Table 5-3 The Layers of the OSI Model–Cisco Communication Principles
| OSI Model Layer | Description |
| 7–Application | The application layer contains protocols used for process-to-process communications. |
| 6–Presentation | The presentation layer provides for common representation of the data transferred between application layer services. |
| 5–Session | The session layer provides services to the presentation layer to organize its dialogue and to manage data exchange. |
| 4–Transport | The transport layer defines services to segment, transfer, and reassemble the data for individual communications between the end devices. |
| 3–Network | The network layer provides services to exchange the individual pieces of data over the network between identified end devices. |
| 2–Data Link | The data link layer protocols describe methods for exchanging data frames between devices over a common media. |
| 1–Physical | The physical layer protocols describe the mechanical, electrical, functional, and procedural means to activate, maintain, and de-activate physical connections for bit transmission to and from a network device. |
Upper and Lower Layers of the OSI Model (5.3.6)
You can visualize how data moves across a network by using the seven layers of the OSI model, as shown in Table 5-3. The OSI model breaks down network communication into multiple processes, as shown in Table 5-4. Each process is a small part of the larger task.
Table 5-4 Common Components of the Layers of the OSI Model
| Group | Layer Number | Layer Name | Common Network Components Associated with This Layer |
| Upper Layers | 7 | Application | Network-aware applications |
| 6 | Presentation | ||
| 5 | Session | Web browsers and servers File transfer Name resolution | |
| Lower Layers | 4 | Transport | Video and voice streaming mechanisms Firewall filtering lists |
| 3 | Network | IP addressing Routing | |
| 2 | Data Link | Network interface cards and drivers Network switching WAN connectivity | |
| 1 | Physical | Physical medium (copper twisted-pair, fiber-optic cables, wireless transmitters) |
For example, in a vehicle manufacturing plant, the entire vehicle is not assembled by one person. Rather, the vehicle moves from station to station where specialized teams add specific components. The complex task of assembling a vehicle is made easier by breaking it into manageable and logical tasks. This process also makes troubleshooting easier. When a problem occurs in the manufacturing process, it is possible to isolate the problem to the specific task where the defect was introduced and then fix that problem.
In a similar manner, the OSI model helps you troubleshoot by focusing on a specific layer to identify and resolve network problems. Networking teams often refer to different functions occurring on a network by the number of the OSI model layer that specifies that functionality. For example, the process of encoding the data bits for transmission across the media occurs at Layer 1, the physical layer. The formatting of data so it can be interpreted by the network connection in your laptop or phone is described at Layer 2, the data link layer.
OSI Model and TCP/IP Model Comparison (5.3.7)
Because TCP/IP is the protocol suite in use for Internet communications, why do you need to learn the OSI model as well? The TCP/IP model is a method of visualizing the interactions of the various protocols that make up the TCP/IP protocol suite. It does not describe general functions that are necessary for all networking communications. It describes the networking functions specific to those protocols in use in the TCP/IP protocol suite. For example, at the network access layer, the TCP/IP protocol suite does not specify which protocols to use when transmitting over a physical medium, nor the method of encoding the signals for transmission. OSI Layers 1 and 2 discuss the necessary procedures to access the media and the physical means to send data over a network.
The protocols that make up the TCP/IP protocol suite can be described in terms of the OSI reference model. The functions that occur at the Internet layer in the TCP/IP model are contained in the network layer of the OSI model, as shown in Figure 5-7. The transport layer functionality is the same between both models. However, the network access layer and the application layer of the TCP/IP model are further divided in the OSI model to describe discrete functions that must occur at these layers.
Figure 5-7 The OSI and TCP/IP Models
The key similarities are in the transport and network layers; however, the two models differ in how they relate to the layers above and below each layer:
- OSI Layer 3, the network layer, maps directly to the TCP/IP Internet layer. This layer is used to describe protocols that address and route messages through an internetwork.
- OSI Layer 4, the transport layer, maps directly to the TCP/IP transport layer. This layer describes general services and functions that provide ordered and reliable delivery of data between source and destination hosts.
- The TCP/IP application layer includes several protocols that provide specific functionality to a variety of end-user applications. The OSI model Layers 5, 6, and 7 are used as references for application software developers and vendors to produce applications that operate on networks.
- Both the TCP/IP and OSI models are commonly used when referring to protocols at various layers. Because the OSI model separates the data link layer from the physical layer, it is commonly used when referring to these lower layers.