Download Troubleshooting Local

Network
Troubleshooting
by Othmar Kyas
Infrastructures
6 Cable
in Local-Area Networks
An Agilent Technologies Publication
Agilent Technologies
SECTION II
Troubleshooting Local-Area
Networks
6
Cable Infrastructures
in Local-Area Networks
“Always expect the worst and you will never be disappointed.”
PETER WASTHOLM
The main prerequisite for trouble-free operation of a local-area network is the
use of a high-quality cable infrastructure with specified data transmission parameters. Estimating the overall transmission characteristics of network cable
infrastructures is an extremely complicated task. In practice, the only way to
design a physical foundation for reliable data communications is through strict
adherence to accepted standards for the cabling. The ISO/IEC IS 11801 specification, from which the European standard EN 50173 is derived, is an internationally recognized standard that describes customer premises cabling. The
corresponding North American standard is the EIA/TIA 568A Commercial
Building Telecommunications and Wiring Standard.
The ISO/IEC IS 11801 specification distinguishes between three types of cable
by range: campus backbone, for use over distances of up to 1,500 meters; vertical
backbone, for up to 500 meters; and horizontal cabling, for connecting components on the same floor over distances of up to 90 meters. ISO/IEC IS 11801 also
N e tw o rk
C o m p o n e n t
N e tw o rk
C o m p o n e n t
G
C a m p u s
d is tr ib u to r
N e tw o rk
C o m p o n e n t
F
B u ild u n g
d is tr ib u to r
E
F lo o r w ir in g
c lo s e t
E n d s y s te m
W a ll ja c k
D
1 5 0 0 m
c a m p u s
b a c k b o n e
C
5 0 0 m
b u ild in g
b a c k b o n e
B
9 0 m
flo o r
c a b lin g
A
A + B + E £ 1 0 m
C , D
< 2 0 m
F , G
< 3 0 m
Figure 6.1 Campus, vertical and horizontal cabling according to ISO/IEC IS 11801
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CABLE INFRASTRUCTURES IN LOCAL-AREA NETWORKS
6
defines the maximum permissible lengths for cabling systems, as well as the
upper limits for work area and device cabling (see Figure 6.1).
The transmission media described in the cable performance categories are
twisted copper wire and single-mode and multimode fiber-optic cabling. Coaxial
cable, or coax, is used in 10Base2 and 10Base5 networks, but is not suitable for
structured cabling and thus not in the specifications mentioned previously. The
only standard that defines coaxial cabling is EIA/TIA 568, which describes 50 Ω
coax as an option for horizontal cabling.
The different types of twisted-pair data cable available are divided into seven
performance categories, numbered 1 through 7. Twisted-pair cabling can be
used over various distances in any of the six ISO/IEC application classes
( A through F) defined for copper cabling, depending on the transmission characteristics of each category. A seventh application class has been defined for the
use of optical transmission media. Bandwidth ranges and typical applications
are also defined for each class. Class A applications, for example, require
bandwidths of up to 100 kHz; this class includes such applications as X.21/V.11
and ISDN S0. Class F applications can require up to 600 MHz bandwidth, and
include Gigabit Ethernet and 622 Mbit/s ATM (see Figure 6.2).
A p p lic a tio n c la s s e s
p e r IS O /IE C IS 1 1 8 0 1
D e fin itio n
C la s s A
A p p lic a tio n s u p to 1 0 0 k H z
( a n a lo g te le p h o n y , X .2 1 , IS D N S 0 , e tc .)
C la s s B
A p p lic a tio n s w ith b a n d w id th s o f u p to 1 M H z
(X .2 1 , IS D N S 0)
C la s s C
A p p lic a tio n s w ith b a n d w id th s o f u p to 1 6 M H z
( IS D N S 2 M , 1 0 B a s e - T , 4 /1 6 M b it/s T o k e n R in g )
C la s s D
A p p lic a tio n s w ith b a n d w id th s o f u p to 1 0 0 M H z
( 1 0 0 B a s e - T X , A T M 1 5 5 M b it/s , G ig a b it E th e r n e t)
C la s s E
A p p lic a tio n s w ith b a n d w id th s o f u p to 2 0 0 M H z
( A T M 1 5 5 M b it/s , G ig a b it E th e r n e t)
C la s s F
A p p lic a tio n s w ith b a n d w id th s o f u p to 6 0 0 M H z
( A T M 1 5 5 M b it/s , G ig a b it E th e r n e t)
O p tic a l d a ta tr a n s m is s io n
D a ta c o m m u n ic a tio n s o v e r m u ltim o d e
a n d s in g le - m o d e o p tic a l fib e r
Figure 6.2 Application classes for structured cabling per ISO/IEC IS 11801
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6.1
6.1.1
CABLE INFRASTRUCTURES IN LOCAL-AREA NETWORKS
6
Coaxial Cable
Coaxial Cable:
Specification and Implementation
Coaxial cable has a copper inner conductor at the center and an outer conductor
that acts as shielding. Between the two conductors is an insulator, and the whole
is encased in a plastic jacket. Called “coax” (two syllables) for short, coaxial
cable can provide data speeds of several gigabits per second and is used for both
digital data and television signals.
In n e r c o n d u c to r D ie le c tr ic
B r a id e d o u te r c o n d u c to r
P la s tic ja c k e t
Figure 6.3 Structure of coaxial cable
The types of coax most commonly used for data communications, besides the
traditional 50 Ω cable of 10Base2 and 10Base5 Ethernet networks, are RG-62
coaxial cable, which has an impedance of 93 Ω (used to connect IBM 3270
terminals), RG-6 and RG-11 coax, with 75 Ω impedance, and twin-axial cable,
with an impedance of 105 Ω (AS/400, IBM /36 /38 series). Detailed specifications for the various types of cable are found in the MIL-C-17G standard (see
Figure 6.4).
