The Effect of Network Cabling on Bit Error Rate
Performance
By Paul Kish NORDX/CDT
April '00
Table of Contents
Introduction............................................................................................. 2
Probability of Causing Errors....................................................................... 3
Noise Sources Contributing to Errors........................................................... 4
Bit Error Rate due to NEXT.......................................................................... 4
Insertion Loss Deviation (ILD).......................................................................6
Bit Error Rate due to ILD.............................................................................. 9
Conclusion............................................................................................... 12
Page 1 of 12
INTRODUCTION
There is a new buzzword in the cabling industry called “zero bit error rate”. What
do we mean by zero bit error rate? How do we measure bit error rate? What
causes errors in digital transmission systems? How can we relate the number of
bit errors to the performance of a channel? This white paper will look into the
whole concept of bit error rates, first to provide an understanding of the
underlying fundamentals and second to try and put a handle on this elusive
concept. So let’s have a closer look at what we mean by bit error rates. Is it an
issue that we need to be concerned about for Category 5, Category 5e or
Category 6 channels?
The issue of bit error rate (BER) is addressed by the Institute of Electrical and
Electronic Engineers (IEEE), which is the organization in the industry that sets
the standards for data networking. The IEEE has published a number of
standards over the last 10 years. One of the most influential data-networking
standards in the industry is 10BASE-T Ethernet, which is designed to run over
minimum Category 3 twisted pair cabling. It has gained a reputation in the
industry as being a very robust and reliable network. Today, there is a migration
from 10BASE-T Ethernet to fast Ethernet (100BASE-TX), which is designed to
run over minimum Category 5 cabling. There is a perception in the industry that
fast Ethernet is not as robust and is more susceptible to errors due to cabling and
equipment related imperfections. In this paper we will examine the types of
cabling imperfections that can generate errors. The reader may be surprised at
the outcome.
A realistic objective for 100BASE-TX is a worst case BER of 10-10. This means
that the number of bit errors should not exceed 1 error in 10,000,000,000 bits (10
billion bits) of information. For an information transfer rate of 100 Mb/s, this
translates to a maximum of 1 error every 100 seconds. Within the Ethernet
protocol, bits of information are packaged into frames. Any error that is detected
within a frame would signal a retransmission of the affected frame. A maximum
BER of 10-10 is a reasonable upper limit for errors. A BER significantly in excess
of this will start to affect data throughput and slow down network performance.
Page 2 of 12
Probability of Causing Errors
Before a signal is transmitted over a channel, the bits of information are coded
into symbols using digital modulation techniques. One common technique is
called Pulse Amplitude Modulation (PAM). For this modulation scheme, a
symbol is encoded into discrete signal levels. For example, a two-level symbol
can be used to represent one bit of information (binary 1 or 0). The conversion
or coding of these symbols into real, temporal waveforms to be transmitted over
a channel is called line coding.
Error Probability (PAM)
0
5
10
15
20
25
30
1.00E+00
1.00E-01
Symbol Error Probablilty
1.00E-02
1.00E-03
1.00E-04
1.00E-05
1.00E-06
1.00E-07
P(e) M=2
P(e) M=3
P(e) M=5
1.00E-08
1.00E-09
1.00E-10
1.00E-11
1.00E-12
1.00E-13
1.00E-14
1.00E-15
1.00E-16
Signal-to-Noise Ratio (dB)
Figure 1 - Probability of error, P(e), for PAM signals, where M is the discrete
number of signal levels assuming a white Gaussian noise process.
The performance of a modulation scheme is measured by its symbol error
probability P(e), which is the probability that a waveform is detected incorrectly.
There is a fundamental relationship between the probability of error P(e) and the
signal-to-noise ratio (SNR) of a channel. The Symbol Error Probability is shown
in Figure 1 as a function of the SNR for PAM signals having (M) discrete signal
levels.
The bit error probability Pb(e), which is equal to the BER, is derived from the
P (e )
symbol error probability using the simple relationship
≤ Pb (e) ≤ P (e)
log 2 M
Page 3 of 12
From the above, we can conclude that there is no such thing as a zero bit error
rate. In practice, if the noise level is sufficiently small, then the probability of
causing an error is some very small number such as 10-12 or less but not zero.
We can also conclude that in order to achieve a BER of 10-10 the SNR needs to
be greater than 13 dB for PAM-2, 16 dB for PAM-3 and 18 dB for PAM-5 coding,
respectively.
