superchannelsa - infinera · moving toward a new type of dwdm ... plexed quadrature phase-shift...
TRANSCRIPT
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Reprinted with revisions to format, from the March/April 2012 edition of LIGHTWAVECopyright 2012 by PennWell Corporation
By GEOFF BENNETT
Superchannels will save
carriers from the dilemma
of how to flexibly scale
capacity, particularly as
requirements exceed
100 Gbps.
A S THE NEED for ever-
increasing amounts of
DWDM transmission
capacity shows no sign of waning,
the optical transport industry is
moving toward a new type of DWDM
technology – the “superchannel.”
A superchannel is a set of DWDM
wavelengths generated from the
same optical line card, brought
into service in one operational
cycle, and whose capacity can be
combined into a higher-data-rate
aggregate channel. It’s the DWDM
industry’s answer to the question,
“What comes next after 100 Gbps?”
Scaling optical-fiber capacityThe capacity and service flexibility
of optical fiber is remarkable, but
still governed by strict rules of
physics and engineering practicality.
Although written in 2006, Emmanuel
Desurvire’s paper still gives an excel-
lent overview of those limits, while
a more recent paper by Adel Saleh
and Jane Simmons points out that
increases in the spectral efficiency of
optical transport systems ultimately
provides the biggest “bang for the
buck” in terms of capacity scaling
to meet growing internet demand.1,2
But what neither of these papers
covers is that, despite 40% compound
growth in demand over the past
five years (equivalent to a factor of
five increase), service providers
are not able to hire an army of extra
network engineers. In fact, in most
cases headcount will be frozen.
So it’s clear that the optical
transport networks of the future
must be capable of turning up
much larger amounts of DWDM
capacity for a given operational
effort without sacrificing optical
Superchannelsto the rescue!
GEOFF BENNETT is the director of solutions and technology for Infinera. He has more than 20 years’ experience in the data communications industry, including IP routing with Proteon and Wellfleet, ATM and MPLS with FORE Systems, and optical transmission and switching with Marconi as distinguished engineer in the CTO Office.
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FEATURE Superchannels to the rescue!
reach or total fiber capacity. Today
that capacity unit in long-haul
networks is 100 Gbps – a data rate
enabled by a series of advances
in optical transmission, namely:
• High-order phase modulation
(typically polarization-multi-
plexed quadrature phase-shift
keying, or PM-QPSK).
• Coherent detection using a very
stable local oscillator laser.
• Advanced digital signal process-
ing in the receiver to compensate
for fiber impairments.
• High-gain forward error correc-
tion (FEC), including soft-decision
FEC that can offer more than 11
dB of gain for a typical span.
Let’s refer to the combination of
these four items as “coherent techno-
logy,” which offers a quantum leap
in terms of optical performance
compared to non-coherent systems.
While there will likely be incre-
mental improvements in future
coherent technology, these advan-
ces alone are unlikely to keep
up with bandwidth demands.
It’s interesting to note that
computer manufacturers are
facing a similar problem.
You may be aware that CPU
clock speeds appeared to
stop getting faster about
five years ago. Yet the
famous Moore’s law remains
valid in that the number of
transistors on a chip is still
increasing. CPU and GPU
(graphics- processing-unit)
manufacturers are using
those additional transistors
to build multiple cores, rather than
running individual cores at faster
data rates. But the chips they produce
appear as a single unit of processing
capacity to the operating system.
Likewise, a DWDM superchannel
consisting of multiple wavelengths
appears as a single unit of operatio-
nal capacity to the network engineer.
This analogy is shown in Figure 1.
Implementing superchannelsSo what’s the best way to implement
coherent superchannels? Let’s
assume that a service provider
needs to turn up a terabit of optical
capacity in a single operatio-
nal cycle. Today that would mean
installing ten 100G transponders
– an approach that actually takes
more than 10X the effort of a single
transponder because each time
a transponder is added it affects
the existing wavelengths in the
fiber. Since this approach offers
no value for operational scaling,
we will not consider it further.