There are two ways to connect network components to coaxial cable in busbased Ethernet networks (10Base2, 10Base5). The first method consists of
cutting the cable and inserting a T-connector. The second method involves
drilling a hole in the coaxial cable until the inner conductor is just reached, and
then inserting a special connector known as a tap or vampire tap. This method is
used with 10Base5 cabling (“yellow cable” or “thick Ethernet”), while T-connectors are commonly used in 10Base2 networks (“thin Ethernet” or “cheapernet”).
The advantage of vampire taps is that new components can be installed without
interrupting network operation. This can be an important factor, especially in
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6
C a b le d e s ig n a tio n
R G -5 8
1 0 B a s e 2 c a b le ,
th in E th e r n e t, C h e a p e r n e t
R G -8 A /U
1 0 B a s e 5 c a b le ,
th ic k E th e r n e t, y e llo w c a b le
U s e
1 0 B a s e 2
1 0 B a s e 5
Im p e d a n c e (9 )
5 0
5 0 ± 2
A tte n u a tio n ( d B /1 0 0 m )
4 .6 a t 1 0 M H z
1 .7 a t 1 0 M H z
V e lo c ity fa c to r
0 .7 7
0 .8 3 – 0 .8 6
In n e r c o n d u c to r (m m )
0 .9 4
2 .7
In s u la tio n ( m m )
2 .5 2
6 .1 5
P V C o u te r ja c k e t ( m m )
4 .6 2
1 0 .3
B e n d in g r a d iu s ( c m )
5
2 5
Figure 6.4 Specifications for RG-58 and RG-8A/U coaxial cable
(10Base2 and 10Base5)
production networks. Care must be taken in using this method, however, as the
cable may break if the hole is drilled too deep. If the hole is not deep enough, the
connection will be unstable. In either case, serious network problems can result.
10Base5 cable is usually equipped with N-type connectors; 10Base2 cable with
BNC connectors. To prevent reflections, each cable end must be connected to a
50 Ω terminating resistor. It is essential that one of the terminating resistors be
grounded, and equally important that the second one not be grounded. If neither
terminating resistor is grounded this could result in an electrical charge throughout the entire network. If both are grounded and the two grounds have different
electrical potentials, considerable current on the line may result.
In addition to observing segment length limitations (185 meters for 10Base2;
500 meters for 10Base5), it is especially important to respect the minimum
bending radius for coaxial cable (5 cm for 10Base2; 25 cm for 10Base5) and to
install pressure relief fixtures over the cables where necessary to prevent
damage caused by equipment housings, other objects resting on the cables or
other mechanical stress.
1 0 B a s e 2
1 0 B a s e 5
M a x im u m
s e g m e n t le n g th
1 8 5 m
5 0 0 m
M a x im u m
b e n d in g r a d iu s
5 c m
2 5 c m
M a x im u m
n u m b e r o f n o d e s p e r s e g m e n t
3 0
1 0 0
M a x im u m
a tte n u a tio n p e r s e g m e n t
8 .5 d B
8 .5 d B
M a x im u m
tr a n s m is s io n d e la y p e r s e g m e n t
9 5 0 n s
2 ,1 6 5 n s
Figure 6.5 Design guidelines for 10Base2 and 10Base5 networks (IEEE 802.3)
SECTION II
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TROUBLESHOOTING LOCAL-AREA NETWORKS
149
CABLE INFRASTRUCTURES IN LOCAL-AREA NETWORKS
6.1.2
Troubleshooting Coaxial Cable
(10Base2, 10Base5)
The first steps in tracking down the source of a problem in coaxial cable include
checking the voltage, resistance, terminating resistors, noise, cable lengths,
reflections and attenuation. When these tests are performed conscientiously,
they often lead directly to the cause of the problem.
6.1.2.1
Measuring the Voltage in Network Cabling
The easiest way to measure the voltage in a segment of network cabling is to use
a voltmeter on an unused T-connector or vampire tap. If the voltage is less than
±100 mV (±200 mV at one end of the cable), then there is probably nothing wrong
in this area. If the voltage exceeds 100 mV—especially if the reading is several
volts—then it is likely that the supply voltage of a media access unit (MAU) or
other component on the cable segment is leaking into the network cable. In this
case, disconnect the MAUs one at a time and repeat the voltage measurements
after each unit is removed. Because more than one MAU may be defective, do not
re-connect the deactivated units to the network until the source of the problem
has been isolated. If the voltage is still too high even after all components have
been disconnected, the cable is probably grounded in more than one place, with
different electrical potentials at each ground. Remove all grounds except one.
Sometimes a second ground is created by a defective network component that
R
In n e r c o n d u c to r
r
1
M u ltim e te r
R
d e v l
r
d e v n
B r a id e d o u te r c o n d u c to r
r
d e v n
R
R
1
a n d r
u n d R
a r e o th e r d e v ic e s c o n n e c te d to th e L A N c a b le
2
d e v l
a r e te r m in a tin g r e s is to r s
R
R
2
2
R
+ R
1
o r 2 5 9
1
Figure 6.6 Measuring resistance in coaxial cable
R
2
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6
does not electrically isolate network components from the data transport medium. If this is the case, a simple network interface card may be the cause of the
voltage problem.
6.1.2.2
Measuring the Cable Resistance
All network components must be switched off before performing resistance
measurements. Measure resistance at a T-connector or vampire tap (see Figure 6.6, 6.7).
If the values measured are between 24.0 and 26.6Ω for 10Base5 or between 24.4
and 26.6Ω for 10Base2, then the level of resistance in the cable is not the root of
the problem.
T h in E t h e r n e t c a b le
w ith B N C c o n n e c to r o r T - c o n n e c to r
L A N c a b le r e s is ta n c e m e a s u r e m e n t
Figure 6.7 Test setup for measuring the resistance of coaxial cable
If the resistance is high, the cable is broken somewhere along its length. In this
case, isolate the defective cable segment by repeating the resistance measurements on neighboring T-connectors or vampire taps until you detect normal
values.
If the resistance is very low, the cable is short-circuited. You can quickly
pinpoint the location of the short using a time-domain reflectometer (TDR).
