Cabling Infrastructure
10GigE just fine on copper cabling
10GBASE-T enables network managers to scale their networks to 10-gigabit speeds.
by George Zimmerman and Bruce Tolley
Ratified in 2006, the IEEE
802.3an 10GBASE-T specification for
10-Gigabit Ethernet (10GigE) on unshielded
twisted pair copper cabling offers two
important attributes to network managers and
IT professionals planning data centers and
enterprise networks. First, it supports
legacy copper cabling, and for new
installations maintains the structured
copper cabling paradigm, as well as support
for RJ-45 connections and patch panels.
Secondly, 10GBASE-T will enable over time
lower-cost 10GigE interconnects by enabling
high-density 10-gigabit switches.
Because of its lower cost
and plug-and-play simplicity, unshielded
twisted pair (UTP) copper cabling remains
the media of choice for in-building
horizontal runs and data center cabling. As
much as 80 percent of the cabling inside
buildings today is Category 5e or better,
and Category 5e channels can be qualified to
support
10GBASE-T operation. On Category 6 links,
the standard supports distances from 55
meters to 100 meters. In addition, augmented
Category 6 or Category 6a supports 10GBASE-T
links up to 100 meters.
The challenge of echo and near-end crosstalk cancellation in 10GBASE-T is one of the major complexity
challenges to implementation.
10GBASE-T enables network
managers to scale their networks to
10-gigabit speeds, while taking advantage of
their investment in installed copper cabling
infrastructure. For new installations, it
leverages the cost-effectiveness of copper
structured cabling.
To increase Ethernet data
rates to 1 Gbps, 1000BASE-T Ethernet uses
four pairs in a Category 5e cable, with
bidirectional signaling. The transceivers
are required to cancel both echo and
near-end crosstalk (NEXT) on each pair of
wires, and cancellation of far-end
crosstalk, while recommended, was not
required.
To get another order of
magnitude in the bit rate, 10GBASE-T takes
this several steps further, both expanding
the signaling rate and increasing the number
of levels in the transmitted signal.
In order to achieve this,
a low-parity density check code is employed,
as well as substantial improvements to
receiver sensitivity, echo and crosstalk
cancellations. Challenges consist of
impairments related to the propagation of
the signals themselves, such as insertion
loss and intersymbol interference, that are
caused by the limited bandwidth and real
impedance of the cable itself, plus
degradations due to interfering effects,
such as echo, near-end crosstalk and far-end
crosstalk. In addition, background noise and
other radiated signals, such as alien
crosstalk, can reduce the received
signal-to-noise ratio.
Vulnerable to noise
With wired Ethernet
systems, as frequency increases, the
received signal will become weaker, making
it more vulnerable to noise, culminating in
high power loss at 400 MHz. In addition to
the need to encode the 10GigE signal to fit
within this limited bandwidth, the slope of
the attenuation, from less than 3 decibals
at low frequencies to a 40-decibal loss at
400 MHz, drives the need for
efficient equalization. Because hardware
trades will affect robustness, physical
layer evaluators should test not only the
longest links a physical layer can operate
on, but also numerous intermediate length
and patch cord configurations.
Beginning with
1000BASE-T, and continuing to 10GBASE-T, the
effects of echo and NEXT have been cancelled
substantially in the receiver. The practical
implication is that in addition to cabling
connectors and interfaces, echo and NEXT
formed by the transmitted signal reflecting
off minor imperfections in the twists should
be cancelled. As a result, echo cancellers
can be required to have taps covering the
entire length of the channel. These taps
should continue to adapt to changes in the
channel in order to respond to mechanical
changes in the wire.
The echo and NEXT
cancellation in 10GBASE-T is one of the
major complexity challenges to
implementation. The techniques used in
1000BASE-T solutions, if implemented in a
straight-forward way, result in a complexity
increase. Because of the inherent
variability and randomness of the impulse
responses, simple techniques to extend the
length of the impulse response cancelled,
such as continuous-time analog filters or
digital filters, do not easily lend
themselves to flexible solutions that will
work on a variety of wiring.
To gain both the
advantage of highly precise digital signal
processing and efficient analog processing,
one approach involves both analog and
digital signal processing. This approach
allows for robust cancellation of NEXT and
echo by implementing cancellers hundreds of
taps long, and sharing computation between
all 16 echo and NEXT cancellation paths.
Because of the trades
made in the design of echo and near-end
crosstalk cancellers, testing the system's
ability to respond to perturbations in the
cable is important. Bit errors may occur
from the transients involved in bending a
cable, but the system should recover after
adjusting to the new echo and NEXT
environment.
The final within-cable
impairment is far-end crosstalk (FEXT). FEXT
is interference produced by signals
emanating from adjacent transmitters at the
far end of the 10GigE link. For 10GBASE-T,
however, FEXT interference produces a
significant loss in the signal-to-noise
ratio that would prevent 10GBASE-T
transmission at intermediate lengths, as
well as on long lines.
Performance testing recommended
Because FEXT is dispersed
similar to the desired received signal,
canceling it can require a significant
amount of signal processing hardware. One
tradeoff is to combine equalization and FEXT
cancellation, resulting in a four to six
times reduction in FEXT canceller
complexity. Because levels of FEXT can vary
significantly depending on cabling and
connectors, testing the performance of these
transceivers at all lengths is important.
FEXT is a dominant issue for
intermediate-length cabling, between
20 meters and 50 meters, with two to four
connector channels.
Since the advanced signal
processing in 10GBASE-T manages to mitigate
the sources of noise and distortion within
an individual four-pair cable sheath, this
ultimately leaves the system limited in
performance by external crosstalk from other
cables in close proximity, called alien
crosstalk. Alien crosstalk can come either
from sources at the same end of a
transmitting link (alien NEXT), or from
sources transmitting at the other end, or
somewhere along the length of the link
(alien FEXT).
Alien NEXT sources are
effectively mitigated through cabling
design, qualification and installation
practices. Alien FEXT, however, has proven
to be a tougher problem for cabling. To
avoid the issue of a short-distance link
far-end cross talking into a link on a long
line, the 10GBASE-T standard requires
transmitters to reduce their transmit power
to only the level needed on their link.
Thus, short-link transmitters will transmit
with a "power backoff" from the nominal
power used for longest, 100-meter links.
Three cabling systems
exceed the 10GBASE-T alien crosstalk
requirements: Category 6 cable constructed
with a foil screen (FTP), augmented Category
6 cabling and Category 7 cabling,
supporting links up to 100 meters. For new
cabling installations, customers should be
evaluating Category 6a cabling,
and for those customers who prefer screened
cabling, Category 7 or
Category 6 FTP.
George Zimmerman and Bruce Tolley are with Solarflare Communications, Irvine, Calif.
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