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Tips on Computer Network Cabling |
First the Basics
We've lost track
of how many times we've been on a customers site and found that
they have re-used the old Cat 3 and Cat 4 patch leads simply
because they didn't have any Cat 5 leads at the time.
These leads almost never get changed because it hasn't
made any noticeable difference to the operation of the
network.
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Years later of course, things start to go wrong and the old
leads aren't even suspected because "they've not been a
problem in the past".
You can use the old leads on voice systems but never use them
on data networks. It is like throwing a load of rocks into a
smooth running stream, the data will probably still get
through at first but when you increase 'the flow', they will
start to impede the throughput.
Is the problem only at one PC?
If the problem
only affects one PC, take that PC to the patch cabinet and
plug it directly into the hub. This may seem like an obvious
thing to try but it has to be said as it proves whether the
fault is actually a cabling problem or not. You should use a
known good port on the hub and a known good patch lead, if the
machine works then the problem is either the original hub
port, the patch lead, the drop lead or the fixed cabling. From
here it is easy to eliminate each part of the link, but it
must be carried out methodically, one component at a time.
Another point worth mentioning is that a patch lead that works
fine in a Token Ring network may not work in an Ethernet
network, this is because they use different pins on the RJ45
plug. Token Ring uses pins 3, 4, 5 & 6 whereas 10BaseT
Ethernet uses 1, 2, 3 & 6, so any cable with a fault on pins 1
& 2 will work for Token Ring but not for Ethernet. Although a
Cat 5/5E tester can be very expensive, you can buy a simple
continuity tester for under £100 which will test for shorts,
opens and crossed pairs, this will not prove your cabling is
up to standard but they are quick and easy for finding faulty
Cat 5 patch leads.
Another thing to
watch out for is the wiring configuration, there are actually
two different schemes allowed under the 568A standard. These
are called 258A (or T568B), and 258B (or T568A). Pin for pin
they are the same but with the orange and blue pairs swapped
over, so as long as you have the same type of jack at each
end, no problem. However, if you have 258A on one end, and
258B on the other then you have a crossed pair.
Check the time of day!
The time of day
may indicate another cause of network problems. If the problem
only occurs at a certain time, it maybe that the network is
slowing due to an increase in traffic say at 9:00am or 5:00pm.
If, for instance, the drawing office starts at 9:00am and
twenty draughtsmen are all trying to pull large drawing files
from a server which is on the general network, this will
impact the rest of the company's business. Likewise at 5:00pm
when they are all saving their work back to the server the
sudden increase in traffic may cause so many collisions that
the network to grinds to a halt. A process of elimination is
easy to implement and if this is the cause, it is time to put
the drawing office and the CAD server on to its own hub or
switch.
Electrically 'noisy'
environments
Another common
cause is electromagnetic interference from electric motors and
sources of high frequency radio waves. The more obvious things
to look for are cable routes that pass too close to lift
motors, arc and spot welders, heavy plant machinery which use
large electric motors, and fluorescent light fittings. All of
these things, if situated close to the data cabling, could
induce spikes into the network. Take a walk around the
building and make a note of all possible causes, and then try
to eliminate them one at a time.
What next?
Eliminating possible
causes one at a time is the ideal, but sometimes the problem
occurs so infrequently that it is almost impossible to track
down, and specialist equipment and engineers have to be
employed. 'Network Sniffers' and mains monitors can be hired
from companies such as Livingston Hire, and if you are
confident that you can correctly interpret the results, do it
yourself. If not, well its the end of the line and time to
call in the specialists.
To summarize, the following
considerations should be taken into account:
1. Low grade
patch leads and/or drop leads
2. Time of day
3. Increase in
network traffic from other sources
4. Electromagnetic
interference
Ethernet
Most of the points
mentioned above should find Ethernet problems, however, there
are a couple of things that should be taken into consideration
when dealing with Gigabit Ethernet.
Although
Gigabit Ethernet was designed to run on 100MHz cable, problems
may arise with older Cat 5 systems. The more stringent Cat 5E
standards take into consideration that Gigabit Ethernet uses a
four pair transmission method, but this was not part of the
test parameters with Cat 5. If you are trying to run Gigabit
Ethernet over standard Cat 5 cabling, then the whole system
should be tested to confirm that it meets the new Cat 5E
standard.
It used to be
said that multimode fibre was good for 2km, but recently it
has been found that for Gigabit Ethernet applications the
length limit is right down to around 220m over 62.5/125 fibre.
The only way to prove if a fibre is good enough for Gigabit
Ethernet is to use a certification tool. These are fairly
expensive test instruments, but you should be able to hire one
from a specialist hire company. The results give a clear 'pass
or fail' for different applications, but bear in mind that one
dirty connector can affect the results considerably.
Type 1 cable
was designed for Token Ring and is a very robust system,
however, some of the data connector termination's I've seen
leave a lot to be desired. It is possible to re-terminate
these connectors, so if you have some that are looking
decidedly worn or broken, get them fixed. Because of their
large size, data connectors tend to get knocked and bumped and
this can be the cause of a lot of problems. Also remember that
Type 1 is a shielded system which has to be grounded properly
at the patch cabinet, ground continuity should also be carried
right through to the PC. Failure to ensure proper grounding
can cause a multitude of problems.
The data
connector was designed to 'loop back' when unplugged to ensure
that if the main ring cabling is interrupted the network will
stay up. If you look into the end of a data connector you will
see two gold strips one behind the other These 'short circuit'
the transmit and receive pairs when the connector is
unplugged, and loops the signal back the other way thereby
maintaining ring integrity. This very clever piece of design
does have a slight draw back in that a disconnection on the
main ring path can go unnoticed because the data connector is
doing its job.
This means that if you have a fault on the ring and it is
already working on loop back, you will only know when a second
fault occurs. In theory, if your Token Ring is working
perfectly and the ring is complete, you should be able to
disconnect the main ring path anywhere without disrupting the
network. If, on the other hand, your Token Ring is already
running on loop back due to a fault that you are not aware of,
disconnecting the main ring will effectively split the network
in half.
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