Consultant Programme
Article 18 / 2007
July 2007
A review of TSB 155 – The first Augmented
Cat 6 cabling standard
TSB 155 and 10GBASE-T
The Cat6A story really started last year
(2006) when the IEEE 802.3an standard
was published for 10GBASE-T.
This
standard describes how ten gigabit
Ethernet
over
copper
cable
works
and the minimum electrical performance
required of the cabling plant to achieve
satisfactory operation.
This will be followed over 2007 and 2008
with another six cabling related standards.
Three of them will relate to new
Augmented Category 6 cabling products
and three will consider testing and
characterising
the
cable
plant
to
determine if it is 10GBASE-T compliant.
10GBASE-T Panel from Connectix
The first of these standards to be published was TSB 155, which came out in March 2007.
TSB means ‘Telecommunications System Bulletin’ which is a series of publications to give
extra information in the use and understanding of existing cabling standards. They are
produced by the same American TIA 568 committee as the rest of the mainstream
standards in the ANSI/TIA/EIA-568-B family.
The guidelines of this Telecommunications Systems Bulletin (TSB) contain additional
recommendations to further characterise existing Category 6 cabling plants, as specified
in ANSI/TIA/EIA-568B.2-1 for supporting 10GBASE-T applications. This TSB includes
field test procedures and field tester guidelines that can be used for this assessment.
©2007 Connectix Limited
Consultant Programme
Article 18 / 2007
This TSB describes additional guidelines to support the IEEE 802.3an 10GBASE-T
standard for using installed 100Ω, 4-pair category 6 cabling meeting the requirements of
ANSI/TIA/EIA-568- B.2-1. This TSB characterizes the crosstalk coupling between 4-pair
category 6 cabling in close proximity referred to as alien crosstalk and provides additional
guidelines for field test equipment and field test methods and alien crosstalk mitigation. It
also provides guidelines on internal parameter mitigation. These guidelines provide
additional information on the extended frequency transmission performance expected of
category 6 cabling from 250 MHz to 500 MHz. The transmission parameter recommendations included herein provide a means to assess installations of category 6 cabling, as
specified in ANSI/TIA/EIA-568-B.2-1 and corresponding addenda, up to and including
500 MHz and for the additional parameters necessary to support 10GBASE-T. This TSB
does not specify any requirements for category 6 cabling systems or components,
beyond those already given in ANSI/TIA/EIA-568B.2-1.
As we can see from the quote above there is a lot of focus on the issue of Alien Crosstalk
or ANEXT. Alien crosstalk is the limiting factor in high speed data transmission over
unscreened copper cable, as the sophisticated error correcting and digital signal
processing techniques available only have limited success in this area.
ANEXT can come from any source external to the cable but the largest contributor will be
cables lying adjacent to the ‘victim’ cable. Thus long parallel cable runs are going to be
at the most risk of interfering with one another.
This is of course only an issue with unscreened (also known as unshielded or UTP)
copper cable. Correctly installed screened cable, and of course optical cable, will not
suffer from alien crosstalk.
TSB 155 introduces a range of new terms to address the issue of alien crosstalk
and we can see that it is quite a complicated issue.
AELFEXT Alien equal level far-end crosstalk
AFEXT Alien far-end crosstalk
ANEXT Alien near-end crosstalk
AACRF Attenuation to alien crosstalk ratio far-end
PSAACRF Power sum attenuation to alien crosstalk ratio far-end
PSAELFEXT Power sum alien equal level far-end crosstalk
PSAFEXT Power sum alien far-end crosstalk
PSANEXT Power sum alien near-end crosstalk
©2007 Connectix Limited
Consultant Programme
Article 18 / 2007
One can measure the effect of one cable interfering with another adjacent cable but a
more realistic test is to add up all the interference coming from the six cables that would
surround any one cable in the middle of a bundle of cables. This is what is meant by the
Power Sum measurements. It is the realistic sum total of all the surrounding interfering
fields.
TSB 155 contains all the mathematical models and justifications to calculate the
parameters of the cable channel over the frequency range 1 MHz to 500 MHz (standard
Cat 6 defines a 250 MHz channel) and to be fair these will only be of passing interest to
the general user. It is these figures however that an automated cable tester will be using
when it attempts to test and characterise a cable channel for 10GBASE-T. One example
is given below; the equation to determine Power Sum Equal Level Far End Crosstalk at
any frequency.
The practical uses of TSB 155 come from determining what length of cable channels are
likely to work, methods of field testing for alien crosstalk and an explanation of the mitigation
factors required to assist unscreened cable installations in reaching those targets.