Noise Sources Contributing to Errors
There are different types of noise sources that can contribute to errors in digital
transmission systems. These can be either externally generated or internally
generated. For the purpose of this paper we will look at internally generated
noise due to transmission impairment of a channel. Fast Ethernet (100BASETX) operates over a channel that uses two pairs, one pair for each direction of
transmission. For this application, the dominant internally generated noise
sources include 1) Near End Crosstalk (NEXT) interference, and 2) an added
noise component that is related to Insertion Loss Deviation (ILD). The latter
noise source may come as a surprise to some readers since it is not a specified
parameter for Category 5 channels. In fact, this paper will show that ILD noise
can be, by far, much worse than NEXT for worst case Category 5 channels.
Bit Error Rate due to NEXT
The noise component due to NEXT is illustrated in Figure 2 below for
the100BASE-TX application
R2
IL2
Insertion Loss
100 Mb/s
S2
100 Mb/s
NEXT
N32
100 Mb/s
100 Mb/s
S3
Figure 2 – Near End Crosstalk noise between a transmit pair and a receive pair for 100BASE-TX
Page 4 of 12
For the configuration shown in Figure 2, the SNR in dB is determined using
equation (1), i.e. by subtracting the total noise power due to NEXT from the total
signal power at the receiver.
SNR = 10 log( ∫
150
1
S 2 − IL 2
S 3 − N 32
150
10 10 df ) − 10 log(∫1 10 10 df )
…(1)
where, S2 ~ S3 = Power spectrum of the transmit signal on pair 2 and 3, respectively
For this calculation, the power spectrum of the noise and receive signals are
given by the terms (S3 – N32) and (S2 – IL2), respectively. The total power of the
noise and receive signals is determined by integration over the frequency band
from 1 to 150 MHz. To perform this calculation, the power spectrum of a
100BASE-TX signal was obtained from measurements using a spectrum
analyzer as illustrated in Figure 3.
Signal Spectral Density
Calculated Power = 11.52 dBm
0
Spectrum Setting for trace measurement
Center Freq. 1
Span Freq. 150
Resolution BandWidth 300 kHz
Reference Level 0
-10
-20
dBm
-30
-40
-50
-60
-70
0
20
40
60
80
100
120
140
160
Frequency (MHz)
Figure 3 – Power spectrum of a 100BASE-TX signal measured at the output of the transmitter
Using the worst case parameters for a Category 5 channel the signal-to-noise
ratio due to NEXT works out to be 30 dB over the complete frequency band from
1 to 150 MHz. This is significantly better than the16 dB threshold that is required
to achieve a BER of 10-10 for MLT3 (3-level coding). From this, it is concluded
that NEXT for a Category 5 channel is not a major contributor to system error
rates for the 100BASE-TX application.
Page 5 of 12
Note: For comparison purposes, the SNR due to NEXT was calculated by
substituting a Category 3 cable instead of Category 5 (keeping the same
connectors). For this case, the SNR works out to be 10 dB. This is 6 dB
worse than the required SNR to achieve a BER of 10-10. From Figure 1,
this would lead to an excessively high BER of ~ 10-3.
Insertion Loss Deviation (ILD)
Insertion Loss Deviation (ILD) is a new parameter for many readers. It is
important to get a good understanding of what it is and why it is so important. A
channel, as specified in the TIA/EIA 568-A standard, is made up of components
that include cords, connectors and cables. In a worst case configuration, a
channel can include up to four connectors (two at each end), an equipment cord,
a patch cord, a horizontal cable, a furniture cable (part of horizontal) and a work
area cord. All these components can be characterized as having an insertion
loss (also called attenuation) and an impedance.
Traditionally, the total insertion loss of a channel is determined by adding up the
loss of all the components and calling it the channel insertion loss. All the
formulas for a Category 5 and Category 5e channel model in the TIA standard
make the assumption that the whole is equal to the sum of the parts. The
problem is that it is only an approximation. It is a good first approximation, but
the channel insertion loss is in fact higher than the just adding up the loss of each
component. The additional losses are due to signal reflections and re-reflections
at the boundaries between different components, as illustrated in Figure 4.
