Download The Myth of Zero Bit Error Rate Transmission

Consultant Programme
Article 25 / 2008
April 2008
The Myth of Zero Bit Error Rate Transmission
A powerful
way
of
measuring
the
performance of a digital communications
channel is to measure its bit error rate.
This is a ratio of how many digital ‘bits’ of
information have been sent compared to
those successfully and correctly received
at the other end. It is termed BER (for Bit
Error Rate) and expressed as ten to the
power of a number where that number
expresses how many bits of data can be
sent before one error is allowed, e.g. a
BER of 10-9 means that for every
thousand million bits sent (i.e. one with
nine noughts after it) only one will be
received in error.
Ethernet protocols, as described in the
IEEE 802.3 series of Standards, call for
performance ranges typically in the order
Fluke DTX-1800 testing TIA Cat 6 Perminant Link
of 10-10 to 10-12. A gigabit Ethernet signal,
transmitting a thousand million bits of data
determines a number which is then added
per second at a bit error rate of 10-12
to the end of the frame. The receiving
would therefore expect to see an error not
circuitry does the same process and
more than once every thousand seconds,
checks to see if the number it has
or about four per hour, and still be within
calculated is the same as the number
specification. Ten gigabit Ethernet, also
appended to the transmitted frame. If it is
expecting 10-12, would expect an error not
then it is most likely that the frame has
more often than once every 100 seconds.
arrived uncorrupted.
If the number is
different there is a very high likelihood that
What happens if there is an error?
the frame has been corrupted en route.
Modern communications systems usually
The receiving station can then choose to
contain some form of error correcting
ignore it or ask the transmitting station to
protocol whereby a mathematical process
retransmit that packet.
on the number of bits sent in each frame
The end user of the data would probably
© 2008 Connectix Limited
Consultant Programme
Article 25 / 2008
each
layer
depends
upon
correct
operation of the parameters below, all the
way down to the fundamental layer of the
channel’s electrical performance. The
electrical parameters are those measured
when a cable tester is connected to each
end of the link and automatically cycles
through the following measurements,
across the bandwidth of interest, e.g. 250
MHz for Category 6:
properly received frames.
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•
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There are a number of ways of measuring
Ultimately every communications channel
the performance of a communications
is defined by its Signal to Noise Ratio.
channel.
This is as it sounds: a measure of the ratio
Connectix 10G patch panels and cables
notice nothing but if the number of lost
packets or retransmits was high then the
system would appear to slow down.
A real-time digital video signal would
freeze-frame and then jump on to the next
Attenuation (insertion loss)
NEXT
Return Loss
Power Sum Next (PSNEXT)
Far End Cross Talk (FEXT)
PSFEXT
PSELFEXT
Resistance
Delay
Differential delay
Attenuation to Crosstalk Ratio (ACR)
Power Sum ACR (PSACR)
of the wanted signal compared to the
• The electrical (or optical) parameters,
unwanted noise trying to drown it out.
e.g. attenuation, Near End Cross Talk
There will come a point where the
etc (NEXT)
receiving
circuitry
can
no
longer
• The signal to Noise Ratio (SNR)
differentiate between the legitimate signal
• The Bit Error Rate
and the background noise and no
• Packet transmission checking using the
transmission is possible regardless of the
LAN protocol required, e.g. Ethernet
error correcting protocols in place.
Digital signals do not degrade gracefully,
We can consider this to be a hierarchy as
© 2008 Connectix Limited
Consultant Programme
Article 25 / 2008
unlike analogue signals, they tend to work
corrupted at gigabit transmission speeds.
perfectly then reach a point where the
SNR is so poor that the BER starts
It is worth noting here that the higher the
to
speed of transmission then the more
decline
rapidly,
and
then
they
fail completely.
sensitive to noise (and therefore leading to
poor SNR and BER) they become. A lowly
The Attenuation to Crosstalk (ACR) test of
LAN protocol, like 10BASE-T transmits a
the cable tester is analogous to the SNR
data stream of ‘ones’ and ‘zeroes’ as a
measurement, though not exactly the
series of positive and negative voltage
same thing. ACR is literally a measure of
levels, starting out at about 1V for a ‘one’
the ratio of the Attenuation of the link to
and minus one volt for a ‘zero.’
the Near End Cross Talk, which is usually
the largest and dominant source of a
This
noise in a cable link.
modulating information onto an electrical
simple
amplitude
method
of
signal is not the only way. Information may
For the BER to be adequate the SNR
be encoded by using many different levels
must be within specified limits.
