CompTIA Network+ Certification Study Guide part 60 ppt

We’ll be using the ISO’s OSI model to help us in troubleshooting network connectivity issues.. When one computer communicates with another one, data at the sending computer is passed fro

The first networking protocols were proprietary; that is, each networking vendor developed its own set of rules Computers using an individual vendor’s protocol would be able to communicate with each other, but not with computers that were using the networking product of a different vendor This had the effect of locking a business into a particular product; the business would always have to use the same vendor’s products to maintain compatibility

The solution to this problem was the development of protocols based

on open standards Organizations such as the International Organization

for Standardization (ISO) were charged with overseeing the definition and control of these standards and publishing them so they would be available

to any vendor that wanted to create products that adhered to them The advantage to the consumer is that no longer is he forced to patronize a single vendor The advantage to the vendor is that its products are more widely compatible, and therefore can be used in networks that started out using a different vendor’s products

A model provides an easy-to-understand description of the networking

architecture and serves as the framework for the standards The OSI model has become a common reference point for discussion of network protocols and connection devices

As we look at the OSI and DoD models, you’ll see that they both use

layers to represent areas of functionality In OSI terms, each of the layered specifications uses the services of the layer below it to build an enriched service The layered approach provides a logical division of responsibility,

where each layer handles only the functions that are specific to that layer You can think of this like the teamwork exhibited by a good assembly-line crew that’s building an automobile One worker may be responsible for fitting a wheel onto the axis, another for inserting and tightening the screws, and so forth There are several advantages to this type of working model:

Each worker only has to be concerned with his or her own area of

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responsibility

Each worker becomes extremely proficient at his or her particular

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job through constant repetition

Working together in sequence, the team of workers is able to

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produce the final product much more quickly and efficiently than one person or than a group of people with no assigned responsibilities could

How to Use the OSI Model in Troubleshooting 577

If something goes wrong (for instance, if a particular part was

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put on incorrectly), the supervisor knows who is to blame for the

problem

Likewise, when the networking protocols are divided into layers,

communication generally flows more smoothly, and when it doesn’t,

troubleshooting is easier because you are better able to narrow down the

source of the problem to a specific layer We’ll be using the ISO’s OSI model

to help us in troubleshooting network connectivity issues

reviewing the oSI Model

You should remember that the OSI model consists of seven layers When one

computer communicates with another one, data at the sending computer is

passed from one layer to the next until the physical layer finally puts it out

onto the network cable At the receiving end, it travels back up in reverse

order Although the data travels down the layers on one side and up the

layers on the other, the logical communication link is between each layer

and its matching counterpart, as shown in Figure 12.1

Here’s how it works: as the data goes down through the layers, it is

encapsulated or enclosed within a larger unit as each layer adds its own

header information When it reaches

the receiving computer, the process

occurs in reverse; the information is

passed upward through each layer,

and as it does so, the encapsulation

information is evaluated and then

stripped off one layer at a time The

information added by the network

layer, for example, will be read and

processed by the network layer on

the receiving side After processing,

each layer removes the header

information that was added by its

corresponding layer on the sending

side It is finally presented to the

application layer, and then to the

user’s application at the receiving

computer At this point, the data is

in the form it was in when sent by

the user application at the source

machine Figure 12.2 illustrates how

FIGurE 12.1 Each Layer of the OSI Model Communicates with the Corresponding Layer.

Application Sending Computer Receiving Computer

Presentation Session Transport Network Data Link Physical

Network Media

Application Presentation Session Transport Network Data Link Physical

the header information is added to the data as it progresses down through the layers

Establishing a Troubleshooting Strategy

This chapter will provide a number of examples of how to troubleshoot network issues The most important thing that you can do when troubleshooting is

to be organized and methodical in your approach to solving problems If you work in a rushed fashion, you’re likely to miss a crucial troubleshooting step

or forget what you did to solve the problem the next time it occurs

In general, you can break down the items necessary to troubleshoot a network issue into the following seven steps:

1 Identify the symptoms and potential causes The first step is

determining exactly what’s wrong, and ensuring that there is actually a problem For example, if you upgrade the speed of your company’s Internet connection so that a file transfer that used to

FIGurE 12.2 Each Layer Adds its Own Header Information.