6.1.2.3
Testing the Terminating Resistor
If you suspect that you have a defective terminating resistor, make sure you test
the T-connector and the terminating resistor simultaneously. It is possible that
the terminating resistor is in perfect condition, but connected to a defective
T-connector (see Figure 6.9).
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CABLE INFRASTRUCTURES IN LOCAL-AREA NETWORKS
R
R = h ig h
R = h ig h
6
fa u lt
R = h ig h
5 0 9
te r m in a t in g
r e s is to r
R = n o rm a l
F a u lty s e g m e n t
o f L A N c a b le
Figure 6.8 Systematic search for defects in coaxial cable
B N C c o n n e c to r
5 0 9
te r m in a t in g r e s is to r
T h in E t h e r n e t c a b le
w ith B N C c o n n e c to r
B N C T -c o n n e c to r
5 0 9
te r m in a t in g r e s is to r
L A N c a b le r e s is ta n c e m e a s u r e m e n t
Figure 6.9 Setup for testing a terminating resistor
6.1.2.4
Measuring Electromagnetic Interference
When line noise is measured using a cable scanner or a spectrum analyzer, the
frequency of electromagnetic interference in the line can be identified. Noise is
measured between the shielding (outer conductor) and the core (inner conductor). If the noise exceeds permissible levels, check the area around the affected
cable route for possible sources of interference, including production environments, elevators, photocopiers, fluorescent tubes, arc welding equipment and
X-ray devices. If a spectrum analyzer is available you may try to determine the
frequency spectrum of the interfering equipment. Typical interference frequency ranges are listed:
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CABLE INFRASTRUCTURES IN LOCAL-AREA NETWORKS
Potential source of interference
FM radio and TV signals
Operating frequency
1-100 MHz
GSM telephones
800 MHz
Pagers
500 MHz
PCs
100-400 MHz
Fluorescent tubes, electric motors
6.1.2.5
6
10-150 kHz
Measuring Cable Lengths, Reflections and Short Circuits
Cable length, reflections and short circuits can be measured using a timedomain reflectometer or TDR. The TDR transmits a signal and then measures
the time that elapses until the signal’s reflection is received. Any irregularity in
a given cable segment will cause reflections of greater or lesser intensity,
depending on the magnitude of the irregularity. The TDR can be set to wait
either for the largest reflection that occurs or to measure smaller reflections,
such as those that amount to only 30 percent of the original signal. The latter
method allows you to determine the distance to impedance anomalies along the
cable. To determine the overall cable length, remove the far terminating resistor
and set the TDR to measure the greatest reflected signal. If the cable is broken at
any point along its length, however, the reflection will come from the break, not
from the far end. As is the case with most cable measurement procedures, the
cable segment to be tested must be taken out of operation completely before
performing TDR measurements.
6.1.3
Symptoms and Causes: Coaxial Cable
Symptom: Intermittent Loss of Connection
Cause (1):
Cause (2):
Poor or no physical connection to the network cable due to a loose
BNC connector or a vampire tap that does not reach the inner
conductor.
Terminating resistors exceed tolerance limits.
Symptom: Complete Failure of a Segment
Cause (1):
Cause (2):
Short circuit in the cable (outer conductor touching inner conductor) due to kinks, defective cable, or vampire tap drilled too deep.
Voltage on the network exceeds permissible levels due to defective
MAU or lack of grounding.
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6
Symptom: Unusually High Number of Collisions
Cause (1):
Cause (2):
Cause (3):
Cause (4):
Strong reflections in the cable, resulting in collisions.
Check to see if terminating resistor is missing, defective or out of
tolerance.
Segment may have too many MAUs.
Segment grounded in more than one place.
Maximum cable length exceeded.
Symptom: Intermittent or Frequent Collisions and Fragments
Cause:
Electromagnetic interference.
Check whether photocopiers, pagers, elevators or x-ray devices
are being used near the cabling.
Symptom: No Connection or Intermittent Loss of Connection After New
Installation
Cause (1):
Cause (2):
High attenuation in newly installed cables; variations in impedance between connectors or in patch panels.
Impedance of new cable is outside tolerance limits; wrong cable
type installed.
Check tolerances and specifications of all new components.
Gathering Information; Common Errors
Most problems in cabling infrastructures arise in conjunction with external
alterations. Such changes may have been made intentionally or even inadvertently, as can happen during the course of other activities. Specific information
about the context of the problem can provide clues to its exact location and
possible causes. Questions to ask at this stage include:
• Has anyone connected or disconnected a PC (laptop or desktop) or any
other component to or from the network?
• Has anyone installed an interface card in a computer?
• Has anyone stepped on a cable?
• Has any maintenance work been performed in the building recently
(by a telephone company or building maintenance personnel, for example)?
• Has anyone (including cleaning personnel) moved any equipment or furniture?
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6
Figure 6.10 lists the most common causes of problems in coaxial cabling infrastructures:
•
C a b le c o n tin u ity is in te r r u p te d a t a T - c o n n e c to r .
•
C o n tin u ity fa u lt in a c o n n e c to r .
•
C o n n e c to r h a s a s h o r t c ir c u it.
•
C a b le in s u la tio n is d a m a g e d ( s h ie ld in g is v is ib le ) .
•
C a b le is k in k e d o r b e n t to o tig h tly .
•
C a b le is to o lo n g .
•
C a b le is n o t g r o u n d e d .
•
C a b le is g r o u n d e d in tw o p la c e s .
•
C a b le s e g m e n ts lo o k s im ila r , b u t h a v e d iffe r e n t im p e d a n c e s .
•
E le c tr o m a g n e tic in te r fe r e n c e ( e le v a to r s , e le c tr ic m a c h in e r y ) o r
m e c h a n ic a l s tr e s s ( d o o r s , fu r n itu r e ) e x is ts a lo n g c a b le r o u te .
•
Im p e d a n c e in n e w c a b lin g e x c e e d s to le r a n c e lim its , o r th e w r o n g
c a b le ty p e h a s b e e n in s ta lle d .
