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:
- Stage 1 – Encapsulation
- Stage 2 –
Transmission over the physical medium
- Stage 3
– Network devices utilizing control information to deliver data
to the appropriate end-system
- Stage 4 –
Decapsulation of data as needed for use with the target
application
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