How to Use the OSI Model in Troubleshooting 579

take 30 seconds now takes no time at all, a user might think that the

file transfer didn’t work In this case there really wasn’t an error, but

the user thought that there was a problem because something she

was used to had suddenly changed So your first step should be in

determining the exact symptoms of the problem Is a user unable to

access the Internet entirely, or is it only that she is unable to receive

her e-mail while Web browsing is working fine? Ask questions and

get very specific details on the issue Screenshots of error messages

can be helpful as well Having as much information as possible at

hand will help you determine the potential causes of an issue

2 Identify the affected area Is only one user having trouble

accessing the Internet, or is there an issue that’s affecting an

entire subnet or your entire network? If all users on a particular

subnet are experiencing connectivity troubles, you might suspect

the default gateway or the network connection between that

subnet and the rest of your network If the issue is restricted

to a single user, you’ll be more likely to examine the specific

user’s workstation to check for hardware failures or software

misconfigurations

3 Establish what has changed Once you’ve determined what the

problem is, you should try to find out if anything has changed

that may have caused the issue to surface If an issue is impacting

multiple machines, think about what may have changed across

the enterprise; if it is an issue that is isolated to a particular

machine then question the user as to what they may have

changed recently Often times a problem can seem to come out of

nowhere only to find out later on that other administrators were

making changes that you were not aware of Communication

between teams is critical in any organization and being aware

of what changes may be taking place in your organization are

important Knowing what has changed on a network will usually

give you a good starting point from which you can begin the

troubleshooting process

4 Select the most probable cause Based on the information you

gathered in the first three steps, and using the troubleshooting

tools we covered in Chapter 11, try to determine the most likely

reason why you are experiencing the particular problem Keep in

mind that your first guess might be incorrect and that there may be

more than one cause Also, don’t get discouraged if you have to run

through a process of elimination not just a first, but then a second and even third or fourth time before finding the true root cause

5 Implement an action plan and solution, including potential impact The first part of this seems straightforward enough – “Fix

the problem!” But many administrators will overlook the second half of the statement, which is just as important: you don’t want

to do something to fix one problem that will only end up causing another, potentially worse, problem So think carefully about the impact of any troubleshooting steps that you are going to take, especially if it involves steps that you’re taking on a server, router,

or other device that many users rely on Also, once you have devised a course of action for a solution be sure to think through what the ramifications of implementing the solution may be For example, you may decide that you need to stop and restart the WWW publishing service on your Microsoft Web server to solve an issue being experienced by one particular user, but you need to plan this carefully because it will affect every user that is connected to the Web server You should be even more careful in changing any configuration items on a server or router so that the change you’re making doesn’t create additional problems A good rule of thumb is

to follow any change management guidance that your organization may have in place and be thoughtful of any changes you make to production systems

6 Test the result In addition to determining if your solution actually

fixed the problem you were experiencing, you should also look at the overall connectivity of your network to make sure that you didn’t inadvertently create another issue by implementing the fix you chose in Step 5

7 Document the solution and process Even though you probably

think that you’ll never forget how to fix this problem, especially

if it’s one that you had to work at solving for hours and hours, the simple fact of the matter is that if you don’t write down your solution, odds are quite good that you won’t remember if the issue recurs many months down the line or won’t be able to reproduce it precisely You should document the information that you gathered in each of these steps: how the problem manifested itself, the systems that were affected, and the details about the solution and how you implemented it Most importantly, don’t forget to document any side effects that came about as a result of