•
A M A U is d e fe c tiv e .
•
N e w ly in s ta lle d v a m p ir e ta p s a c t a s m in i- a n te n n a e a n d in d u c e
s ig n a ls .
•
N e tw o r k v o lta g e e x c e e d s a llo w a b le le v e ls d u e to a d e fe c tiv e M A U
o r la c k o f g r o u n d in g .
•
N e w c a b le s w ith h ig h a tte n u a tio n h a v e b e e n in s ta lle d ; v a r ia tio n s in
im p e d a n c e b e tw e e n c o n n e c to r s o r in p a tc h p a n e ls .
•
P h y s ic a l c o n n e c tio n to th e n e tw o r k c a b le is p o o r o r d o e s n o t e x is t
d u e to lo o s e B N C c o n n e c to r s o r to v a m p ir e ta p s th a t d o n o t r e a c h
th e in n e r c o n d u c to r .
•
T h e s e g m e n t is g r o u n d e d in m o r e th a n o n e p la c e .
•
S h o r t c ir c u it in th e c a b le . T h is c a n b e c a u s e d b y a n y o f th e
fo llo w in g : k in k s , c a b le d e fe c ts , v a m p ir e ta p d r ille d to o d e e p , o u te r
c o n d u c to r to u c h in g c o p p e r c o r e .
•
T e r m in a tin g r e s is to r is m is s in g o r d e fe c tiv e .
•
T e r m in a tin g r e s is to r s e x c e e d to le r a n c e lim its .
•
T e r m in a tin g r e s is to r is m is s in g , d e fe c tiv e o r n o t to s p e c ific a tio n .
•
T o o m a n y M A U s e x is t o n th e s e g m e n t.
•
V o lta g e in d u c e d b y e le c tr o m a g n e tic in te r fe r e n c e . C h e c k w h e th e r
p h o to c o p ie r s , p a g e r s , e le v a to r s o r x - r a y d e v ic e s a r e in u s e n e a r
th e a ffe c te d c a b le r o u te .
Figure 6.10 The most common causes of problems in coaxial cabling infrastructures
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6.2
6
Twisted-Pair Cable
6.2.1
Twisted-Pair Cable: Specification and
Implementation
Whereas coaxial cabling is unbalanced because the outer conductor is always at
ground potential, twisted pair is balanced cabling because the signals on each
wire of a pair have opposite potentials. Twisted-pair cable can have two or four
wire pairs, which are twisted together with 6 to 26 rotations per meter of cable.
Twisting the conductors together improves the cable’s immunity to electromagnetic interference (EMI). Cable with four wires twisted together is also referred
to as twisted quad. A distinction is made between shielded twisted pair (STP)
and unshielded twisted pair (UTP). STP has braided shielding, metal foil, or a
combination of the two wrapped around each wire pair. In addition to blocking
external EMI, this shielding also significantly reduces interference emitted by
the wire pairs themselves. Metal foil is more effective than braided shielding
against high-frequency EMI, while braided shielding is more effective in absorbing low-frequency radiation. In high-performance Category 7 cabling, for example, which is specified to 600 MHz, each pair of wires is wrapped in metal foil.
Cable with this type of shielding is also referred to as pairs in metal foil (PIMF).
Another cable type, known as screened STP (S/STP), has shielding around each
wire pair plus an additional wrapping of braided shielding around all pairs.
P a ir s h ie ld in g
S h ie ld e d
T w is te d P a ir ( S T P )
U n s h ie ld e d
T w is te d P a ir ( U T P )
C a b le s h ie ld in g
A ll fo u r w ir e s tw is te d
( tw is te d q u a d )
E a c h p a ir tw is te d
E a c h p a ir tw is te d
Figure 6.11 Structure of copper twisted-pair cabling
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The ISO/IEC IS 11801 standard, divides twisted-pair cabling into seven categories according to frequency ratings. Twisted-pair cabling is specified for frequencies of up to 600 MHz. Figure 6.12 shows the different categories with the
maximum distances that they can span.
C a b le ty p e
C la s s A
C la s s B
C la s s C
C la s s D
C la s s E
C la s s F
(1 0 0 k H z )
(1 M H z )
(1 6 M H z )
(1 0 0 M H z )
(2 0 0 M H z )
(6 0 0 M H z )
C a te g o ry 3
( s y m m e tr ic a l
c o p p e r c a b le )
2 k m
5 0 0 m
1 0 0 m
—
—
—
—
C a te g o ry 4
( s y m m e tr ic a l
c o p p e r c a b le )
3 k m
6 0 0 m
1 5 0 m
—
—
—
—
C a te g o ry 5
( s y m m e tr ic a l
c o p p e r c a b le )
3 k m
7 0 0 m
1 6 0 m
1 0 0 m
1 5 0 9
( s y m m e tr ic a l
c o p p e r c a b le )
3 k m
1 k m
2 5 0 m
1 5 0 m
C a te g o ry 6
( s y m m e tr ic a l
c o p p e r c a b le )
—
—
—
—
1 0 0 m
—
—
C a te g o ry 7
( s y m m e tr ic a l
c o p p e r c a b le )
—
—
—
—
—
1 0 0 m
—
—
O p tic a l
lin k s
—
—
—
—
—
M u ltim o d e
fib e r
—
—
—
—
—
—
2 k m
S in g le - m o d e
fib e r
—
—
—
—
—
—
3 k m
Figure 6.12 Performance categories for structured cabling
according to ISO/IEC IS 11801
The key operating parameters of twisted-pair cabling are the signal propagation
speed, attenuation, near-end crosstalk (NEXT) and the attenuation-to-crosstalk
ratio (ACR) (see Figure 6.13).
