Session Layer of the OSI Model: Functions, Protocols & Examples

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  • 0:01 What Is the Session Layer?
  • 0:55 What is the OSI Model?
  • 4:03 Session Layer…
  • 5:11 Lesson Summary
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Lesson Transcript
Instructor: Kent Beckert

Kent is an adjunct faculty member for the College of Business at Embry-Riddle Aeronautical University and has a Master's degree in Technical Management.

In this lesson, we will discuss the session layer, layer 5 of the OSI model. We will talk about how the session layer sets up, manages, and maintains multiple network communicative sessions, and you'll learn about APIs like NetBIOS and TCP/IP.

What Is the Session Layer?

Say you are planning and hosting a large party. Initially, the problems are minor—like small issues with the food or party favors—but as the party activity increases, time and management become critical. To manage the event and keep your party on track, you need additional helpers to interface with the caterer, florist, and baker. With the setup problems solved, the helpers continue providing assistance by giving management support ensuring each activity runs smoothly. At the party's conclusion, the helpers aid in cleaning up, closing party facilities, and terminating connections.

You may be wondering what a party has to do with networking, particularly the session layer? We can make a comparison between setting up, managing, and closing the party with the functions performed by the session layer, which is layer five of the seven layers comprising the Open Systems Interconnection (OSI) model.

What Is the Open Systems Interconnect Model?

Before we can begin our session layer discussion in greater detail, we need to look into the networking environment in which the session layer exists: the OSI model. The OSI model has seven protocol layers, with each layer performing a specific and supportive communicative task. For a brief recap, let's go over each of the layers from top to bottom:

  • Layer 7: the application layer - provides access to available network resources
  • Layer 6: the presentation layer - translates, encrypts, and compresses data
  • Layer 5: the session layer - establishes, manages, and terminates communicative sessions
  • Layer 4: the transport layer - provides reliable process-to-process message delivery and error recovery
  • Layer 3: the network layer, - moves packets from source to destination providing internetworking capabilities
  • Layer 2: the data link layer, - organizes bits into frames providing node-to-node delivery
  • Layer 1: the physical layer - transmits bits over a medium establishing mechanical and electrical specifications

Here, we see an illustration of data flow to and from the sending and receiving computers on a network. As seen here, the session layer provides communicative support to the OSI model's presentation (6) and transport (4) layers.

Understanding the Session Layer

Networking is defined as a grouping of two or more computers being linked together to exchange data. As layer 5 of the OSI model, the session layer is primarily responsible for setting up and managing communicative sessions, thereby enabling the sending and receiving of data for an undetermined length of time—similar to a telephone call. Additionally, the session layer is responsible for terminating sessions between software processes.

As was the case with our party helpers, the primary responsibilities of session layer protocols (our helpers) are to set up, control, maintain, and close communicative sessions. Common session layer helpers are often referred to as application programming interfaces (APIs).

A common example of an API is the network basic input/output system (NETBIOS), which allows applications on separate computers to communicate over a local area network (LAN). Another example is the use of transfer control protocol/internet protocol (TCP/IP), which standardizes communications. APIs such as these are of primary interest to programmers and developers of application software for completing network communication setup and simplifying management.

As part of the OSI model, the session layer determines what format will be used for data exchange between the engaging nodes on a network. As long as the established format is in place, the communications channel will maintain an open status. Looking back at your party, you had to set up a session (the party) by arranging and managing session formats with items such as the guest list, party favors, food, flowers, and baked goodies. Since you had helpers (APIs), you were able to control the environment, had a great session (party), and closed related activities gracefully.

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