In this lesson, we'll explore the seven layers comprising the OSI model with a focus on the transport layer. During our discussion, we'll define the transport layer, examine its functions and go over the supporting network protocols.
Layers of the OSI Model
Before beginning our transport layer discussion in detail, let's take a look at the networking environment in which the transport layer exists: the OSI or Open Systems Interconnect model. The OSI model includes seven protocol layers, with each layer performing a specific and supportive communicative task. In this table, the names and descriptions of the layers are listed in descending order from the top layer to the bottom layer. Each layer provides inputs and outputs to and from the layers above and below it, if present.
| Layer | Layer Name | Layer Description |
|---|---|---|
| Seven | Application | Provides access to available network resources |
| Six | Presentation | Translates, encrypts and compresses data |
| Five | Session | Establishes, manages and terminates communicative sessions |
| Four | Transport | Provides reliable process-to-process message delivery and error recovery |
| Three | Network | Moves packets from source to destination providing inter-networking capabilities |
| Two | Data Link | Organizes bits into frames providing node-to-node delivery |
| One | Physical | Transmits bits over a transmission medium establishing mechanical and electrical specifications |
Transport Layer
Imagine that you've just sat down at your computer to check your bank account or place an order from your favorite retailer. Initially your request descended through the 'application', 'presentation' and 'session' layers, or layers seven, six and five respectively. Arriving at layer four, or the transport layer, your request was broken down into small data packets and delivered to the remaining three layers: 'network', 'data link' and 'physical', or layers three, two and one respectively, where your request is set for delivery. When the data packets, formulated at the transport layer, arrived at the financial institution's internet server, the packets were processed by each layer in ascending order, beginning with the physical layer and ending with the application layer. As a result, you can view your requested information on your computer.
Services
As layer four of the OSI model, the transport layer is responsible for providing communication services between computers on a network. For example, as the data packets transit between the originating or sending computer and the destination or receiving computer, the transport layer performs error checking and data packet routing services. Additionally, while working with the three lower and three upper layers, the transport layer continues to provide services, even though different application software processes are running simultaneously on multiple computers.
Functions
There are six major functions performed by the transport layer; these functions are identified and described in this table:
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