Essential factors for trouble-free network operation include not only the use of
high-quality cable of the appropriate category, but also connectors of the right
category. When components of different categories are used in the same network, the lowest-performance component on a given transmission path determines the overall transmission characteristics of that path. As with coaxial
cable, it is also important to make sure the minimum bending radius allowed for
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=
1 0
C a t. 3
2 0
a tte n u a tio n
n e a r-e n d
c r o s s ta lk lo s s
C a t. 5
3 0
= [d B /1 0 0 m ]
=
N E X T =
6
4 0
5 0
C a t. 7
6 0
7 0
8 0
1
1 0
3 1 :2 5
1 0 0
3 0 0
6 0 0
N E X T [d B ]
Figure 6.13 Specifications concerning attenuation and NEXT for cabling
of Categories 3, 5 and 7
C a b le ty p e
M in
b e n
d u r
u n d
im
d
in
e
u m
in g r a d iu s
g in s ta lla tio n
r te n s io n
1 0 0 9 , 1 2 0 9
tw is te d p a ir
8 t im e s t h e c a b le 's
o u ts id e d ia m e te r
M in im u m
b e n d in g r a d iu s
in in s ta lle d s ta te
M in im u m
o n e -tim e
b e n d in g r a d iu s
6 t im e s t h e c a b le 's
o u ts id e d ia m e te r
( in b a c k b o n e s )
N /A
4 t im e s t h e c a b le 's
o u ts id e d ia m e te r
(w o rk g ro u p a re a s )
1 5 0 9 tw is te d p a ir
N /A
7 .5 c m
2 c m
Figure 6.14 Minimum bending radii for twisted-pair cabling (ISO/IEC IS 11801)
the cable is low enough for your installation requirements, and to install protective fixtures where necessary to prevent undue mechanical stress on the cables.
6.2.1.1
Grounding Twisted-Pair Cable Infrastructures
Proper grounding of the cabling infrastructure is absolutely essential. Cable
shielding must be grounded both in the patch panel, which should have a ground
bus connected to the building ground, and in the wall jack. Connecting cables
and patch cables must be grounded in both the wall jack and the end device. It is
important that the potential in the wall jack ground is the same as that in the
patch panel and terminal device. A simple measurement with a multimeter is
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6
sufficient to determine whether there is a difference in potential between the
wall jack shielding and the patch cable shielding. If a noticeable voltage level is
detected, the grounding of the individual components must be reviewed so that
they all end at the same grounding block. Fiber-optic cabling is used to connect
floor wiring closets to the building distributor. This prevents current due to
varying ground potentials. This is even more important in connecting building
distributors to the campus backbone cable because there can be significant
differences in ground potentials from one building to another.
6.2.1.2
Electromagnetic Interference
It is important to make sure that the cable is installed at a sufficient distance
from potential sources of electromagnetic interference, especially when using
unshielded twisted-pair cables. The North American specification EIA 569 describes the following guidelines for determining appropriate distances:
S o u rc e o f
e le c tr o m a g n e tic
in te r fe r e n c e
M in im u m d is ta n c e
a t lin e p o w e r < 2 k V A
M in im u m d is ta n c e
a t lin e p o w e r 2 to 5 k V A
M in im u m d is ta n c e
a t lin e p o w e r > 5 k V A
U n
o r
n e
c a
s h
e le
a r
b le
ie ld e
c tr ic
o p e n
d u c
d p o w e r lin e s
a l e q u ip m e n t
o r n o n - m e ta llic
ts
5 in
1 ft
2 ft
U n
o r
n e
c a
s h
e le
a r
b le
ie ld e
c tr ic
m e ta
d u c
d p o w e r lin e s
a l e q u ip m e n t
llic , g r o u n d e d
ts
3 in
6 in
1 ft
—
6 in
1 ft
T ra n s fo rm e rs ,
e le c tr ic m o to r s
—
—
3 ft
F lo u r e s c e n t tu b e s
—
—
1 ft
P o w e r
o r e le c
g ro u n d
n e a r m
c a b le d
lin e s
tr ic a l e q u ip m e n t in
e d m e ta llic s h ie ld in g
e ta llic , g r o u n d e d
u c ts
Figure 6.15 Guidelines for minimum distances between cabling and potential sources
of electromagnetic interference (EIA 569)
6.2.2
Troubleshooting Twisted-Pair Cable
The main task in troubleshooting twisted-pair cabling infrastructures is the
measurement of key operating parameters, such as cable length, attenuation,
NEXT, ACR, and signal-to-noise ratio (SNR). To eliminate the possibility of
incorrect pin assignments right at the outset, however, it is a good idea to begin
by comparing the pin assignments in the patch panel with those in the corresponding wall jacks. This type of wiring fault can result from incorrect manual
wiring, but may also be due to the use of different color-coding systems. The
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6
specifications TIA/EIA 568A and TIA/EIA 568B, for example, define different
systems of color-coding for twisted-pair wiring. Figure 6.16 points out typical
wiring errors, as well as the color-coding systems used in TIA 568A and in
TIA 568B.
P a ir
2
P a ir
3
P a ir
1
1
2
3
4
T IA /E IA 5 6 8 A c o lo r c o d e
(o p tio n 1 )
T IA /E IA 5 6 8 A c o lo r c o d e
(o p tio n 2 )
5
P in
C o lo r
1
w h
g re
w h
b lu
w h
o ra
w h
b ro
P a ir
4
6
7
8
2
3
4
5
T 5 6 8 A
6
7
8
P a ir
3
P a ir
1
P a ir
2
1
2
3
4
5
7
P in
C o lo r
re e n
1
ra n g e
3
b lu e
o ra n
b la c
re d
g re e
y e llo
b ro w
g ra y
2
4
lu e
5
6
ro w n
7
8
n
w
n
T IA /E IA 5 6 8 B c o lo r c o d e
(o p tio n 2 )
P in
C o lo r
1
w h
o ra
w h
b lu
w h
g re
w h
b ro
8
2
3
4
5
T 5 6 8 B
6
7
8
it e /o
n g e
it e /g
e
it e /b
e n
it e /b
w n
P in
C o lo r
ra n g e
1
re e n
3
b la c
y e llo
b lu e
re d
g re e
o ra n
b ro w
g ra y
2
4
lu e
5
6
ro w n
7
8
P in 1
C o r r e c t w ir in g
g e
k
T IA /E IA 5 6 8 B c o lo r c o d e
(o p tio n 1 )
P a ir
4
6
it e /g
e n
it e /o
e
it e /b
n g e
it e /b
w n
R e v e r s e d p a ir
k
w
n
g e
n
P in 8
S p lit p a ir
T r a n s p o s e d p a ir
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
6
6
6
6
6
6
6
6
7
7
7
7
7
7
7
7
8
8
8
8
8
8
8
8
Figure 6.16 Color coding and wiring errors in twisted-pair cabling
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6
To test for faulty pin assignments in the cable, connect the wall plug to a number
of different terminating resistors in turn, and test the wire pairs coming from
the patch panel with each of the resistors. Reversed pairs are the most common
wiring errors and are caused by a simple reversal of the wires, usually at the
patch panel. Some wiring errors, such as split pairs, cannot be detected using
this procedure. Split pairs can only be detected indirectly, when NEXT tests
produce unusually poor results. Split pairs are usually the result of incorrect
installation by technicians unfamiliar with twisted pair. Transposed pairs usually result from counting pins from the wrong side of the jack.