Troubleshooting the Physical Layer 581

implementing the fix Taking the time to go through this step at

the time of resolution can save you hours down the road when you

encounter a similar issue and find that you need to retrace your

steps

In the next sections, we will begin discussing the layers of the OSI model

We will map each one back to the troubleshooting methodology discussed in

this section We will start with Layer 1 and work our way through Layer 7, and

within each section we will take the time to discuss components that exist

at each tier and how you may approach them Being able to bring together

the concepts of the OSI model, a solid troubleshooting methodology, and the

appropriate tools are the magical combination of ingredients that will enable

you to successfully tackle just about any network issue that may arise

TrouBlEShooTING ThE phySICAl lAyEr

In many ways, the physical layer is the easiest to troubleshoot because it deals

with devices and concepts that are concrete and tangible – you can actually

see the network cabling and NICs that exist at the physical layer, whereas

concepts such as IP addresses and Media Access Control (MAC) addresses are

intangible items that are simply configured on the physical devices

The physical layer deals with such things as the type of signal transmission,

cable type, and the actual layout or path of the network wiring These are

things we can see, touch, or at least easily represent with a drawing or

diagram Because of this, the functions of the physical layer devices, which

include NICs, cables, connectors, hubs, and repeaters, are also relatively

easy to understand

Physical layer devices are mostly items that you’ll find in any networking

equipment catalog The basics are deceptively simple: you insert a network

card into an expansion slot on each computer, plug a piece of cable into each

network card, and plug the other end of each cable into a hub But leafing

through any type of equipment catalog will reveal that physical layer issues

are a little more complex Some cable manufacturers offer literally thousands

of different cables, and the variety of available network cards and connectivity

devices is just as overwhelming Getting a network up and running at the

physical level requires a good bit of knowledge about what works with what

and which hardware type is best for your particular situation

The NIC is the hardware device most essential to establishing

communication between computers Although there are ways to connect

computers without a NIC (using a modem over the phone lines or via a serial

null modem cable, for instance), in most cases where there is a network, there

is a NIC for each participating computer There are some basic guidelines to consider when selecting a NIC for any device The first thing is that the NIC must match the bus type for which you have an open slot in the computer

It also must be of the correct media access type, and the chosen NIC must have the correct connector for the cable your network uses Transfer speed

is an additional important consideration, and the NIC should be rated to transfer data at the proper speed Ethernet normally transmits at 10, 100, or

1000 Mbps, and Token Ring runs at 4 or 16 Mbps

The network media is the cable or wireless technology on which the

signal is sent Cable types include thin and thick coaxial cable that is sim-ilar to cable TV cable They include twisted-pair, such as used for modern telephone lines and available in both shielded and unshielded types, or fiber optic that sends pulses of light through thin strands of glass or plastic for fast, reliable communication Fiber optic is expensive and can be diffi-cult to work with Wireless media can include radio waves, laser, infrared, and microwave

Hubs and repeaters are devices that operate at the physical layer Repeaters

connect two network segments (usually thin or thick coax) and boost the signal so the distance of the cabling can be extended past the normal limits

at which attenuation, or weakening, interferes with the reliable transmission

of the data Hubs are generally used with Ethernet twisted-pair cable, and most modern hubs are repeaters with multiple ports Hubs also strengthen the signal before passing it back out to the computers attached to it Hubs can be categorized as follows:

Active hubs are the ones just described They serve as both a

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connection point and a signal booster Data that comes in is passed back out on all ports

Passive hubs serve as connection points only; they do not boost the

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signal before passing it on Passive hubs do not require electricity and thus won’t have a power cord as active hubs do

Intelligent or “smart” hubs include a microprocessor chip with

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diagnostic capabilities, so that you can monitor the transmission on individual ports

The NIC is another device that functions at the physical layer, and it is responsible for preparing the data to be sent out over the network media Exactly how that preparation is done depends on which media is being used;

a Token Ring NIC is different from an Ethernet NIC, for example They use different access methods, and even though 10Base2, 10Base5, and 10BaseT Ethernet networks all use Carrier Sense Multiple Access/Collision Detection