Signal in
Signal out
re-reflected
signal
ILD = additional losses due to signal reflections
Figure 4 – Insertion Loss deviation (ILD) is due to impedance mismatch between components
Page 6 of 12
The greater the impedance mismatch between components, the higher the
mismatch losses. What makes it even more interesting is that at some
frequencies these signal reflections can add in phase and at other frequencies
they can add out of phase. Therefore, these additional losses are not uniform,
and will vary depending on the length of patch cord, the number of connectors,
the length of channel, etc. The difference between the actual Insertion Loss as
measured on a channel and the Insertion Loss as determined by adding the
component losses is called the Insertion Loss Deviation.
What is the expected ILD for Category 5, Category 5e and Category 6 channels?
This was determined using a channel model as described in Annex H of TIA/EIA
568-A-5. It consists of a series of concatenated transmission lines where each
component is modeled by its own transmission matrix [T], where [T] is described
in terms of its A,B,C,D parameters. The overall transmission matrix of a channel
is then determined by matrix multiplication. For the results reported in this paper,
the following channel configuration was used for modeling purposes and was
maintained fixed throughout, except for changing out the components.
Signal In
Equipment
2m
2m
Outlet
Cross-connect
CP
85 m
6m
5m
Signal
Out
Figure 5 – Channel configuration used for ILD modeling including 4 connectors (100 meters)
Page 7 of 12
The ILD was determined for the following cases:
1.
2.
3.
4.
5.
Cat 5 cable (105 Ω), cord (90 Ω) & connector (14 dB RL @ 100 MHz)
Cat 5 cable (105 Ω), cord (85 Ω) & connector (14 dB RL @ 100 MHz)
Cat 5e cable (105 Ω), cord (95 Ω) & connector (20 dB RL @ 100 MHz)
Cat 6 cable (103 Ω), cord (97 Ω) & connector (24 dB RL @ 100 MHz)
Cat 6 cable (101 Ω), cord (99 Ω) & connector (24 dB RL @ 100 MHz)
The ILD results are presented in Figure 6. The modeling results for a Category
5 channel, labeled as Cat 5-1 and Cat 5-2, show a considerable deviation with
significant peaks of approximately 0.3 dB in the range from 10 to 20 MHz and 1.4
dB at 100 MHz. The cord length of 4 meters (2 m + 2 m) and the difference in
impedance between the cord and the cable of 15 Ω and 20 Ω respectively were
deliberately chosen at the extreme end of the tolerance range for Category 5.
This was done in order to simulate a worst case condition for the 100BASE-TX
application where the bulk of the signal energy is less than 30 MHz. The results
for a Category 5e channel are much better with an ILD of less than 0.1 dB below
30 MHz and maximum ILD of less than 0.5 dB at 100 MHz. The Category 6
results are also reported for comparative purposes. They do not represent the
worst case, but are representative of the type of results that can be achieved
using very well matched components.
Insertion Loss Deviation
2.5
2
ILD Cat6-1
ILD Cat6-2
ILD Cat5e
ILD Cat5-1
ILD Cat5-2
dB
1.5
1
0.5
0
0
20
40
60
80
100
120
140
160
Frequency (MHz)
Figure 6 – Insertion Loss Deviation Modeling Results for Category 5, 5e and 6 channels
Page 8 of 12
Bit Error Rate due to ILD
Now comes the interesting part. Let’s have a closer look at the ILD results in
Figure 6. The obvious question is does it really matter? So what if there is a
deviation (additional losses) of something like 0.5dB over most of the operating
frequency range for 100BASE-TX. Is it really significant? It seems like such a
small number.
The answer comes by asking another question. What causes ILD? ILD is
caused by another signal that is superimposed or is riding on top of the receive
signal. This superimposed signal appears as a noise source at the receiver. In
this paper it is referred to as an equivalent ILD noise source and is designated by
the term (ILX). ILX is the level of noise in dB that when added (as a power sum)
to the primary signal produces the observed value of ILD.
Note: It should be noted that the results obtained using this approach
tend to be conservative, i.e the equivalent noise level may be higher than
what is obtained when ILD is determined using a least squares curve fit
through the measured insertion loss data. This means that the BER
performance for a given mismatch condition may be better than what is
predicted, i.e. the error is on the safe side.
Table 1 below illustrates how this calculation is performed. The first column is
the primary signal at the output of a channel having an Insertion Loss (IL), where
it is assumed that all components have the same impedance. The second
column is the noise component ILX that is riding on top of the signal and
represents the effect of impedance mismatches. The third column is the
resultant signal and is calculated as the power sum of the primary signal plus the
noise component. The fourth column is the Insertion Loss Deviation (ILD) and is
calculated as the difference between the third column and the first column.
