RJ45, the
eye of the needle: the tighter, the better
Cat. 6 products from R&M, “de-embedded”
tested and certified / full compatibility
and full performance in Class E
Cables and components of Category 6 are
meant to enable Class E cabling systems,
with a specified transmission frequency
range going up to 250 MHz. As long as installers
and users stay true to one manufacturer,
this does work. But if a third-party supplier
is involved, there could be trouble. The
RJ45 connection is usually the eye of the
needle, where bandwidth and performance
get bogged down. With this in mind, Reichle
& De-Massari (R&M), the leading
Swiss manufacturer of cabling systems, arranged
for their RJ45 connection
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modules to undergo de-embedded testing and certification
by an independent, accredited laboratory. The
result is unrestrictedly compatible components
and guaranteed transmission parameters for the
network. If you take out your notebook PC and
plug the standard patch cable into the standard
socket, you will expect the LAN connection to
work. If it doesn't, it isn't usually the fault
of the twisted copper wires that were originally
invented for telephone traffic and that now –
four pairs in one cable – form the skeleton of
the cabling system, at least on each floor. The
eye of the needle is the plug connector.
In trunk connections, it was the last mile that
limited the subscriber's bandwidth. On a LAN,
it's the last few millimetres in the connection
modules and plugs. This is where signals mutually
affect each other and reflections originate, distorting
the transmitted signal to the point of indecipherability
for the receiver. Near-end crosstalk (NEXT), far-end
crosstalk (FEXT) and return loss (RL) are therefore
key parameters.
These effects can be partially compensated for
by electrical means, and this has already been
done for category 5 and 5e (enhanced) components.
The lower bandwidth (up to 100 MHz actually used,
but tested up to 125 MHz) made it possible to
set quite generous tolerance limits: so generous,
in fact, that interoperability between components
from different manufacturers was a question of
luck.
The tighter, the better
Manufacturers, installers and users eagerly anticipated
the publication of the more stringent Cat. 6 limit
values. They were published in America in June
2002 as TIA/EIA 568-B.2-1. Since then, this norm
has been listed online by TIA as at the head of
the “top selling standards”, and Frost & Sullivan,
an American market research institute, expects
it to give a boost to the industry. The second
edition of the corresponding European norm for
application-independent communications cable systems
was ratified by CENELEC in November 2002 as EN
50173-1. The second edition of the related ISO
/ IEC 11801 standard was published in September
2002.
This puts a limit on the problem, but doesn't
eliminate it. The reason is that crosstalk is
measured via the commercial RJ45-plug, which at
250 MHz hits the limits of its physical performance.
And this is where the question arises: are we
actually measuring the test specimen – a connection
module, for example – or are we measuring the
entire test setup? Because of this doubt, some
countries are still reluctant to accept the IEC
version of the TIA norm.
This goes to the heart of the problem, the measurement
method. Up to now, “mated” plugs and connection
modules have usually been measured, i.e. it is
the properties of the connection as a whole that
are certified. And of course each manufacturer
can select ideally matched pairs of plugs and
sockets from his production line, and send them
for testing. Consequently, the test result shows
only a small excerpt from reality. It doesn't
prove that all of the manufacturer's products
invariably guarantee Cat. 6 performance right
up to their performance limits.
This increases the risks for installers and users
of network operations. If they stay faithful to
one manufacturer, things mostly go well. If they
go to a third party, things may go wrong. Worse
still: if the user inserts a “bad” (but still
compliant) Cat. 5 plug into a Cat. 6 socket, overcompensation
may mean that the combination no longer meets
Cat. 5 specifications.
De-embedded measurements,
independently certified
R&M already put their products through a one-hundred-per-cent
check. This makes them unique worldwide. And,
in addition, R&M use a manufacturing process
that guarantees the highest transmission quality
between terminal block and socket.
Now, R&M are sending their Cat. 6 connection
modules for de-embedded measurement and certification.
This implies that the plug and the connection
module are measured separately, i.e. “de-embedded”.
Thus, for the crosstalk of a connection module
to be certified, suitably checked testing plugs
are required.
“For this purpose, we don't simply use plugs with
average crosstalk values. We actually use plugs
with values at the limit. This corresponds to
the norm, and is appropriate to the situation
in practice”, says Rene Trosch, head of development
and the testing laboratory at R&M. “This is
the only way to ensure that our Cat. 6 solutions
are unrestrictedly interoperable, cross-compatible
and backwards compatible. And we have it tested
and certified by a neutral, independent body.”
R&M has entrusted the Danish laboratory DELTA
with the task of certification. Among other things,
DELTA are accredited for the type approval of
components and systems used in application-independent
cabling. “We have long been testing mated Cat.
6 connections”, says Erik Bech, Testing Manager
at DELTA. “The manufacturer supplies plug and
socket, and the test verifies the transmission
characteristics of the pair. We are now also running
genuine de-embedded tests, using testing plugs
whose crosstalk corresponds exactly with the limits
of the norm.”
Twelve cases have to be covered in the measurement
of near-end crosstalk alone, so in the extreme
case twelve plugs with the corresponding properties
have to be found. That can't be done overnight.
“We had to refine our testing methods over and
over again”, reports Bech. “The procedure has
now been finalized. We know the crosstalk in the
plugs over the whole frequency range up to 250
MHz, with enough accuracy to achieve precise and
reproducible results.” These results are fed back
into product development at R&M.
Back in 2001, R&M became the first company
in the world to obtain Cat. 6-certification from
CENELEC (in accordance with EN-Norm) for their
connection modules, on the basis of tests made
by an independent laboratory. But this was still
a “mated” measurement. R&M will be presenting
the new de-embedded result for the first time
at CeBIT 2003 in Hanover.
Why Category 6? Why
Class E?
The two terms are usually equated. The difference
is that the category specifies the components,
whereas the class specifies the network. A Class
E network has specific transmission data up to
250 MHz, and this is achieved reliably with components
of Category 6. But where's the need for 250 MHz,
when even Gigabit Ethernet manages with a bandwidth
of approximately 80 MHz?
The answer is that transmission rate is not the
only factor. In present-day applications, reliability
counts. The permissible bit error rate under the
Ethernet standard (10 EXP -8, i.e. one error in
100 million bits) has long been outdated and can
interfere with time-critical applications such
as production control or audio, video and VoIP
transmission. What is being sought is an error
rate of 10 EXP -12, i.e. a further reduction by
a factor of ten thousand.
In the near future even 10-Gigabit Ethernet will
be pushed through copper conductors, and this
is also meant to operate with Cat. 5e components.
But the transmission process is even more sensitive.
The installation may simply not be good enough.
In parallel with this, consideration is being
given to using the four pairs of conductors in
the cable in simplex, i.e. with two pairs for
each direction rather than all four being used
out and back. This would relax the requirements
regarding reflection and crosstalk, but the lines
could not be used with the present-day Gigabit
Ethernet at the same time. A proposal is being
worked on in the ANSI/TIA/EIA-854 committee.
Cat. 6 certification makes planning more reliable
We should not spend much time wondering whether
we really need these high transmission rates.
So far, Moore's law, which says that the transistor
density in integrated circuits doubles every two
years, has also been true of network bandwidth
requirements.
How soon will we suddenly find we want to link
not only a PC into the network but a remote file-server
or a new application server? In that sort of case,
an installation with de-embedded certified Cat.
6 components will give planning a high degree
of reliability. All of this is also supposed to
operate over Cat. 5e, as mentioned earlier. With
a certified and entirely Cat. 6 solution, you're
on the safe side.
