link type has been selected. Also, the test
standards have evolved sufficiently that the requirements for a
particular test, such as ISO Class D, may no longer be the same
as what is loaded in the software of the tester. Check the
website of the manufacturer of the tester for new tester
software two or three times per year. Most wiremap failures
occur at cable terminations, either at the RJ-45 (plug or
jack), or at an intermediate crossconnect or patch panel.
Faults at the RJ-45 can usually be seen by checking the wire
colors carefully against T568A or T568B pinout colors, or by
checking the RJ-45 plug for wires that did not seat fully to
the end of the connector when it was crimped. Another source of
RJ-45 related problems is how well the connector was crimped.
Propagation Delay and Delay Skew
TIA/EIA-568-B
permits up to 498 ns of propagation delay for the Permanent
Link and up to 555 ns of propagation delay for the Channel
Link, for all Categories. It is unlikely that this parameter
could fail without other parameters failing as well. Failing
propagation delay suggests an inappropriate or bad cable in the
link. Check the overall length of the cable. Inspect the cable
closely to see if the correct type of cable was installed. If
this is the only parameter that failed, it will probably be
necessary to replace the cable link. Delay Skew
Delay skew occurs as a result of different wire pairs
within a cable being insulated with different materials. This
could occur if there is an industry supply problem for a
favored insulating material. In this case the critical RX and
TX pairs may be coated with the favored insulative materials
and the pairs not utilized in TIA/EIA-568-A or TIA/EIA-568-B
standards are coated using sub-optimal insulative materials.
TIA/EIA-568-B permits up to 44 ns of delay skew for the
Permanent Link and up to 50 ns of delay skew for the Channel
Link, for all Categories. Both of these numbers are quite
generous. It is fairly difficult to fail delay skew if good
materials were used in the link. A delay skew failure is
possible if wire pairs in a single cable have different
insulative material on some pairs. A failure is also possible
if various lengths of twisted wire pairs were used as a patch
cable or jumper at a connection point. Varying the lengths of
pairs at any point along the link probably indicates bad
workmanship, as individual pairs should never be used for
networking applications. This situation should cause other
parameters to fail too. Inspect the connection points in the
link, and if the workmanship appears reasonable, there may be
little choice but to replace the entire cable run. Test a
sample of the new cable before installing it to be sure that
the materials are not causing the problem.
Content
3.5 Isolating Physical Layer
Problems 3.5.4 Cabling incorrect Check for
wires that were not fully seated in the crimping process. Also
check to see if the correct type of RJ-45 was used, stranded or
solid wire pins. This is difficult once the end has been
crimped. Using the wrong style of pin may cause intermittent
connections after a period of time, though the cable usually
works immediately after it is made. If a non-proprietary
terminator with a non-standard pinning is used, the connection
may look the same but the internal wiring could be off. Check
on the standard used. There have been notable cases where large
campus sites have been wired by two different groups of
contractors. If the cabling process is not coordinated
correctly different contractors may use different wiring
standards in their cabling. This would result in incompatible
wiring schemes in the two halves of what was meant to be a
single network. Cabling length is also a major issue. Cabling
guidelines are constructed to take into account factors such as
propagation delay and signal attenuation, which are related to
the length of the cable used. - Do not install long
cables. Make all runs as short as possible, certainly no longer
than is permitted by the media access protocol being used. For
example, never install UTP runs longer than 100 meters.
- Whether building cables onsite or buying pre-made cables,
be sure to test them with a reliable cable tester before use,
especially if already in troubleshooting mode.
Other issues such as grounding and ability to exclude internal
and external noise from interfering with data transmission must
also be taken into account. UTP is not the correct cable type
to be laid near strong sources of external EMI, such as
elevator cavities, power lines, radio, microwave, television
signal, and transmission stations. In these types of
environments where redirection of the data cables cannot occur,
then either shielded CAT5 or optic fiber must be considered as
a replacement. More dangerous is where there is a different
electrical potential at each end of cabling runs. The
terminating device may become live and capable of discharging
significant voltage to other attached equipment, or even
personnel. In this case, fiber-optic cable is to be used
instead of UTP or STP, because it does not carry an electronic
signal. It uses nonelectrically conductive material that
transmits modulated light signals instead.
Content
3.5 Isolating Physical Layer
Problems 3.5.5 Interface configuration
Prior to examining the interface configurations on network
devices, it is important to discount the physical causes first:
- Verify cable connectivity.
- Verify that the
power supply is on and running.
- Verify the router LED
status. If all LEDs are down, it is most likely an issue with
the power supply of the device, or with incorrectly seated
modules in a modular router or switch.
When
examining interfaces on a Layer 2 switching device:
- Start by looking for duplex and speed mismatches.
- Ensure that the correct VLAN encapsulation method has been
applied if the port is a trunking port, and that the port has
been designated for trunking rather than access.
- Ensure that individual ports have been assigned to the
EtherChannel group for trunking ports combined to form an
EtherChannel.
- Ensure that the port has not been
accidentally assigned an IP address. On 3550 and 6500 series
switches, the no switchport command and an IP address
converts the switch port to a routing port.
- Look
under port configuration to make sure it has not been assigned
as a RSPAN reflector port.
When examining a Layer 3
device: - Ensure that the IP addressing on both sides of
the link is within the same subnet.
- Ensure that the
interface is not administratively shut down.
- Ensure
that serial ports designated as Data Clocking Equipment (DCE)
end are assigned the correct clock rate for the link.
Console Not Responding
Console problems occur
when the router becomes unresponsive to input at the console
port. If the console is not responsive, it means that a high
priority process prevents the console driver from responding to
input. If traffic is still flowing through the device, try
disconnecting network interfaces and see if the router starts
responding. Many times the router thinks it is doing something
too important to service exec sessions. Another possibility is
that one of the device interfaces may be down. There are
multiple events that could cause this, such as a wrong
configuration command, or a hardware failure of the interface
or the cable. If some interfaces appear down when using the
show interfaces command, try to find out what caused it.
Content 3.5 Isolating Physical Layer
Problems 3.5.6 Operational statistics A
variety of end-station and network device commands can be used
to gather operational statistics on a network or on specific
component links and devices. These commands have been discussed
in section 3.3 and also in 3.4.8 which details CPU statistics
over time. Operational statistics can also be gained from