client B will receive an email notification displaying a message to indicate that a new email message has been received.

Familiarize yourself with the OSI model and each layer's responsibility. You should be able to recognize the function of each layer of the OSI model. The seven layers of the OSI reference model are typically divided into two categories: upper layers (layers 4 through 7) and lower layers (layers 1 through 3).

As you can determine from the example of encapsulation, the OSI model provides a service that allows information to flow smoothly from one layer to another. Eventually, the information will be presented to the end device in a readable format.