Special testing equipment can perform most measurements on twisted-pair
cables. Cables are directly connected to a tester using an RJ-45 connector. In
some cases, it is also necessary to perform TDR measurements to create a
detailed impedance profile for a given length of cable. This requires an oscilloscope, which can easily be connected to a wire pair by means of an RJ-45
breakout box. The pulse for the TDR test can be produced by most cable testing
devices. Figure 6.17 shows typical TDR measurement results. Each deviation in
R e fle c te d
p u ls e
T e s t p u ls e
R e fle c te d
p u ls e
T e s t p u ls e
D is c o n tin u ity
C a b le u n d e r
te s t
R J -4 5
R J -4 5
P u ls e g e n e r a to r
( C a b le te s te r )
C o n n e c to r p r o b le m
R e fle c te d
p u ls e
T e s t p u ls e
S h o r t c ir c u it
Figure 6.17 TDR measurements on twisted-pair cables
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6
impedance along the cable under test (interruptions, connector problems, short
circuits, etc.) causes a reflection of the test pulse. The oscilloscope display
shows the type and location of the impedance deviation. Often a number of
minor impedance anomalies are detected, none of which would cause a problem
on its own, but which are compounded with other errors to cause serious
malfunctions.
6.2.3
Symptoms and Causes: Twisted-Pair Cable
Symptom: Diminished Network Performance, Collisions,
Frame Check Sequence (FCS)Errors
Cause (1):
Cause (2):
Cause (3):
Cause (4):
Cause (5):
Crosstalk due to split pairs, untwisted cable segments in patch
panels or Token-Ring concentrators.
Crosstalk due to insufficient cable quality for the data speeds
used.
Crosstalk and reflections due to unsuitable connector systems
(connectors, wall jacks, etc.) for high data speeds.
Electromagnetic interference: cables near photocopiers, power
lines, x-ray systems, pagers, production environments or other
source of EMI.
Cable and wall plug shielding not grounded or grounded to different potentials.
Symptom: Unusually High Number of Collisions and Fragments
Cause (1):
Cause (2):
Cable impedance exceeds tolerance limits.
This can be caused by the poor quality of the cable itself or by
poor installation (cabling bundled too tightly or bent).
Faulty wiring in patch panel; faulty or loose connectors.
Symptom: Intermittent Loss of Connection or No Connection After New
Installation
Cause (1):
Cause (2):
Cause (3):
Excessive attenuation in newly installed cables; variations in impedance between connectors and cables or in patch panels; inadequate wiring through several patch panels.
Wiring faults (reversed pairs, split pairs, transposed pairs).
Patch faults at patch panel.
Symptom: Complete Failure of a Station
Cause (1):
Cause (2):
Faulty cable; bent cable.
Faulty connection between cable and connector due to defective
or low-quality crimp components.
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6
Twisted-pair cable can have either solid or stranded wire. If RJ-45
plugs designed for stranded wire cables (primarily used as patch
cables) are used for solid wire, the crimp contacts just touch the
surface of the wire and lose contact over time (see Figure 6.18).
a )
b )
Figure 6.18 Crimping solid and stranded wire cables
Cause (3):
Short circuit in the cable due to kinks, excessive bending, defective material, or a nail inadvertently driven through the cable.
Gathering Information; Common Errors
As with coaxial cable, comprehensive information about the context of a problem with twisted-pair cable provides a detailed description of the symptoms and
clues to possible causes. Questions to ask at this stage include:
• Has anyone connected or disconnected a PC (laptop or desktop) or any
•
•
•
•
•
other component to or from the network?
Has anyone installed an interface card in a computer?
Has a telephone or fax machine been installed?
Has anyone stepped on a cable?
Has any maintenance work been performed in the building recently (by a
telephone company or building maintenance personnel, for example)?
Has anyone (including cleaning personnel) moved any equipment or furniture?