Page 9 of 12
For example, using Table 1, if the measured Insertion Loss is 20.5 dB and the
expected Insertion Loss assuming matched components is 20 dB, then the ILD of
0.5 dB and would be caused by an ILX noise component of ~ 30 dB.
IL
(dB)
20.0
20.0
20.0
20.0
20.0
20.0
20.0
20.0
20.0
20.0
20.0
20.0
20.0
ILX
(dB)
40.0
38.0
36.0
34.0
32.0
30.0
28.0
26.0
25.0
24.0
23.0
22.0
21.0
Psum( IL + ILX )
(dB)
20.04
20.07
20.11
20.18
20.28
20.46
20.75
21.26
21.65
22.20
23.02
24.33
26.87
ILD
(dB)
0.04
0.07
0.11
0.18
0.28
0.46
0.75
1.26
1.65
2.20
3.02
4.33
6.87
Table 1 – Determination of the equivalent noise (ILX) that yields a corresponding value of (ILD)
The SNR due to ILD noise is derived using equation (2) and is similar to the
derivation for NEXT noise except that the term (S2 – ILX2) is substituted for the
term (S3 - N32).
SNR = 10 log( ∫
150
1
S 2 − IL 2
S 2 − ILX 2
150
df
)
10
log(
−
10 10
∫1 10 10 df )
…(2)
where, S2 = Power spectrum of the transmit signal on pair 2
The results of this derivation are shown in Figure 7. Using the worst case
parameters for a Category 5 channel, the signal-to-noise ratio (SNR) due to ILD
noise works out to be 14.2 dB for a 20 Ohm impedance mismatch between the
cord and the cable and 16.5 dB for a 15 Ohm impedance mismatch. This
illustrates that under certain worst case conditions, a Category 5 channel is really
borderline compliant for 100BASE-TX application, Surprise! What does this
mean in practice? This situation is unlikely to occur in practice except when the
user cords and cables are at the extreme ends of the Category 5 tolerance
range. That is why it is so important that all the components are designed to
work together as part of an end to end solution.
Page 10 of 12
ILD Equivalent Noise
70
60
SNR=31.6 dB
Att dB
ILX Cat6-1
IL Cat6-1
IL Cat5-1
ILX Cat5-1
IL Cat5-2
ILX Cat5-2
NEXT Cat5
ILX Cat5e
ILX Cat6-2
50
SNR=25.5 dB
dB
40
30
SNR=21 dB
SNR=16.5 dB
20
SNR=14.2 dB
10
0
0
20
40
60
80
100
120
140
160
Frequency (MHz)
Figure 7 – Insertion Loss Deviation shown as an equivalent noise source for different channels
and the corresponding SNR for these channels for a 100BASE-TX application
The other important result is that a Category 5e channel, which is specified with
tight tolerances on component return loss (a good measure of impedance
mismatch), achieves a SNR of 21dB. This provides a 5 dB margin above the
SNR objective for 100BASE-TX and a 3 dB margin for 1000BASE-T. A Category
6 channel (proposed), using well-matched components, provides much higher
margins.
Page 11 of 12
Conclusion
Network managers are concerned about the reliability and performance of their
data networks, which are increasingly becoming an essential lifeline to their dayto-day business operations. Loss of a network or a slowdown in performance
can be very costly. Network performance depends on the ability of the network
cabling to support data rates of 100 Mb/s today and 1000 Mb/s or higher
tomorrow. An important question to ask is - Does my cabling system have the
built-in performance margin to deliver this information without stumbling? One
measure of network performance is the “bit error rate” (BER). From the results
presented in this paper, it is concluded that component impedance mismatch is
the major contributor to bit errors and can cause some Category 5 channels to
exceed the BER objective for 100BASE-TX applications. It is also concluded that
channels and components that are certified to meet Category 5e requirements
have an inherent margin of at least 3 dB beyond the minimum required to meet
the system bit BER objectives, i.e. less than 1 error in 10 billion bits of
information transmitted (BER ≤ 10-10).
The bottom line is that this whole issue of “zero bit error rate” or should I say
“virtually zero bit error rate” has been satisfactorily addressed in the standards
with the publication of the Category 5e addendum (TIA/EIA 568-A-5) in January
2000. This standard includes very tight requirements for patch cord impedance
stability and cable return loss. The user can be assured that a channel or
components that are certified to meet Category 5e requirements per TIA/EIA
568-A-5 will not contribute to excessive bit errors.
Page 12 of 12