Troubleshooting the Physical Layer 583

(CSMA/CD) as their access method, they use different cable and connector

types; however, it is possible to get a “combo” card that has connectors for

all three, though not common nowadays

Another consideration at the physical layer is whether the signaling

method will use the entire bandwidth of the cable to transmit the data, or

only use one frequency When all frequencies are used, the transmission

method is called baseband If only part of the bandwidth is used (thus

allowing other signals to share the bandwidth), it is referred to as broadband

Traditionally, baseband transmission has been associated with digital

signaling and broadband with analog, but this does not always hold true

For instance, digital subscriber line (DSL) is a high-speed technology

offered by many telephone companies for Internet connectivity DSL is a

broadband technology It uses only a part of the wire to transmit data Voice

communication can take place simultaneously on the same cable, using a

different frequency than is being used by the data communications Cable

television is another example of broadband transmission, bringing dozens of

different channels into your home on just one coaxial cable

Another important physical layer issue is the layout, or topology, of the

network This refers to whether the cables are arranged in a line going directly

from computer to computer (bus), in a circle going from computer to computer

with the last connecting back to the first (ring), or in a spoke-like fashion

with each connecting directly to a central hub (star) A fourth topology, the

mesh, is used when every computer is connected to every other computer,

creating redundant data pathways and high fault tolerance, at the cost of

increasing complexity as the network grows Wireless communications can

use the cellular topology that is widely used for wireless telephone networks

In this case, an area is divided into slightly overlapping cells, representing

connection points

The physical layout of the network will influence other factors, such as

which media access method (and thus which cable type) is used All of the

physical layer factors, such as the cable type, access method, topology, and

so on, when considered together, define the architecture of the network

Popular network architectures include Ethernet, ARCnet, Token Ring, and

AppleTalk

layer 1 Troubleshooting

When troubleshooting the physical layer, you’ll be most concerned with

NICs, network cables, and hubs We’ll start by looking at potential issues

with NICs, and ways to troubleshoot problems with them Configuring the

NIC at the physical layer is the first step in creating a TCP/IP connection

Although an improperly configured card is not a protocol-specific issue, it may be mistaken for one, and you can lose a great deal of time trying to troubleshoot TCP/IP when the problem actually lies with the NIC itself Thus, it’s important for you to know how to determine when a TCP/IP connection

is failing due to a lower-level problem related to physical components such

as the computer’s NIC or physical cabling

One easy way to determine that the problem lies at the physical layer

is to attempt to establish a connection using a different protocol If your computer is unable to communicate with others on the network using TCP/

IP, but can make the connection when NetBEUI or NWLink is installed on the machines, you can surmise that the problem lies in the way that TCP/IP

is configured, and you can start troubleshooting the protocol configuration accordingly If you still have no luck in making a connection with other network transport protocols, it is likely that you have a problem with the hardware or the hardware drivers This simple test can save you much time and effort when you begin the troubleshooting process

In addition, when you are troubleshooting physical devices like cables or NICs, it’s also helpful to have spare equipment to swap out so that you can quickly determine the cause of the problem If you suspect that the network cable is at fault, switch it with one that you know works; if connectivity is restored, then you know that the cable was the culprit If switching the cable doesn’t help, try using a different wall jack or another port on the switch

or hub If switching to a different port solves the problem, then you have isolated the point of failure

The Role of the NIC

The NIC, also called the network adapter or just the network card, plays an essential role in TCP/IP and other network communications The NIC is the device that physically joins the computer and the cable or other network media, but its function is more complex than that Network cards also

have memory chips, called buffers, in which information is stored so that

if the data comes in or goes out too quickly, it can “rest” there while the bottleneck clears until there is room for it to pass onto the cable or up into the computer’s components

It is essential that you ensure that the NIC installed in the computer is the proper type for both the media and architecture used by your network For instance, Ethernet and Token Ring require different types of NICs This

is because of the different ways in which the media access methods function And, of course, the card must have the proper connector for the cable type

Troubleshooting the Physical Layer 585

being used These are basic, relatively straightforward issues, but don’t

overlook them when troubleshooting connectivity problems

If you are installing a NIC on a Windows-based computer, be sure to

check the Windows Hardware Compatibility List (HCL) or Windows catalog

to ensure that your card is supported The list can be accessed from the

Microsoft Web site at www.microsoft.com/hcl Although devices not listed

may still work with Windows, if your card is on the list you can be confident

that it has been tested and is compatible with the operating system

Driver Issues

Like other hardware devices, the NIC requires a software driver to provide

the interface between the operating system and the card Be sure the driver

that is designated for your specific model of NIC is installed and that it is the

most recent version In many cases, simply installing an updated NIC driver

can solve countless connection problems

Recent Windows operating systems such as Windows XP, Windows

2003, and Windows Vista support a large number of common brands and

models of NICs, and the drivers are included on the Windows installation

CDs However, these may not be the latest versions Always check the

manufacturer’s Web site for a download area where you can obtain the

latest drivers Because Windows 2000, XP, 2003, and Vista are plug-and-play

operating systems, supported cards are more likely to be automatically

detected and the drivers installed from the Windows installation files You

may be prompted to supply the disk or network location if the driver

can-not be automatically located, but compared to systems like Windows NT

4.0, this is still a huge step up Windows NT 4.0 does not include native

plug-and-play functionality Just be cautioned that drivers installed by the

operating system may be outdated, and even if a device does successfully

plug-and-play, it is still wise to verify that most current drives are in use In

Exercise 12.1, we’ll walk through the steps of updating a driver for a NIC

installed on a Windows Vista PC

ExErcisE 12.1 Updating Nic Drivers

Click

1 Start |Control Panel |System Select the Device Manager in the upper left

hand corner The list of installed devices will be displayed as shown in Figure 12.3.

Click the

2 + sign next to Network Adapters Double-click the NIC installed in this computer

and select the Driver tab You’ll see a screen similar to the one shown in Figure 12.4.