October 2015
28
E
mbedded
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omputing
Identifying challenges in future
100G backplane designs
By Sergej Dizel,
Pentair
The telecom and datacom markets
are the most data-hungry industries.
This article explains the new PICMG
100G-ATCA specification which is not
yet complete, but will enable future
100 Gbps data rates on backplanes.
Since the introduction of AdvancedTCA,
which was specified in 2002, the race for more
data transfer speed on the backplane has been
on. The telecom and datacom markets have
a hunger for processing power that is never
satisfied. Today processor technologies with
Multicores, GPGPUs and powerful co-proces-
sors can satisfy the demands of full HD video
on demand, increasing data-hungry services
for smart phones and other services.
In addition to the processing power needed for
those applications, infrastructure must sup-
port this huge amount of data traffic as well.
Coming from packet switching backplanes
with 1Gbps of data transfer in the beginning
of this millennium, today AdvancedTCA back-
planes support 40Gbps of data transfer. This
is achieved by four ports each having a trans-
mit and receive differential pairs transferring
10Gbps each. The four ports together are capa-
ble of transferring 40Gbps. Even this is not suf-
ficient to feed applications today. Nowadays
Dual Star backplanes are often used where
two switches work in parallel to increase the
data traffic between transmitter and receiver
to 80Gbps. This is certainly not the end point
of demand for data speed. The IEEE specifi-
cation for 100Gbps Ethernet over copper was
released at the end of 2014. Today, the PICMG
working group is defining 100G Ethernet for
AdvancedTCA based on the IEEE spec.
100Gbps data rates create many new chal-
lenges for backplane design. The graphs in fig-
ure 1 and figure 1.1 show the thresholds for
insertion and return loss of the IEEE802.3ap
specification which defined 40G Ethernet
and the new standard IEEE802.3bj, which
defines 100G. At 100G, the IEEE defined
two coding methods, 100GBASE-KR4 and
100GBASE-KP4. The thresholds of both
methods are shown in the graphs. As shown,
the thresholds for both new methods of 100G
are defined for much higher frequencies. This
means for the backplane that all components
such as connectors, the bare board and the
copper trace structures have to be designed
for those high frequencies.
The ZD and ZD+ connectors as defined in
the AdvancedTCA specification has not been
for those data rates. A few connector vendors
have been working on a solution. The chal-
lenge for the new 100Gbps AdvancedTCA
connector was to achieve a much more homo-
geneous impedance transition between board
and backplane and keep the crosstalk as low
as possible. Among other improvements this
was achieved by decreasing the size of the
through holes in the backplane. This method
creates another advantage for the backplane
routing as the smaller holes sizes create a
larger routing channel in between the connec-
tor pins. This leads to a more homogeneous
impedance of the traces and a lower cross-talk
between connector vias.
Figure 3 shows the simulation of insertion
loss (Sdd21) of a 30mm long differential
pair in an AdvancedTCA backplane PCB
without connectors assembled. The differ-
ence between the current configuration with
0.6mm through hole and a reduced through
hole size of 0.36mm is obvious. A reduction of
the distance of the press fit pins within the dif-
ferential pair from 1.5 to 1mm is able to fur-
ther reduce the losses of the transmission line.
The next important part of the backplane to
address is the trace structure, especially their
dimensioning with regard to losses and cross
talk. Not every differential pair has similar
properties. For example, when comparing two
differential stripline pairs with each having
100 ohm impedance but different trace widths
(and thus different layer stackup), they will
have different behavior with regard to trans-
mission line losses and cross-talk (emission
and immission). At 40G backplane data trans-
fer this issue is solved, but there are new chal-
lenges with 100G. This became visible when
conducting the first simulations for 100G
data transfer. The impedance discontinuities
between connector and the differential pair
need to be evaluated very carefully. This part
of the transmission line already plays a signif-
