After all symptoms are gathered, if no solution is identified, the network administrator compares the characteristics of the problem to the logical layers of the network to isolate and solve the issue.
Logical networking models, such as the OSI and TCP/IP models, separate network functionality into modular layers. These layered models can be applied to the physical network to isolate network problems when troubleshooting. For example, if the symptoms suggest a physical connection problem, the network technician can focus on troubleshooting the circuit that operates at the physical layer. If that circuit functions as expected, the technician looks at areas within another layer that could be causing the problem.
OSI Reference Model
The OSI reference model provides a common language for network administrators and is commonly used in troubleshooting networks. Problems are typically described in terms of a given OSI model layer.
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 upper layers (5 to 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 to 4) of the OSI model handle data-transport issues. Layers 3 and 4 are generally implemented only in software. The physical layer (Layer 1) and data link layer (Layer 2) are implemented in hardware and 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.
Figure 1 shows some common devices and the OSI layers that must be examined during the troubleshooting process for that device. Notice that routers and multilayer switches are shown at Layer 4, the transport layer. Although routers and multilayer switches usually make forwarding decisions at Layer 3, ACLs on these devices can be used to make filtering decisions using Layer 4 information.
TCP/IP Model
Similar to the OSI networking model, the TCP/IP networking model also divides networking architecture into modular layers. Figure 2 shows how the TCP/IP networking model maps to the layers of the OSI networking model. It is this close mapping that allows the TCP/IP suite of protocols to successfully communicate with so many networking technologies.
The application layer in the TCP/IP suite actually combines the functions of the three OSI model layers: session, presentation, and application. The application layer provides communication between applications, such as FTP, HTTP, and SMTP on separate hosts.
The transport layers of TCP/IP and OSI directly correspond in function. The transport layer is responsible for exchanging segments between devices on a TCP/IP network.
The TCP/IP Internet layer relates to the OSI network layer. The Internet layer is responsible for placing messages in a fixed format that allows devices to handle them.
The TCP/IP network access layer corresponds to the OSI physical and data link layers. The network access layer communicates directly with the network media and provides an interface between the architecture of the network and the Internet layer.