OSI Model

The OSI Model is an abstract reference that makes it easy for professionals to talk about network operation and performance.

OSI Model Introduction

OSI stands for Open Systems Interconnection. The OSI Model is a reference model that makes it easy for professionals to talk about the way networks operate by breaking them down into seven layers that describe both hardware and software. Each layer provides specific services to the layer above it and either sends or accepts data from the layer below it depending on whether we are referring to the sending device or the destination that receives the data.

Protocol Data Units, or PDUs, are the packets of data that we can refer to at each layer. As data is sent, it begins at the top of the OSI model and works its way down through each layer until it reaches the physical medium. Each layer adds its own PDU to the existing data along with other helpful information that directs the data where to go and how to get there. On the receiving end, the data is separated as it works its way up to the the end user. This process is called encapsulation, and is very similar to stuffing an envelope with data and then adding the necessary information so the post office knows exactly what to do with it. I will provide more specific details about this process in a later lesson.

Below is a table that lists the seven layers of the OSI model, briefly describes what they do and shows you the Protocol Data Unit associated with each layer. Often times, networking professionals will refer to devices and functions by their layer number instead of the layer name, so it is important to fully understand what number belongs to what layer. For example, switches generally work at Layer 2, but at times can work at Layer 3. Such a device would be referred to as a Layer 3 switch.

OSI Model Layers

# Layer Name Function PDU
7 Application Determines what interfaces can be used for network communications and tells the Presentation layer what data format is needed for various uses . Application PDUs
6 Presentation Formats data based on the needs of the Application layer and manages the way it is presented as it travels up and down the OSI model. Presentation PDUs
5 Session Creates, maintains, and terminates connections, allowing communications to start or stop. Session PDUs
4 Transport Ensures reliable connections and transmission of data between the sender and receiver. Segments
3 Network Responsible for addressing and routing of data between the sender and receiver. Packets
2 Data Link Manages point to point connections over the physical medium and takes care of the sequencing of data. Frames
1 Physical The actual medium being used to transmit data, such as cables or wireless radio waves that send signals back and forth. Bits

The OSI Model is a very common network reference model, but it is not the only one. The next lesson will be about the four layers of the TCP/IP Model along with the functions and protocols associated with each layer.