Cable lengths
TSB 155 gives an opinion on the possible operating distances when using 10GBASE-T
over Category 6 cable.
Remember that the target distance for full specification
Augmented Cat 6 cable is 100 metres.
10GBASE-T should operate over channel lengths of up to 37 meters of
Category 6 cabling, and should operate over channel lengths between 37 and
55 meters of Category 6 cabling depending on the alien crosstalk environment.
Channel lengths over 55 meters may require mitigation.
Note the use of the word ‘should’ here. But it appears existing Cat 6 should be able to
handle 10GBASE-T up to 37 m, but only after testing to ensure that. In the computer room
environment 37 m may well be adequate. The existing IEEE 802.3an standard contains
the more optimistic 55 m figure.
©2007 Connectix Limited
Consultant Programme
Article 18 / 2007
Testing
The first diagram shows the method of testing two cables for ANEXT. Note the cables have
to be terminated with resistors to ensure they behave like ‘real’ cables. One test instrument
injects a noise signal onto a cable and another test instrument, connected to the adjacent
cable, picks up the induced ‘alien’ noise. A synchronisation cable links the two instruments
so that the two units can compare injected noise with induced noise.
The next diagram shows the method of testing for Far End Alien Crosstalk. One test
instrument injects the noise signal onto one cable and the other instrument is now located
at the far end of the adjacent cable link to pick up the sum total of the induced noise along
the whole length of the cable. A third cable is used as the synchronisation channel
between the two test instruments. Once again the cables have to be terminated with
resistors as well.
It is clear from the above that there will be a great deal of combinations to test in a large
cable installation. The leading manufacturer of test instruments has stated that in a
bundle of 24 cables the PSANEXT calculation for the pairs in one “victim” link requires
ANEXT measurements of 368 combinations taking an estimated 17 minutes (allegedly).
©2007 Connectix Limited
Consultant Programme
Article 18 / 2007
The PSANEXT measurement combinations, when every link must be considered a victim
link, requires ANEXT link-to-link measurements of 276 link combinations {4,416 pair-topair combinations} taking an estimated 207 minutes or 3 hours and 45 minutes.
It is clear that it is impractical to test every cable link for alien crosstalk in a real
installation. Many customers will ask for a sample test, but which links should be chosen?
Who will decide which are the most at risk adjacent cables and who indeed will even
know? What are the contractual liabilities for the installer if just some of the links fail?
It is interesting to note that the cable manufacturers offering unscreened cable solutions,
along with a system guarantee, do not require any alien crosstalk testing at all. Perhaps
they see this as tempting fate…
Mitigation techniques
Mitigation means a number of measures needed to overcome the alien crosstalk effects
of adjacent unscreened cables.
There are only two mitigation techniques that work; sheer physical distance between the
interfering and victim cables, or metallic screening placed between them. Screened
cables do not require mitigation techniques by virtue of the screening elements in the
cable. Good installation is still required however when using screened cable which
includes properly trained installers.
The mitigation techniques offered by TSB 155 all centre on the prevention of forming long
adjacent cable runs or even preventing cables from touching at all. They are summarised below.
1.
2.
3.
4.
5.
6.
7.
8.
Utilise nonadjacent patch panel positions (patch panel adjacency should also be
checked at the rear of the patch panel)
Separate the equipment cords
Unbundle the horizontal cables
The number of disturbed channels to be tested should be determined using
statistical sampling techniques based upon the intended confidence level.
An alternative to separating equipment cords is to utilise equipment cords
sufficiently specified to mitigate the alien crosstalk coupling such as Category 6
screened cords or Category 6A cords.
Reconfigure the cross-connect as an interconnect
Replace connectors with Category 6A connectors
Replace the horizontal cable with Category 6A horizontal cable
The channel performance after mitigation should be verified.
©2007 Connectix Limited
Consultant Programme
Article 18 / 2007
Rear of 10GBASE-T Panel from Connectix
Conclusion
TSB 155 is the first of the 10GBASE-T cabling standards to be published.
We can see that 37 metres distance should be achievable when using standard Cat 6
UTP. Unscreened cables will always struggle though with high speed data flows using a
frequency range up to 500 MHz (UHF).
Alien crosstalk is the limiting factor when using 10GBASE-T on unscreened cable and it is
extremely difficult to realistically test this on site. Mitigation techniques may improve the
performance of unscreened cables but they are hardly conducive to low cost and rapid
installations. Connectix recommends the use of screened cable or optical fibre when
contemplating the use of ten gigabit Ethernet to simplify and increase the reliability of the
cable network.
UK Head Office:
Connectix Limited
33 Broomhills Industrial Estate
Braintree, Essex CM7 2RW
©2007 Connectix Limited