Figure 6.19 lists the most common causes of problems in twisted-pair cabling
infrastructures:
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163
CABLE INFRASTRUCTURES IN LOCAL-AREA NETWORKS
•
W ir in g e r r o r s in p a tc h p a n e ls
•
W ir in g e r r o r s in c o n n e c to r s
•
C r o s s ta lk d u e to s p lit p a ir s
•
N e a r - e n d c r o s s ta lk d u e to in fe r io r c a b le u n s u ita b le fo r h ig h - s p e e d d a ta
•
C o n tin u ity fa u lts d u e to p o o r c r im p c o n n e c tio n s in c o n n e c to r s
•
C o n tin u ity fa u lts d u e to b e n d s , k in k s , lo o s e c o n n e c to r s
•
S h o r t c ir c u its d u e to m e c h a n ic a l s tr e s s o r m a te r ia l d e fe c ts
•
N e a r - e n d c r o s s ta lk d u e to fa u lty in s ta lla tio n o f p a tc h p a n e ls o r c a b le r o u tin g
•
N e a r - e n d c r o s s ta lk d u e to u n tw is te d p a tc h in g ( in p a tc h p a n e ls o r T C U s )
•
Im p e d a n c e a n o m a lie s d u e to c r a m p e d c a b le b u n d le s
•
E x c e s s iv e a tte n u a tio n b e c a u s e c a b le s a r e to o lo n g
•
E le c tr o m a g n e tic in te r fe r e n c e ( e le v a to r s , e le c tr ic m a c h in e r y )
o r m e c h a n ic a l s tr e s s ( d o o r s , fu r n itu r e )
6
Figure 6.19 The most common causes of problems
in twisted-pair cabling infrastructures
6.3
6.3.1
Fiber-Optic Cable
Fiber-Optic Cable:
Specification and Implementation
In recent years fiber-optic cabling has proven to be a sturdy, high-performance
transmission medium for use in both primary (campus) and secondary (building or vertical) cabling. The fiber-optic cables most commonly used are multimode cables with wavelengths of 850 nm or 1,300 nm. At power levels of
-10 dBm to -20 dBm, signals can travel distances of some 5 kilometers. The
dynamic range of such segments is between 15 dB and 20 dB. With the trend
toward data speeds of 622 Mbit/s and 2.4 Gbit/s, however, even LAN infrastructures are increasingly coming to rely on single-mode, fiber-optic cabling with a
wavelength of 1,550 nm.
Two types of connectors are used in fiber-optic LANs: those that make contact
between one fiber and the next fiber (mated connections), and those that leave
an air gap between the fiber end and the connector (unmated connections).
Unmated connections are easier to work with because they are not as easily
contaminated as mated connections, but they have a higher insertion loss.
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6
Figure 6.20 lists the types of fiber-optic cabling and connection systems used in
various LAN topologies.
L A N to p o lo g ie s :
E th e rn e t
F O IR L
W a v e le n g th
(n m )
8 5 0
8 5
(7 9 0 – 9 1 0 ) 8 0
(a
8 2
(p
0
0 –
c tiv
0 –
a s s
9 1 0
e )
9 1 0
iv e )
S ig n a l p o w e r
(d B m )
-1 2 to -1 5
-1
(a
-1
(p
2 to
c tiv
1 to
a s s
-2 0
e )
-1 5
iv e )
R e c e iv e r
s e n s itiv ity
(d B m )
- 9 to -2 7
M a x im u m n e tw o r k
le n g th ( k m )
4 .5
M a x im u m
s e g m e n t
le n g th ( k m )
1
C o n n e c to r ty p e s
S T
1 0 B a s e -F
T o k e n B u s
T o k e n R in g
F D D I
( m u ltim o d e )
8 5 0
8 0 0 – 9 1 0
8 5 0
8 0 0 – 9 1 0
1 3 0 0
1 3 0 0
(1 2 7 0 – 1 3 8 0 ) (1 2 7
C a t.
(1 2 9
C a t.
-7 to -1 1
-1 3 to -2 2
-1 4 to -2 0
-1
(C
-1
(C
4 to
a t.
5 to
a t.
-2 0
1 )
-3 7
2 )
-1 2 to -3 2 .5
( a c tiv e )
- 2 7 to -4 1
( p a s s iv e )
-3 1 /4 1
to -1 1 /-2 1
-1 2 to -3 0
-1 4 to -3 1
-1
(C
-1
(C
4 to
a t.
5 to
a t.
-3 1
1 )
-3 7
2 )
4 .5
4 .5
4 .5
2 0 0
2 0 0
2
F -S M A ,
S T
S C
1
S T , F -S M A
E 2 0 0 0
2
M IC ; S T ;
B ic o n ic ,
F -S M A ,
M in i- B N C
M in i- B N C
2
M IC ; S T ;
B ic o n ic ,
F -S M A ,
M in i- B N C
F D D I
F ib r e
( s in g le - m o d e ) C h a n n e l
0 - 1 3 4 0 ,
1 )
0 - 1 3 3 0 ,
2 )
2
M IC , S T
F -S M A
1 3 0 0
G ig a b it
E th e rn e t
8 5 0
( m u ltim o d e )
1 3 ,0 0 0
( s in g le - m o d e )
1 ( 2 0 0 M b it/s ) 0 .5 5
> 1 0
( m u ltim o d e 5
( 8 0 0 M b it/s )
s in g le - m o d e )
S M -M IC
E s c o n
S C
S C D u p le x
S C
D u p le x - M IC
F D D I
Figure 6.20 Fiber-optic cabling and connectors in local-area networks
The permissible length for a fiber-optic cable is derived from the available
attenuation budget, which in turn depends on the performance of the transmitting device and the sensitivity of the receiver. Each splice in a cable increases the
attenuation by about 0.1 dB; each connector by up to 0.5 dB.
6.3.2
Troubleshooting Fiber-Optic Cable
The most important tasks in diagnosing problems with fiber-optic cable involve
checking the power and attenuation, and taking measurements with an optical
time-domain reflectometer (OTDR).
Attenuation is measured in order to determine whether or not the total attenuation of a cable segment exceeds the attenuation budget. The first step is to set
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6
up the testing conditions in accordance with IEC 874-1, Method 6. This involves
inserting a reference cable in place of the cable segment to be tested, between
the transmitter and receiver of the measuring instrument. A reference measurement is taken from this cable and compared with the results measured on the
actual network cable. The reference values serve to offset the influence of the
test location and the test connector. Once the reference values have been
recorded, the same measurements are performed on the cable to be tested (see
Figure 6.21).
