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Computer Repair with Diagnostic Flowcharts
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Copyright 2012 by Morris Rosenthal
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LAN with Cat 5 Network Cards and Hubs Diagnostic Flowchart
Wireless networks are replacing wired LANs at a rapid rate, but all computers
are still equipped with an RJ-45 port for high speed networking. I still
use an RJ-45 Cat 5 cable to connect my laptop directly to the DSL modem/router
while I'm at home just for the speed advantage. The flowchart below is one
of 17 from my book "Computer Repair with Diagnostic Flowcharts Revised Edition"
which has been widely used in training new computer technicians. If you find
the approach works for you, the 118 page printable PDF eBook can be downloaded
instantly or you can order the book through any bookstore.
Note that these steps correspond with decision points on the flowchart and
are reached directly by clicking on the diamond symbols. The text below cannot
be read sequentially.
Do the networking resources, such as print
servers and file servers, show up in Windows? If you see resources listed,
click on them to make sure they are really connected, because some Windows
software has a way of retaining resources in lists even if the connection
no longer exists.
Did you add a new router or hub to the LAN?
We all fall into the trap of assuming that new equipment will work when we
take it out of the box, especially if the LEDs light up when it's plugged
in. So if you've installed a new hub or router and the workstation you're
having trouble with is wired through it, the first test is to move the
workstation's patch cord to a port on another router and eliminate the port
you were using on the new router as the issue. Just for the record, a network
hub is a switching point within the network, while a router technically comes
between networks. But people have taken to using the two interchangeably
since so many have a router at home to connect the Internet that doubles
as a local hub for a home LAN.
The typical twisted pair LAN uses RJ-45 connectors (the 8 wire phone connectors)
for 100BaseT and 1000BaseT (Gigabit). You're not very likely to encounter
10BaseT anymore unless you're servicing a small business or home office that
hasn't updated anything in twenty years. Network hubs often give you the
choice of a straight through uplink port or a crossover port for daisy chaining.
It doesn't matter which one you use as long as you have the right cable.
Straight through cables in 100BaseT world have 1-1 correspondence right through
the connector, but the pairs are shared between 1 and 2, 3 and 6, 4 and 5,
7 and 8. It's based on the AT&T 258A spec, don't ask me why. Crossover
cables allow you to connect two PCs without a router or hub in the middle.
Crossover cables are wired so the 3 and 6 pair on the first end go to the
1 and 2 pair on the second end, while the 1 and 2 pair first end go to the
3 and 6 pair on the second end. The other two pairs aren't always used, and
they don't change in the basic crossover cable.
All modern motherboards include the network
adapter in the I/O core, but they don't all include the status LEDs. Network
cards generally had one or more exposed LEDs on the exterior to show the
link status and network traffic, but being on the back of the case which
is often buried in office furniture, they aren't always visible. If the link
status LED isn't green, it means there's an open circuit between the network
port and the hub, unless the cable is simply made wrong. An open circuit
doesn't always mean an broken wire or connector, it could be the distance
is just too great. Since you only have one network adapter in the PC, try
swapping the other end of the cable to another hub or patch panel port in
the wiring closet. If you encounter an ancient coaxial network running thin
Ethernet, make sure there are 50 Ohm terminators at the end of any run.
Did you carefully note all of the settings
for the workstation software from a PC without connection problems and transfer
those settings to the problem PC? Generally speaking, everything except the
IP address, the workstation name, and the group name (if there is one) should
be the same for similar workstations. Especially when setting up new PCs
it's easy to make a spelling mistake that prevents the workstation from joining
the network. Software troubleshooting for networks is a subject for many
thousand page books, I'm sticking with hardware problems here.
For Windows computers, does Device Manager
report any problems with the network adapter? If the network adapter isn't
detected or if a problem is reported, the first thing to do is reboot, reinstall
the driver, and see if it clears up. In some cases, it may help to disable
the motherboard network adapter in CMOS Setup (or remove the old network
card) and shut down, go through a reboot cycle, shut down again, reenable
the integrated network adapter (or reinstall the card), and then try reinstalling
the device driver. If Windows still doesn't detect the adapter, it's either
bad, incompatible, or there's a hardware resource conflict. For older PCs,
try the flowchart for troubleshooting hardware conflicts, but for motherboard
integrated network ports, all you can do make sure the driver and BIOS are
up to date.
