3.6
Given a scenario, troubleshoot common physical connectivity problems
Today's networks can be much more complex than networks of the
past, but the rationale behind them is still the same: to provide connectivity
so that users can share information and resources. When physical connectivity is
affected, the users are brought to a standstill, and productivity quickly
declines. Although some issues can be very complex and require extensive
knowledge of software and configuration, others are as simple as having the
right physical pieces connected in the proper way. In this section, I will
discuss common physical connectivity problems including bad connectors, bad
wiring, open short, split cables, DB loss, TXRX reversed, cable replacement,
EMI/Interference, distance, and crosstalk.
Bad connectors
Over time, all connectors can suffer physical fatigue and
damage. This is especially true with those that are more fragile, such as some
fiber-optic cables. If you are experiencing a complete or even intermittent lack
of connectivity, then you should check the connectors to make sure they are not
cracked or broken and that the wires or fiber are securely and properly in
place. This is one reason that it's important to know what they are supposed to
look like, which is why I included all of the figures earlier in this
chapter.
Bad wiring
Bad wiring issues can be caused by using the incorrect cable
type or even by the improper installation of connectors. You might think that it
would be impossible for a cable to allow a connection that lights the link-light
but still isn't proper. In fact, that is possible if the person who created the
cable did not properly align the wires in the connector. The power and ground
could be correct without the transmit and receive being correct as well. In this
case, you would get a link-light, but you would not have a connection.
Open short
A short circuit, which is sometimes called an open short, is a situation that allows current to flow on a
different path than was originally intended in the circuit. In networks, shorts
are typically caused by a physical fault in the cable. They can be detected
using circuit-testing equipment. Often, the best and fastest remedy will be to
use a different cable until the cable that has the short can be repaired or
replaced.
Split cables
Sometimes you might want to split a connection into separate
directions such as for a coaxial connection to TV and Internet. You should take
care to use a high-quality splitter to prevent signal loss. This loss can be
measured as I will discuss next.
DB loss
Without getting into the complex math, which you don't need to
know, DB loss is an algorithm that is used to calculate the difference between
the signal at the source and the signal at the destination. A DB loss of 0 would
be perfect but also impossible. There will be some DB loss in every network
media, but the idea is to keep the number to a minimum. Depending upon the type
of media that you are using, there are tables for acceptable DB loss per 100
feet. Using best practices for each type of connector will keep DB loss within
acceptable levels. The exact measurements for DB loss for each media are beyond
the scope of this book.
TXRX reversed
Transmit (TX) should connect to receive (RX) for each pair of
wires in the network cables, such as patch cables. Earlier in this chapter, when
I discussed crossover cables, I mentioned that they are used to connect similar
devices. This is because using a normal patch cable to connect similar devices
would cause a connection of TX to TX and RX to RX, which would not function
properly. This type of reversal can also be caused by the improper connecting of
wires on a wall jack or patch panel. Some devices have the capability to
autosense the reversal and make the correction, but some devices do not. If you
are experiencing this type of problem, it will show itself as a complete lack of
connectivity, not an intermittent one.
Cable placement
Proper cable placement in your datacenter or network closet and
throughout your building is essential for reliable network communication. Cables
should be run either under the raised floor of a datacenter or in the ceiling
where they are safe and out of the way but accessible if needed. You should take
care to keep them away from power cables, whenever possible, and if you have to
cross a power cable, then cross at a 90-degree angle to minimize cross-talk. You
should also consider other sources of interference such as EMI, which I will
discuss next.
EMI/Interference
Since wires might take on an additional current if placed near
any source of magnetism, you have to be careful where you run your
communications cables. This property of being affected by external magnetism is
referred to as electromagnetic interference (EMI). You can
avoid EMI by taking care to keep copper cables away from all powerful magnetic
sources. These may include electric motors, speakers, amplifiers, fluorescent
light ballasts, microwave ovens, refrigerators, freezers, copy machines, and so
on. Anything that could generate a magnetic field should be avoided when
positioning a cable.
Distance
Often we decide what cable type to use based on the topology of
a network and the distance between its components. This is because some network
technologies can run much farther than others without communication errors. All
network communications technologies suffer from attenuation, which is the degradation of signal because of
the medium itself. Attenuation is much more pronounced in some cable types than
in others. For example, a good rule of thumb and best practice is that any
network using twisted-pair cable should have a maximum segment length of only
100 meters (328 feet). Each cable type has its own limitations because of
attenuation, which I will discuss in more detail in the next section.
Crosstalk
Crosstalk is the occurrence of signal bleed between two wires
that are carrying a current and are adjacent to each other. It can cause network
communications to become slow or to not function at all. Network cable designers
minimize crosstalk inside network cables by twisting the wire pairs together, in
effect putting them at a 90-degree angle to each other. The tighter the wires
are twisted, the less the crosstalk will affect them. Newer cables, such as CAT
6 cable, minimize the effect of crosstalk with a tighter twist, but it still
exists and can affect network communications when network speeds are very high.
To prevent crosstalk, you should use the appropriate cable for the speed of your
network. To completely eliminate it, consider using a fiber-optic cable. Since
fiber uses light instead of electricity, it is completely immune to
crosstalk.
Exam Essentials
Check the physical connections and cables
first Network problems are often caused by bad connectors and bad wiring.
Check the physical connections before you suspect a more complex problem.
Consider the cable configuration Take into
account that trying to do too much with the signal without the proper connectors
can result in DB loss, which could cause poor performance in all areas. If you
decide to split the cable, then use a high-quality cable splitter. Be careful to
use the right types of cable and to install wall jacks and patch panels properly
to prevent TXRX reversal. This will cause a total lack of connectivity. Use the
proper cable for your network speed to prevent crosstalk.
Consider cable placement and distance Your
network cables should be under the floor, in the ceiling, or in the walls so as
to keep them safe from damage. In addition, they should be kept away from
electrical cables and away from any large electric motors or magnets to prevent
EMI. Finally, you should take into account the proper distance for the type of
cable and technology that you are using, such as 100 meters maximum for all
twisted-pair cable.