Instead, Figure 2 shows three
engineering options – A, B, and C
– that we will consider. All three
ServicesO/S
Bandwidthvirtualization
layer
Processingvirtualization
layer
Multi-coreCPU Multi-carrier
superchannel
FIGURE 1. Virtualized parallel processing in the CPU and GPU world (left) and virtualized multi-carrier superchannel in the DWDM transport world (right).
10 lasers40 modulators32-Gbaud electronicsPhotonic ICsTime to market: ~2 years
375 GHz375 GHz
2 lasers8 modulators160-Gbaud electronics~16-nm siliconTime to market: ~7 years
1 TbpsPM-QPSK
375 GHz
1 laser4 modulators320-Gbaud electronics~ 11-nm siliconTime to market: ~10 years
Option A Option B Option C
C-band
FIGURE 2. Comparison of spectral efficiency and electronic-component performance for single-carrier, dual-carrier superchannel, and 10-carrier superchannel implementations.
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FEATURE Superchannels to the rescue!
examples will use PM-QPSK as
the modulation technique:
• Option A is a single-carrier
(i.e., one wavelength) transpon-
der operating at 1 Tbps.
That’s effectively a 100G
transponder where the
electronics run 10X faster.
Unfortunately, electronics
(particularly the analog-
to-digital converter and
DSP chips) that run at the
320-Gbaud rate required
will not be available for
another decade, according to
certain industry roadmaps.
• Option B is a superchannel imple-
mentation consisting of two
500-Gbps “subcarriers,” which
are electronically combined in the
transponder card to appear as a
1-Tbps superchannel. The advan-
tage is that the performance of
the electronics is halved to 160
Gbaud. Unfortunately, we still
have to wait about seven years
before chips with this performance
level are available for products
(they may be available for hero
experiments before this, of course).
• So let’s take that to the next step
with Option C, a superchannel with
10 subcarriers, which divides the
electronics performance by 10
also – and 32-Gbaud electronics is
actually available today. However,
10 subcarriers imply 10 optical
circuits, and coherent technology
already requires a rather large
number of high-quality and there-
fore expensive optical components
even for a single optical circuit.
In fact, a 10-carrier 1-Tbps super-
channel line card would involve
around 600 optical functions in total
for the transmitter and receiver
circuit – quite impractical if built
using discrete optical chips.
Fortunately, DWDM systems
based on large-scale (i.e., multi-
carrier) photonic integrated circuits
(PICs) have been commercially
available since 2004. These PICs
predate the more recent move
toward coherent technologies,
and many skeptics in the DWDM
industry had initially expressed
their doubts that such an advanced
level of optical performance could
be delivered in a commercial PIC.
During the course of 2010 and 2011,
however, a series of incremental field
trials was completed culminating in
a terabit of superchannel capacity
being transmitted over a production
DWDM fiber link between San Jose
and San Diego on the TeliaSonera
International Carrier network last
November. The TeliaSonera trial used
twin pre-production 500G coherent
superchannel line cards, thanks to
large-scale PIC technology. Turning
up this 1 Tbps of capacity took two
operational cycles, one for each
500 Gbps of capacity. This imple-
mentation compares much more
favorably to the “multiple rack”
implementation that’s typical for a
discrete-component superchan-
nel demonstration requiring 10 line
cards of 100 Gbps of capacity.
High-order modulationThose of you with a cable, wireless,
or xDSL technology background
may already be familiar with higher-
order phase modulation. Figure 3
shows the basic principle. Binary
phase-shift keying (BPSK) uses two
phase states per modulation symbol,
which encodes 1 bit in that symbol.
By adding polarization multiple-
xing, PM-BPSK encodes 2 bits per
symbol. We can add phase states to
each symbol to encode additional
bits, enabling us to transmit higher
data rates with much better spectral
efficiency. PM-BPSK will deliver 4
Tbps in the C-band, while PM-16QAM
increases that to about 16 Tbps.
But higher-order modulation comes
at a price. Because optical fiber is a
non-linear medium, each modula-
tion symbol can only be transmitted
at a certain power level before
non-linear effects are triggered.
A series of incremental field
trials culminated in 1 Tbit of
superchannel capacity transmitted
over a production DWDM
fiber link.
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FEATURE Superchannels to the rescue!
While PM-BPSK superchannels could
well be used for transpacific subma-
rine links, the reach of a PM-16QAM
superchannel may be limited.