T e s t c a b le , te s t c o n n e c to r
S te p 1 :
C a lib r a tin g th e te s t s e tu p
T r a n s m is s io n p a th
u n d e r te s t
S te p 2 :
M e a s u r e m e n t a tte n u a tio n
o v e r th e tr a n s m is s io n p a th
Figure 6.21 Attenuation measurement with test setup calibration
in accordance with IEC 874-1
OTDR measurements are used to detect individual components of the overall
attenuation along a segment of fiber-optic cable, and to precisely locate each
component. When performing OTDR measurements, keep in mind that most
OTDR devices have a “dead zone” at close range. The dead zone is the range that
cannot be reliably evaluated due to reflections in the device connector and an
overdriven OTDR. The sensitivity of the OTDR determines the maximum amplitude difference that the testing device can detect between the signal pulse and
the reflected signal. Over longer cable segments, a higher dynamic means the
ability to measure greater distances. With the relatively short distances found in
LANs, high dynamics are also an advantage because they allow the use of
shorter pulses, resulting in higher resolution. The shorter the signal pulse, the
easier it is to detect separate reflections that arrive in rapid succession.
Regular maintenance of fiber-optic cables is also important. Cable maintenance
tasks include regular measurements of the signal strengths of all active components, as well as checking the connector contacts for crossed fibers and contamination.
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166
6.3.3
CABLE INFRASTRUCTURES IN LOCAL-AREA NETWORKS
6
Symptoms and Causes: Fiber-Optic Cable
Symptom: No Connection
Cause:
Line break due to external shock or extreme bending in the cable
The bending radius of the cable must not be less than 20 times the
cable diameter. Thus for internal fiber-optic cables with a diameter of 3 mm, for example, the minimum bending radius allowed is
6 cm. Check all bend radii; if necessary, switch to an intact pair
for the connection in question.
Use an OTDR to check for breaks in the cable. If no suitable test
device is available, the “lighter test” can be used for cable segments of up to several hundred meters: hold a lighter to the cross
section of the fiber at one end and verify that light is visible with
the naked eye at other end.
Symptom: No Connection or Intermittent Connection Problems
Cause (1):
Cause (2):
Cause (3):
Attenuation too high due to poor workmanship on splices or too
many splices.
Check the signal strength using an optical source and power meter.
Contamination of connectors (dust, fingerprints, humidity, etc.)
Transmitter signal strength too low.
Increase light power or replace the LED/laser module.
Symptom: No Connection After New Installation
Cause (1):
Cause (2):
Cause (3):
Faulty connection in wiring closet.
Poor splice; attenuation too high.
Dirty connectors (dust, fingerprints, humidity, etc.).
Gathering Information; Common Errors
As with copper cables, comprehensive information about the context of a
problem with fiber-optic cable provides a detailed description of the symptoms
and clues to possible causes. Questions to ask at this stage include:
• Has anyone connected or disconnected a PC (laptop or desktop) or any
other component to or from the network?
• Has anyone installed an interface card in a computer?
• Has anyone stepped on a cable?
• Has any maintenance work been performed in the building recently (by a
telephone company or building maintenance personnel, for example)?
• Has anyone (including cleaning personnel) moved any equipment or furniture?
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CABLE INFRASTRUCTURES IN LOCAL-AREA NETWORKS
6
The most common causes of problems in fiber-optic cabling infrastructures are
listed here:
•
E x c e s s iv e lo s s d u e to fa u lty s p lic e s o r c o n n e c to r s ;
to o m a n y s p lic e s o r c o n n e c to r s
•
F ib e r b r e a k d u e to m e c h a n ic a l s tr e s s o r in s u ffic ie n t b e n d in g r a d iu s
•
W r o n g fib e r c o n n e c te d in s p lic e tr a y o r a t p a tc h p a n e l
•
In s u ffic ie n t tr a n s m itte r p o w e r
•
E x c e s s iv e lo s s d u e to c o n ta m in a te d c o n n e c to r
•
E x c e s s iv e lo s s d u e to e x c e s s iv e c a b le le n g th
Figure 6.22 The most common causes of problems in fiber-optic cabling infrastructures
SECTION II
TROUBLESHOOTING LOCAL-AREA NETWORKS
Index p1
CABLE INFRASTRUCTURES IN LOCAL-AREA NETWORKS
Index of chapter 6
A
Air gap 163
Application classes for structured cabling 146
Attenuation budget 164
B
BNC connectors 148
C
Campus backbone 145
Category 7 cabling 155
Cheapernet 147
Coaxial cable 147
Color coding 159
D
Dead zone 165
E
EIA/TIA 568A 145
Electromagnetic interference 158
EN 50173 145
F
Failure of a segment 152
Fiber-optic cable 163
Fiber-optic cabling and connectors 164
G
Grounding twisted-pair cable 157
H
Horizontal cabling 145
I
IEC 874-1 165
Insertion loss 163
ISO/IEC IS 11801 145
6
SECTION II
TROUBLESHOOTING LOCAL-AREA NETWORKS
Index p2
CABLE INFRASTRUCTURES IN LOCAL-AREA NETWORKS
K
Kinks 152
L
Loss of connection 152
M
Mated connections 163
Measuring cable lengths 152
Measuring electromagnetic interference 151
Measuring resistance in coaxial cable 149
Measuring the voltage in network cabling 149
MIL-C-17G 147
N
N-type connectors 148
O
OTDR measurements 165
P
Pairs In Metal Foil (PIMF) 155
R
RG-6, RG-11 147
RG-62 coaxial cable 147
S
Screened STP (S/STP) 155
Search for defects in coaxial cable 151
Shielded Twisted Pair (STP) 155
Short circuit 152
Solid wire 162
Stranded wire 162
T
Testing the terminating resistor 150
Thick Ethernet 147
Thin Ethernet 147
Troubleshooting coaxial cable 149
Troubleshooting fiber-optic cable 164
Troubleshooting twisted-pair cable 158
Twisted-pair cable 155
6
SECTION II
TROUBLESHOOTING LOCAL-AREA NETWORKS
Index p3
CABLE INFRASTRUCTURES IN LOCAL-AREA NETWORKS
U
Unmated connections 163
Unshielded Twisted Pair (UTP) 155
V
Vampire tap 152
Vertical backbone 145
W
Wiring errors 159
Y
Yellow cable 147
6