technique is that twice the signaling bandwidth is required, since there must be two pulses for every bit transmitted. As a result, 10-Mbps Ethernet actually works with a 20 MHz serial data signal. Data moving through the physical layer medium from the source to the destination is the end product of the encapsulation process.
Content 2.1 Using a Layered Architectural Model to Describe Data Flow 2.1.3 Network devices utilize control information Layer 2 network devices utilize the control information within a frame to assess where a frame is physically destined to on a local network segment. The physical address, or MAC address, of the destination network adapter, or interface, is read so that the proper decision on switching to an appropriate port can be made. In addition to addressing information, the Layer 2 device can check on the validity of the frame by recalculating the frame check sequence (FCS) and matching it with the FCS included as part of the encapsulation process at the data-link layer. Layer 3 network devices are responsible for determining logical paths between networks through an internetwork. Layer 3 devices read the networking address of a destination contained within the control information of packets, and then forward them to an appropriate interface. Layer 3 addressing is hierarchical so that intermediate devices need only know which network the destination device is a member of in order to deliver the packet to the correct location. Data flow alternates between the physical medium which is stage two of data flow, and Layer 2 and 3 devices representing the third stage in the flow of data from a source to a target end-system.
Content 2.1 Using a Layered Architectural Model to Describe Data Flow 2.1.4 Decapsulation When the interface of an end-system receives data from the physical medium, frames must be extracted from the bit-stream so that the end-system can verify that the destination physical address of the frame equals its own. When the physical address is verified, the packet is decapsulated from the frame control information and the packets logical control information is examined. Data is further decapsulated from packets as needed for use with the target application. This represents the fourth stage in the layered model of data flow. Data returned to the original sender goes through the same process:
Content 2.1 Using a Layered Architectural Model to Describe Data Flow 2.1.5 OSI model versus TCP/IP model Similar to the OSI networking model, the TCP/IP networking model divides networking architecture into modular layers. Figure 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 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. TCP/IP Internet layer relates to the OSI Network layer. The Internet layer of the TCP/IP protocol model is responsible for placing messages in a fixed format that allows devices to handle them. The transport layers of TCP/IP and OSI directly correspond in function. The transport layer is responsible for exchanging packets between devices on a TCP/IP network. The application layer in the TCP/IP suite actually combines the functions of the three OSI model layers which are session, presentation, and application. The application layer provides communication between applications such as FTP, HTTP, and SMTP on separate hosts.
Content 2.1 Using a Layered Architectural Model to Describe Data Flow 2.1.6 Position of network devices in layered model The ability to identify which layers pertain to a networking device gives a troubleshooter the ability to minimize the complexity of a problem by dividing the problem into manageable parts. For instance, knowing that Layer 3 issues are of no importance to a switch, aside from multilayer switches, defines the boundaries of a task to Layer 1 and Layer 2. Given the fact that there is still plenty to consider at only these two layers, this simple knowledge can prevent the wasting of time troubleshooting irrelevant possibilities and will significantly reduce the amount of time spent attempting to correct a problem. However, it is still important to note that there are network applications that are part of these devices that move into Layers 4-7.
Content 2.2 Troubleshooting Approaches 2.2.1 General troubleshooting process The stages of the general troubleshooting process are: Step 1 Gather symptoms Step 2 Isolate the problem Step 3 Correct the problem The stages are not mutually exclusive. At any point in the process, it may be necessary to retrace to previous steps. For instance, it may be required to gather more symptoms while isolating a problem. Additionally, when attempting to correct a problem, another unidentified problem could be created. As a result, it would be necessary to gather the symptoms, isolate, and correct the new problem. A troubleshooting policy should be established for each stage. A policy will give a consistent manner in which to perform each stage. Part of the policy should include documenting every important piece of information. Gathering Symptoms – To perform the "Gathering Symptoms" stage of the general troubleshooting process, the troubleshooter gathers and documents symptoms from the network, end systems, or users. In addition, the troubleshooter determines what network components have been affected and how the functionality of the network has changed compared to the baseline. Symptoms may appear in many different forms. These forms include alerts from the network management system, console messages, and user complaints. While gathering symptoms, questions should be used as a method of localizing the problem to a smaller range of possibilities. However, the problem is not truly isolated until a single problem, or a set of related problems, is identified. Isolation of Problem – To perform the "Isolate the Problem" stage of the general troubleshooting process, the troubleshooter identifies the characteristics of problems at the logical layers of the network so that the most likely cause can be selected. At this stage, the troubleshooter may gather and document more symptoms depending on the problem characteristics that are identified. Correct the Problem – To perform the "Correct the Problem" stage, the troubleshooter corrects an identified problem by implementing, testing, and documenting a solution. If the troubleshooter determines that the corrective action has created another problem, the attempted solution is documented, the changes are removed, and the troubleshooter returns to gathering symptoms and isolating the problem.
Content 2.2 Troubleshooting Approaches 2.2.2 Bottom-up When applying a bottom-up approach towards troubleshooting a networking problem, the examination starts with the physical components of the network and then is worked up through the layers of the OSI model until the cause of the problem is identified. It is a good approach for a troubleshooter to use when the problem is suspected to be physical. Most networking problems reside at the lower levels, so implementing the bottom-up approach will often result in effective results.The downside to selecting this approach is that it requires checking of every device and interface on the