Going gridlessIn explaining Figure 2, I had said
that 1 Tbps of capacity will require
about the same amount of fiber
spectrum regardless of how many
subcarriers make up the super-
channel. That’s not true if the
channels are “forced apart” to
comply with a fixed grid ITU-T
G.694.1 spacing. This recommenda-
tion defines several grid spacings,
including 25 and 50 GHz. If we
assume a 10-carrier superchan-
nel of 100G per subcarrier, using
PM-QPSK, then the carrier width is
about 37 GHz. That’s too wide for
a 25-GHz grid, yet using a 50-GHz
grid will “waste” about 25% of
the available fiber spectrum.
For this reason ITU-T has updated
G.694.1 to include a “flex grid”
option based on a 12.5-GHz granu-
larity. The spectral width for a
1-Tbps gridless superchannel varies
from about 750 GHz (PM-BPSK)
to about 200 GHz (PM-16QAM).
But all of these superchannels
can be accommodated efficiently
using a multiple of 12.5 GHz.
In the short term, however,
service providers will need a super-
channel that can be deployed on
an existing grid-based DWDM line
system. So the first generation of
commercial superchannel products
will use “split spectrum” super-
channels, a term coined by the IETF.
Split spectrum superchannels could
potentially be designed to operate
on 25- or 50-GHz G694.1 grid line
systems and will provide a seamless
migration from a grid-based to
gridless architecture. Meanwhile,
they’ll also offer the required opera-
tional scaling benefits and only
sacrifice about 20–25% of the
maximum ideal fiber spectrum
(assuming PM-QPSK modulation).
OTN flexibilityAn interesting technical challenge
that results from superchannel
architectures is the need for more
f lexibility for Optical Transport
Network (OTN) transport contai-
ners. The current OTN hierarchy
defines ODU0 (1.25G), ODU1 (2.5G),
ODU2 (10G), ODU3 (40G), ODU4
(100G), and ODUflex (n x 1.25G).
ODUflex was ITU-T’s response
for more f lexible, lower-data-rate
containers. Since superchannels
may vary in their total capacity,
depending on the balance of
capacity and reach needed by the
network designer, it’s necessary to
define an “adaptable” OTN contai-
ner that can be sized accordingly.
At last December’s ITU Study
Group 15 meeting, an “OTUadapt”
proposal gained widespread
support from vendors, component
companies, and service provi-
ders. This f lexibility would help
to solve the nagging problem that
OTN containers are often “out of
sync” with next generation Ethernet
services. Gigabit Ethernet (GbE),
10GbE, and 40GbE all had diffe-
rent but significant issues in their
OTN mapping. OTUadapt will
avoid these issues in the future –
especially since the data rate for
Ethernet services beyond 100GbE
has not yet been defined (note
the IEEE standard is expected
in the 2016-17 timeframe).
Flexible capacityDWDM superchannels potenti-
ally offer an ideal solution to the
twin problems of increasing optical
BPSK1 bit per symbolper polarization
QPSK
8QAM
16QAM
2 bits persymbol perpolarization
3 bits persymbol perpolarization
4 bits persymbol perpolarization
FIGURE 3. Adding more bits to a symbol increases spectral efficiency, but the total power per symbol (before non-linear threshold is reached) is shown by the thick black circle.
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FEATURE Superchannels to the rescue!
transport capacity beyond 100
Gbps and providing the f lexibi-
lity to maximize the combination
of optical capacity and reach. By
implementing a superchannel
with many optical carriers, we can
reduce the requirement for exotic
electronics, allowing this techno-
logy to be delivered much more
quickly than other options. The
key to a multi-carrier superchan-
nel is the use of large scale PICs to
reduce optical-circuit complexity
and offer the maximum f lexibility
for an engineering design.
References1. E. Desurvire, “Capacity Demand
and Technology Challenges
for Lightwave Systems in the
Next Two Decades,” Journal
of Lightwave Technology, Vol.
24, No. 12, December 2006.
2. A. Saleh, J. Simmons, “Technology
and Architecture to Enable
the Explosive Growth of the
Internet,” IEEE Communications
Magazine, January 2011.