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| 2.1 |  | Using a Layered Architectural Model to Describe Data Flow |
| 2.1.1 |  | Encapsulating data |

Logical networking models separate network functionality into modular layers. These modular layers are applied to the physical network to isolate network problems and even create divisions of labor. For example, if the symptoms of a communications problem suggest a physical connection problem, the telephone company service person can focus on troubleshooting the T1 circuit that operates at the physical layer. The repair person does not have to know anything about TCP/IP, which operates at the network layer, or attempt to make changes to devices operating outside of the realm of the suspected logical layer. The repair person can concentrate on the physical circuit. If it functions properly, then either the repair person or a different specialist looks at areas in another layer that could be causing the problem.

The Open Systems Interconnection (OSI) model provides a common language for network engineers. Having looked at using a systematic approach, documentation, and network architectures, it can be seen that the OSI model is pervasive in troubleshooting networks. The model allows troubleshooting to be described in a structured fashion. Problems are typically described in terms of a given OSI model layer. At this stage it is assumed that there should be an intimate familiarity with the model. Taking a quick look at the OSI model helps clarify its role in troubleshooting methodology.

The OSI reference model describes how information from a software application in one computer moves through a network medium to a software application in another computer. The OSI reference model is a conceptual model composed of seven layers, each specifying particular network functions. With this technique, one transition is guaranteed for each bit cycle, or bit time. The model was developed by the International Organization for Standardization (ISO) in 1984, and it is now considered the primary architectural model for intercomputer communications. The OSI model divides the tasks involved with moving information between networked computers into seven smaller, more manageable task groups. A task, or group of tasks, is then assigned to each of the seven OSI layers. Each layer is reasonably self-contained, so that the tasks assigned to each layer can be implemented independently. This enables the solutions offered by one layer to be updated without adversely affecting the other layers. The figure details the seven layers of the Open System Interconnection reference model.

The OSI model provides a logical framework and a common language used by network engineers to articulate network scenarios. The Layer 1 through Layer 7 terminology is so common that most engineers do not think twice about it any more.

The upper layers (5-7) of the OSI model deal with application issues and generally are implemented only in software. The application layer is closest to the end user. Both users and application layer processes interact with software applications that contain a communications component.

The lower layers (1-4) of the OSI model handle data-transport issues. The physical layer and data-link layer are implemented in hardware and software. The other lower layers generally are implemented only in software. The physical layer is closest to the physical network medium, such as the network cabling, and is responsible for actually placing information on the medium.

When sending data from an application in one host to an application in a second, the network software on the source host takes data from an application and converts it as needed for transmission over a physical network. The process involves:

- Converting data into segments
- Encapsulating segments with header information that includes logical network addressing information, also the process of converting segments into packets
- Encapsulating packets with a header, including physical addressing information, and converting packets to frames
- Encoding frames into bits

The data is now ready for travel over the physical medium as bits. The encapsulation process as a whole represents the initial stage in transferring data between two end systems.