Try a different patch cable to get from the
network port to the wall jack, even if it means stealing one from another
workstation on the network. If the cables are new, it doesn't hurt to inspect
them for the correct pairing, you can usually squint through the plastic
connectors and make sure they're built right and no conductors are missing
at the ends. The proper pairings are given back near the top of the page.
Pick up the PC (after disconnecting all the
cables), walk it over to another desk where the network is working, and swap
all the cables over to your PC. If you immediately get network access, you
know that there's a cabling problem from your old location to the hub. It
could be the patch cable, easy to test now by swapping it into the new location,
it could be the cabling in the wall, or it could be the port that it's ultimately
connecting to at the patch panel or hub. On the other hand, if the PC can't
connect to the network at the new location, you know that the problem is
within the PC, either the network adapter or the software configuration.
Are you (meaning the PC) experiencing
intermittent problems connecting to the network rather than a total failure?
The RJ-45 connector may be at fault as the wires can loosen up in the crimps
and make for a flaky connection. In rare cases, you might be getting so much
noise on a badly routed cable that it's overcoming the network signal level.
But it's more likely that the problem is really software, some other task
essentially locking up the network software you are using which then appears
to be a network failure. See if rebooting helps, and if the problems only
occur after you've used other applications. And talk to other network users
to make sure that it's not a network wide traffic or resource issue that
you or the user of this workstation is just noticing.
Are we talking about an obsolete setup with
shielded twisted pair or another type of shielded cable other than Cat 5?
For grounded cable, the shield can only be grounded at one end or a current
loop will likely ensue, while if neither end is grounded, the shield functions
as an antenna which can weaken the network signal or attract noise. Don't
count on any cable's noise isolation to make it 100% safe to route it through
the noisiest electrical environment, it makes more sense to stay away from
problems.
Don't forget that networks all have distance
limitations, whatever equipment is used. You can't count on using the maximum
distance defined by the network standard, you have to use the lowest number
given by any of the hardware components (hubs, routers, network adapters)
on the circuit. And remember that all distances are actual cable distances,
not measurements from a blueprint. If you're using repeaters to boost the
signal, there are limits there as well since the distances effect transmission
times and can lead to time-out problems if the setup is finicky.
Did you try connecting to a known good port
on the hub? Never assume that a hub or router port is good because it used
to work and the light seems active. Move the workstation cable to a RJ 45
port that's working now.
Can you associate the problems with peak
usage periods for the network? If the problems only show up when everybody
is working on Friday payroll processing or watching Internet videos at lunch,
odds are it's just a bandwidth issue and you need a faster network or to
split the existing network in two. If the network runs a mix of 1000BaseT
and 100BaseT adapters, it's worth updating everybody to Gigabit, even if
it means sticking PCI network adapters in older PCs that have 100BaseT integrated
on the motherboard.
Are you sure the problems only show up when
the PC is using the network? If the PC is having trouble all the time, you're
really on the wrong troubleshooting flowchart. Run the usual anti-virus and
anti-malware software to make sure the operating system hasn't been compromised,
and then start back at the beginning of the troubleshooting series, especially
the motherboard CPU and RAM and hard drive troubleshooting flowcharts.
I find it really helpful to keep one super
long bypass cable that will let me temporarily connect any workstation to
a hub without using the in-wall wiring. Sometimes it's just more practical
than moving the whole PC, though you shouldn't run a wire on the floor while
other people are working in the office.
Did you try swapping in a cheap PCI network
adapter? PCI network cards are practically free, so any time you're dealing
with PCs rather than laptops, it make sense to stick in a PCI network adapter
to make sure the existing add-in adapter or motherboard network support hasn't
failed. You may have to disable the motherboard network adapter in CMOS to
get the new one recognized. At this point, we've eliminated the physical
layer of the network as being the problem, so if the PC can't connect with
a proven replacement adapter, the problem is software settings.
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