This
of voltages, or using phase or frequency
electrical
components, or any mixture of these. So
within
whereas 10BASE-T uses simple two level
sometimes
coding, 100BASE-T uses three voltage
overlooked though is that the external
levels and 1000BASE-T and 10GBASE-T
electrical noise environment must also be
use many voltage levels and a phase
below certain limits or else electrical noise
component. The downside of this extra
will get into the cabling and degrade
data
the SNR.
bandwidth of the cable is that the required
means
that
parameters
specification.
the
must
cable
all
What
be
is
being
sent
down
the
limited
SNR must go up to compensate as the
Is a zero Bit Error Rate possible? The
higher speed protocols are much more
answer is yes, but only in a laboratory
sensitive to interference than slow ones.
environment
enough
So whereas a certain SNR may result in a
external noise even the most heavily
low BER and zero errors for 100BASE-T,
screened cable will finally succumb.
the same circumstances may lead to an
Experiments have shown that when the
unacceptable level of SNR and Bit
external electrical field strength exceeds
Error Rate in a 1000BASE-T, gigabit
3V/m around the cable then high speed
Ethernet link.
data within the cable starts to get
This argument becomes even more
because
given
© 2008 Connectix Limited
Consultant Programme
Article 25 / 2008
extreme when we get to 10GBASE-T (ten
gigabit Ethernet).
The SNR, and hence the BER will be
optimised by the following:
• Making all the cable components so that
they meet or exceed the individual
electrical specifications as laid down in the
Standards.
• Match the components together so that
they perform as ‘System’.
This is
especially true of the characteristic
impedance.
For structured cabling
components this must be as close to 100
ohms
as
possible.
Mismatched
Connectix cable, IDCs, PCB and sockets,
engineered to precisely match 100 ohms.
components result in Return Loss. This is
a reflection of the signal back down the
most demanding requirements of all
line which will of course corrupt the data.
Ethernet protocols.
• Ensure the ambient electrical noise
noise does not become a negative factor
environment is within the normal ‘benign’
Connectix
limits. This is usually taken as lower than
following:
also
To ensure external
recommends
the
3V/m which is the performance level
electrical equipment on sale in the EU
• Use unscreened cable up to 1000Mb/s
must be able to demonstrate in order to
and when the ambient field strength is
display the CE mark of compliance.
less than 3V/m from zero to 250MHz.
Ambient levels of 3V/m or more definitely
This is the general expectation of the
place us in the realm of screened cable or
ordinary office environment.
even optical fibre.
• Use screened cable up to 10,000Mb/s
Connectix
has
engineered
the
and when the ambient field strength is
components of their cabling systems to
between 3 and 10V/m over 500MHz. This
precisely match each other and can thus
would be considered as an electrically
guarantee a bit error performance rate
noisy industrial environment.
better than 10-12 and thus exceeds the
• Use optical fibre if the ambient field
© 2008 Connectix Limited
Consultant Programme
Article 25 / 2008
strength exceeds 10V/m, e.g. close to
airport radars, radio transmitters and high
voltage equipment such as x-ray sources.
Note that the field strength values quoted
here can be measured approximately by
meters available to the general public or
accurately by specialist EMC consultants
with
very
sophisticated
measuring
equipment.
Is zero bit error rate possible in the real
world? No. Can a superior and more than
adequate BER be achieved with well
engineered products like Connectix? Yes.
Connectix pre-term, tested before leaving factory
If a manufacturer claims a zero bit error
rate performance under all circumstances
ask to see the evidence and remind them
of a quote from the late Arthur C Clarke,
“The laws of physics are not subject to
human appeal.”
UK Head Office:
Connectix Limited
33 Broomhills Industrial Estate
Braintree, Essex CM7 2RW
© 2008 Connectix Limited